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Speaker amps for headphones - Page 114

post #1696 of 3116

I threw up a quick JavaScript Headphone Resistor Network Calculator featuring only the preferred 2 resistor network. It's still pretty basic but it's here: http://robrobinette.com/HeadphoneResistorNetworkCalculator.htm

 

So you guys that don't do spreadsheets should be able to plug in the numbers and play around.


Edited by robrob - 12/2/13 at 10:15pm
post #1697 of 3116

Ergh, a few things...

 

I don't know why the java page has both load impedance and output impedance.

 

"amplifier load impedance" should be "effective speaker load"

"headphone input impedance" should be "effective headphone output impedance"

 

"amplifier attenuation" would make more sense inverted and expressed as a fraction (at least to me it does)

it would also be nice to express it as a change in dB (equation is 20logA where A is the voltage divider ratio)

 

I haven't chequed the actual calculations/equations yet. There seem to be a lot of nested logic statements in there though... but if the equations are set up correctly, you should be able to input values of zero and get valid answers

 

Scenario 0: no resistors at all

- if you don't have a problem with noise and you have enough useful range on the volume pot, then don't mess with a good thing

 

Scenario 1: load resistor

- a safety precaution for transformer coupled tube amps

- also may be used for amps that are designed for specific stable speaker loads

- does not affect output to the headphones at all (in theory) when used by itself

- use this scenario if you already have good control of the pot with no noise, but you're paranoid about damaging your amp because you don't know what kind of load it needs, if any

 

Scenario 1.2: you should add another scenario here which is only a series resistor

- this is a voltage divider with the ratio formed by R2 and the headphone, which means dynamic headphones with changing impedance curves will experience changes to frequency response

- use this scenario if you need more range on the pot and/or want to reduce noise, and you have a planar magnetic headphone

- may also be used with a dynamic headphone, but frequency response may not be accurate (some may prefer this in fact)

 

scenario 2: dual resistor A

- basic voltage divider configuration; ratio controlled primarily by the resistors (and affected only slightly by the headphone)

- use this scenario if you need more range on the pot and/or want to reduce noise, and you have a dynamic headphone and what more accurate response

 

scenario 3: dual resistor B

- the configuration used by the Hifiman adapter box

- essentially scenario 1 and 1.2 together, so a voltage divider plus load resistor for stability

- use this scenario because you like your women like you like your coffee... covered in bees

 

scenario 4: three resistor network

- basically scenario 2 plus scenario 1 to give the amp a more stable load if you're using a dynamic headphone with large impedance curves

- only use this scenario if you need to reduce noise and gain, AND you have a headphone with wild impedance curves, AND you're paranoid about your amp's loading requirements


Edited by Armaegis - 12/2/13 at 11:58pm
post #1698 of 3116

^ What he said!

post #1699 of 3116

Blargh I just went back and edited that previous one with way too much stuff.

post #1700 of 3116
Below is a guide I have found and after reading it , it is clear that what you are doing with the batteries is fine.
Apparently how the batteries are used in the Rc hobby is different then in the application for slow discharge .

there is some voltages I quoted are not correct for the above use.

Sorry ...



Laurel, Maryland USA
Joined Aug 2000
4,291 Posts
Complete Guide to Lithium Polymer Batteries and LiPo Failure Reports
After seeing the many many posts on LiPoly's and answering similar questions time after time I've decided to put up a guide for using LiPoly batteries.

Lithium batteries are the preferred power sources for most electric modelers today. They offer high discharge rates and a high energy storage/weight ratio. However, using them properly and charging them correctly is no trivial task. There are many things to consider before using lithium cells for e-flight. But none is more important than safety.

1. Charging/Saftey IMPORTANT!
Until you are willing to follow all saftey precautions, DO NOT use lithium batteries. If your a type of person that prefers to push the limits of products, or be haphazard about following saftey requirements. Lithium technology is not for you. Read on to find out why.

Lithium cells must be charged very differently than NiCad or NiMH. They require a special charger specifically designed to charge lithium cells. In general any charger that can charge lithium ion can charge lithium polymer, assuming that the cell count is correct. You must NEVER charge lithium cells with a NiCad or NiMH only battery charger. This is dangerous. Charging cells is the most hazardous part of using lithium batteries. EXTREME care must be taken when charging them. It is important to set your charger to the correct voltage or cell count. Failure to do this can cause the battery to spew violent flames. There have been many fires directly caused by lithium batteries. PLEASE BE RESPONSIBLE when charging lithium batteries.

Here are a few MANDATORY guidelines for charging/using LiPos (Lithium Polymer Batteries).


1. Use only a charger approved for lithium batteries. The charger may be designed for Li-Ion or Li-Poly. Both batteries are charged in exactly the same. Some older cell phone chargers may charge the batteries .1 volt to low (4.1 vs 4.2), but that will not harm the battery. However, inexpensive lithium chargers are widely available and the use of cellphone chargers is highly discouraged.
2. Make certain that the correct cell count is set on your charger. Watch the charger very closely for the first few minutes to ensure that the correct cell count continues to be displayed. If you don't know how to do that, get a charger that you do know how or don't charge the batteries.
3. Use the Taps. Before you charge a new Lithium pack, check the voltage of each cell individually. Then do this after every tenth cycle there after. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen. If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced.
Taps are provided on most new lithium packs. Taps give you the ability to check individual cell voltages and charge one cell at a time. Make sure and get the appropriate connector to go into your taps. Don't try to stick you volt meter probes in the taps to measure voltage. They could slip and short your cells. Don't try to charge more than one cell at a time from the taps. Unless you have an isolated ground charging system, you'll short your batteries out. Refer to your individual cell maker for tap pin-outs.
4. NEVER charge the batteries unattended. This is the number one reason for houses and cars being burned to a crisp by lithium fires.
5. Use a safe surface to charge your batteries on so that if they burst into flame no damage will occur. Vented fire safes, pyrex dishes with sand in the bottom, fireplaces, plant pots, are all good options.
6. DO NOT CHARGE AT MORE THAN 1C unless specifically authorized by the pack vendor. I have personally had a fire in my home because of violating this rule. Todays highest discharge batteries can supposedly be safely charged at greater than 1C, however so far in all cases doing so shortens the life of the pack. Better to buy 3 packs than to try to charge 1 pack 3 times quickly. This may change in the future but as of Winter 2005 1C is still the recommended charge rate.
7. DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in the fire safe place for at least 2 hours. Discharge the cell/pack slowly. This can be done by wiring a flashlight bulb of appropriate voltage (higher is voltage is ok, lower voltage is no) up to your batteries connector type and attaching the bulb to the battery. Wait until the light is completely off, then throw the battery away.
8. If you crash with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crash in ANY way carefully remove the battery pack from the aircraft and watch it carefully for at least the next 20 min. Several fires have been caused by damaged cells being thrown in the car and then the cells catch fire later and destroys the car completely.
9. Charge your batteries in a open ventilated area. If a battery does rupture or explode hazardous fumes and material will spew from the battery.
10. Keep a bucket of sand nearby when you are flying or charging batteries. This is a cost effective way to extinguish fires. This is very cheap and absolutly necessary.
11. It can happen to you, do not think to yourself that “it won't happen to me” as soon as you do that it you'll be trying to rescue your kids from your burning house or car. I'm very serious about this.

Now that we have covered that important topic let's move on to lighter matters:

2. Lithium What?
Lithium Polymer batteries are used in many electronic devices. Cell Phone, Laptops, PDA's, Hearing Aids just to name a few. Most, if not all, lithium polymer batteries are not designed for RC use, we use them in different applications than they were designed for. They are similar to Lithium Ion batteries in that they each have a nominal voltage of 3.6 volts, but dissimilar in that they do not have a hard metal casing but rather a flexible material encloses the chemicals inside. The "normal" lithium polymer batteries are thin rectangle shapes with two tabs on the top one positive one negative. The reason we use Lithium cells is that they are significantly lighter than comparable NiCad or NiMH batteries, which makes our planes fly longer and better.

3. Voltage and Cell Count:
LiPolys act differently than NiCad or NiMH batteries do when charging and discharging. Lithium batteries are fully charged when each cell has a voltage of 4.2 volts. They are fully discharged when each cell has a voltage of 3.0 volts. It is important not to exceed both the high voltage of 4.2 volts and the low voltage of 3.0 volts. Exceeding these limits can harm the battery.
The way to ensure that you do not go below 3.0 volts while flying is to set the low voltage cutoff (LVC) of your electronic speed control (ESC). It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC's programming mode to set the LVC to 3.0 volts per cell with a hard cutoff, or 3.3 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.0 volts per cell. You will know when flying that it is time to land when you experience a sudden drop in power caused by the LVC.
If your ESC has an automatic lithium mode. Use it, it will correctly sense the number of cells and set the auto cutoff appropriately.
If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will result in a different number of cells being used. If you had 6 to 7 round cells then 2 lithium polymer cells will correctly duplicate the voltage of those cells. If you had 10-11 cells then 3 lithium polymer cells would be right for you. There are a lot of 8 cell flyer's out there that are stuck between 2 and 3 cells. In my experience the best option is to determine how many watts you were using before and duplicate that with your LiPos, Motor, and Prop. For example. If you were running 8 cells (9.6volts) at 10 amps on a speed 400 airplane, then you have 9.6 x10, 96 watts. So if you went with 2 lithium polymer cells (7.2 volts nominal) then you'd need to change your prop such that you used 13 amps. If you went to 3 LiPoly's (10.8 volts nominal) then you'd need to reduce the amperage to 8.9 amps. These estimates are approximate, and some experimentation is required for best results but conserving Watts is a good way to start.

4.10C from 3S4P? Naming conventions explained.
How fast a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3).
All batteries have limitations on how fast they can discharge. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 1 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPoly batteries at higher currents than single cells could produce.
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it's a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3S pack since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).

5. Which battery should you buy?
With so many choices out there it is difficult to decipher what is marketing hype, what is brand
loyalty, and what is outright lies. Battery manufacturers are constantly trying to one up one another. While capitalism can drive prices down, it also can give cause to false claims about products.
One great way to find out what the best battery is, is to look at graphs of the batteries performance. Looking at how low the voltage of the cell drops at various amperages will give you a metric to compare that battery to similar size/weight batteries.
If graphs aren't your thing then simply look at what other people are using in successful setups that are similar to your application. If a lot of people are reporting long flight times and lots of power from airplane X, with power system Y, and battery Z and you do the same, then if your setup is similar the same battery will probably work well for you.
It pays to learn something about Watts, Volts, and Amps. Understanding these concepts is beyond the scope of this document, but can serve you well in not only figuring out what battery is best but also in your electric aircraft hobby.
I'm not convinced that a 30C battery is really any better than a 10 or 20C battery. Sure a higher C rating means it can discharge faster. But at the same time a battery discharged at 20C continuously will be empty in 3 minutes. Do you really only want to use the battery for 3 minutes? I love having burst power in helicopters and boats, but in almost all other applications actually running a battery at or above 20C is useless to me. I prefer to run batteries at 8-10 C and have a little headroom if I need it.
A final note on choosing a battery. Don't cheap out. Confirm that your batteries are capable of running that the amperage level you plan to use them at. Running a cell at a higher C rating than the battery can handle can not only damage your batteries, but it can also damage your speed control. Castle Creations has an excellent article on how using a weak battery can destroy a perfectly good speed control of any brand. Better to buy a bit better battery than you need than to destroy your electronics.

6. Dealing with temperature.
Lithium batteries like heat, but not too much. In the winter time, try to keep your batteries from the cold as much as possible. Leave them in the car while your flying, or keep them in your cargo pants... etc. At the same time don't let them heat up too much. Try to keep your batteries from reaching 160F after use. This will prolong the life of the cells. A good way to measure temperature is a handheld IR meter, they can be found for around $50.00 at most hobby shops.

If you have any suggestions for future sections or additions to the current document let me know and I'll add the information requested...if I know the answer!

Al D
post #1701 of 3116
Quote:
Originally Posted by Armaegis View Post
 

Ergh, a few things...

 

I don't know why the java page has both load impedance and output impedance.

 

"amplifier load impedance" should be "effective speaker load"

"headphone input impedance" should be "effective headphone output impedance"

 

"amplifier attenuation" would make more sense inverted and expressed as a fraction (at least to me it does)

it would also be nice to express it as a change in dB (equation is 20logA where A is the voltage divider ratio)

 

I haven't chequed the actual calculations/equations yet. There seem to be a lot of nested logic statements in there though... but if the equations are set up correctly, you should be able to input values of zero and get valid answers

 

Scenario 0: no resistors at all

- if you don't have a problem with noise and you have enough useful range on the volume pot, then don't mess with a good thing

 

Scenario 1: load resistor

- a safety precaution for transformer coupled tube amps

- also may be used for amps that are designed for specific stable speaker loads

- does not affect output to the headphones at all (in theory) when used by itself

- use this scenario if you already have good control of the pot with no noise, but you're paranoid about damaging your amp because you don't know what kind of load it needs, if any

 

Scenario 1.2: you should add another scenario here which is only a series resistor

- this is a voltage divider with the ratio formed by R2 and the headphone, which means dynamic headphones with changing impedance curves will experience changes to frequency response

- use this scenario if you need more range on the pot and/or want to reduce noise, and you have a planar magnetic headphone

- may also be used with a dynamic headphone, but frequency response may not be accurate (some may prefer this in fact)

 

scenario 2: dual resistor A

- basic voltage divider configuration; ratio controlled primarily by the resistors (and affected only slightly by the headphone)

- use this scenario if you need more range on the pot and/or want to reduce noise, and you have a dynamic headphone and what more accurate response

 

scenario 3: dual resistor B

- the configuration used by the Hifiman adapter box

- essentially scenario 1 and 1.2 together, so a voltage divider plus load resistor for stability

- use this scenario because you like your women like you like your coffee... covered in bees

 

scenario 4: three resistor network

- basically scenario 2 plus scenario 1 to give the amp a more stable load if you're using a dynamic headphone with large impedance curves

- only use this scenario if you need to reduce noise and gain, AND you have a headphone with wild impedance curves, AND you're paranoid about your amp's loading requirements

 

Scenario 4:

you're over thinking it, mate.

Scenario 2 does all that already.

in Scenario 2, a 2 or 3 or 4 Ohm resistor shunt across the headphone will dominate the headphone impedance and create a "stable" load for the amp to drive even if the 'phone has wild impedance swings.

in scenario 2 it is easy to set up the values such that the load on the amp is CLOSE ENOUGH to 8 Ohms if you are OCD.

example: 10 + 2 = 12 Ohms, close enough to 8 Ohms.

amps ain't that fussy, audiophiles are.


Edited by Chris J - 12/3/13 at 2:21pm
post #1702 of 3116
Quote:
Originally Posted by Armaegis View Post
 

Blargh I just went back and edited that previous one with way too much stuff.

 

^ What he didn't say!

post #1703 of 3116
Quote:
Originally Posted by ALRAINBOW View Post

Below is a guide I have found and after reading it , it is clear that what you are doing with the batteries is fine.
Apparently how the batteries are used in the Rc hobby is different then in the application for slow discharge .

there is some voltages I quoted are not correct for the above use.

Sorry ...

  Great stuff on using LiPo batteries... (Click to show)
Laurel, Maryland USA
Joined Aug 2000
4,291 Posts
Complete Guide to Lithium Polymer Batteries and LiPo Failure Reports
After seeing the many many posts on LiPoly's and answering similar questions time after time I've decided to put up a guide for using LiPoly batteries.

Lithium batteries are the preferred power sources for most electric modelers today. They offer high discharge rates and a high energy storage/weight ratio. However, using them properly and charging them correctly is no trivial task. There are many things to consider before using lithium cells for e-flight. But none is more important than safety.

1. Charging/Saftey IMPORTANT!
Until you are willing to follow all saftey precautions, DO NOT use lithium batteries. If your a type of person that prefers to push the limits of products, or be haphazard about following saftey requirements. Lithium technology is not for you. Read on to find out why.

Lithium cells must be charged very differently than NiCad or NiMH. They require a special charger specifically designed to charge lithium cells. In general any charger that can charge lithium ion can charge lithium polymer, assuming that the cell count is correct. You must NEVER charge lithium cells with a NiCad or NiMH only battery charger. This is dangerous. Charging cells is the most hazardous part of using lithium batteries. EXTREME care must be taken when charging them. It is important to set your charger to the correct voltage or cell count. Failure to do this can cause the battery to spew violent flames. There have been many fires directly caused by lithium batteries. PLEASE BE RESPONSIBLE when charging lithium batteries.

Here are a few MANDATORY guidelines for charging/using LiPos (Lithium Polymer Batteries).


1. Use only a charger approved for lithium batteries. The charger may be designed for Li-Ion or Li-Poly. Both batteries are charged in exactly the same. Some older cell phone chargers may charge the batteries .1 volt to low (4.1 vs 4.2), but that will not harm the battery. However, inexpensive lithium chargers are widely available and the use of cellphone chargers is highly discouraged.
2. Make certain that the correct cell count is set on your charger. Watch the charger very closely for the first few minutes to ensure that the correct cell count continues to be displayed. If you don't know how to do that, get a charger that you do know how or don't charge the batteries.
3. Use the Taps. Before you charge a new Lithium pack, check the voltage of each cell individually. Then do this after every tenth cycle there after. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen. If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced.
Taps are provided on most new lithium packs. Taps give you the ability to check individual cell voltages and charge one cell at a time. Make sure and get the appropriate connector to go into your taps. Don't try to stick you volt meter probes in the taps to measure voltage. They could slip and short your cells. Don't try to charge more than one cell at a time from the taps. Unless you have an isolated ground charging system, you'll short your batteries out. Refer to your individual cell maker for tap pin-outs.
4. NEVER charge the batteries unattended. This is the number one reason for houses and cars being burned to a crisp by lithium fires.
5. Use a safe surface to charge your batteries on so that if they burst into flame no damage will occur. Vented fire safes, pyrex dishes with sand in the bottom, fireplaces, plant pots, are all good options.
6. DO NOT CHARGE AT MORE THAN 1C unless specifically authorized by the pack vendor. I have personally had a fire in my home because of violating this rule. Todays highest discharge batteries can supposedly be safely charged at greater than 1C, however so far in all cases doing so shortens the life of the pack. Better to buy 3 packs than to try to charge 1 pack 3 times quickly. This may change in the future but as of Winter 2005 1C is still the recommended charge rate.
7. DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in the fire safe place for at least 2 hours. Discharge the cell/pack slowly. This can be done by wiring a flashlight bulb of appropriate voltage (higher is voltage is ok, lower voltage is no) up to your batteries connector type and attaching the bulb to the battery. Wait until the light is completely off, then throw the battery away.
8. If you crash with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crash in ANY way carefully remove the battery pack from the aircraft and watch it carefully for at least the next 20 min. Several fires have been caused by damaged cells being thrown in the car and then the cells catch fire later and destroys the car completely.
9. Charge your batteries in a open ventilated area. If a battery does rupture or explode hazardous fumes and material will spew from the battery.
10. Keep a bucket of sand nearby when you are flying or charging batteries. This is a cost effective way to extinguish fires. This is very cheap and absolutly necessary.
11. It can happen to you, do not think to yourself that “it won't happen to me” as soon as you do that it you'll be trying to rescue your kids from your burning house or car. I'm very serious about this.

Now that we have covered that important topic let's move on to lighter matters:

2. Lithium What?
Lithium Polymer batteries are used in many electronic devices. Cell Phone, Laptops, PDA's, Hearing Aids just to name a few. Most, if not all, lithium polymer batteries are not designed for RC use, we use them in different applications than they were designed for. They are similar to Lithium Ion batteries in that they each have a nominal voltage of 3.6 volts, but dissimilar in that they do not have a hard metal casing but rather a flexible material encloses the chemicals inside. The "normal" lithium polymer batteries are thin rectangle shapes with two tabs on the top one positive one negative. The reason we use Lithium cells is that they are significantly lighter than comparable NiCad or NiMH batteries, which makes our planes fly longer and better.

3. Voltage and Cell Count:
LiPolys act differently than NiCad or NiMH batteries do when charging and discharging. Lithium batteries are fully charged when each cell has a voltage of 4.2 volts. They are fully discharged when each cell has a voltage of 3.0 volts. It is important not to exceed both the high voltage of 4.2 volts and the low voltage of 3.0 volts. Exceeding these limits can harm the battery.
The way to ensure that you do not go below 3.0 volts while flying is to set the low voltage cutoff (LVC) of your electronic speed control (ESC). It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC's programming mode to set the LVC to 3.0 volts per cell with a hard cutoff, or 3.3 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.0 volts per cell. You will know when flying that it is time to land when you experience a sudden drop in power caused by the LVC.
If your ESC has an automatic lithium mode. Use it, it will correctly sense the number of cells and set the auto cutoff appropriately.
If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will result in a different number of cells being used. If you had 6 to 7 round cells then 2 lithium polymer cells will correctly duplicate the voltage of those cells. If you had 10-11 cells then 3 lithium polymer cells would be right for you. There are a lot of 8 cell flyer's out there that are stuck between 2 and 3 cells. In my experience the best option is to determine how many watts you were using before and duplicate that with your LiPos, Motor, and Prop. For example. If you were running 8 cells (9.6volts) at 10 amps on a speed 400 airplane, then you have 9.6 x10, 96 watts. So if you went with 2 lithium polymer cells (7.2 volts nominal) then you'd need to change your prop such that you used 13 amps. If you went to 3 LiPoly's (10.8 volts nominal) then you'd need to reduce the amperage to 8.9 amps. These estimates are approximate, and some experimentation is required for best results but conserving Watts is a good way to start.

4.10C from 3S4P? Naming conventions explained.
How fast a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3).
All batteries have limitations on how fast they can discharge. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 1 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPoly batteries at higher currents than single cells could produce.
The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it's a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3S pack since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).

5. Which battery should you buy?
With so many choices out there it is difficult to decipher what is marketing hype, what is brand
loyalty, and what is outright lies. Battery manufacturers are constantly trying to one up one another. While capitalism can drive prices down, it also can give cause to false claims about products.
One great way to find out what the best battery is, is to look at graphs of the batteries performance. Looking at how low the voltage of the cell drops at various amperages will give you a metric to compare that battery to similar size/weight batteries.
If graphs aren't your thing then simply look at what other people are using in successful setups that are similar to your application. If a lot of people are reporting long flight times and lots of power from airplane X, with power system Y, and battery Z and you do the same, then if your setup is similar the same battery will probably work well for you.
It pays to learn something about Watts, Volts, and Amps. Understanding these concepts is beyond the scope of this document, but can serve you well in not only figuring out what battery is best but also in your electric aircraft hobby.
I'm not convinced that a 30C battery is really any better than a 10 or 20C battery. Sure a higher C rating means it can discharge faster. But at the same time a battery discharged at 20C continuously will be empty in 3 minutes. Do you really only want to use the battery for 3 minutes? I love having burst power in helicopters and boats, but in almost all other applications actually running a battery at or above 20C is useless to me. I prefer to run batteries at 8-10 C and have a little headroom if I need it.
A final note on choosing a battery. Don't cheap out. Confirm that your batteries are capable of running that the amperage level you plan to use them at. Running a cell at a higher C rating than the battery can handle can not only damage your batteries, but it can also damage your speed control. Castle Creations has an excellent article on how using a weak battery can destroy a perfectly good speed control of any brand. Better to buy a bit better battery than you need than to destroy your electronics.

6. Dealing with temperature.
Lithium batteries like heat, but not too much. In the winter time, try to keep your batteries from the cold as much as possible. Leave them in the car while your flying, or keep them in your cargo pants... etc. At the same time don't let them heat up too much. Try to keep your batteries from reaching 160F after use. This will prolong the life of the cells. A good way to measure temperature is a handheld IR meter, they can be found for around $50.00 at most hobby shops.

If you have any suggestions for future sections or additions to the current document let me know and I'll add the information requested...if I know the answer!

Al D

 

 

 

Hey Al,

 

First, my hat's off to you for being a man of good integrity and character, but no apology is necessary.  

 

I had never even considered the distinction between slow discharge (as I do) and fast discharge (as these batteries are normally used by RC enthusiasts).

 

There were several golden nuggets in that article you quoted, by the way.  In my three years experience with RC LiPo's I've always understood that one could multiply the C rating by the mAh rating to calculate the highest continuous current output (in mA) that a LiPo battery can sustain without overheating.  But...  I've never read (anywhere) this definition of the C rating - which suddenly makes perfect sense:  

 

Quoting from the spoiler, above:
C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. 

 

And I had never heard of paralleling LiPo packs to increase the total C rating, and I had always wondered why every pack I see for sale is labeled xS1P.  I understood that the first digit referred to the number of series-arranged cells in the pack, but I always wondered why they even bother saying "1P."  Have you ever seen a pack that's sold as an xS2P or xS3P?  (Just curious, as I certainly don't need a higher C rating for my 4-Amp MG3.)  :p

 

By the way, I don't know of even one other person running RC LiPo packs for speaker amps with headphones, so I'm afraid we're off topic.  Shall we continue via PM?  (Where we can freely talk about what everyone else is missing in terms of sound quality, and expound on the sheer joy of simply surrounding oneself with lots of batteries, sophisticated balance chargers, cables, connectors, alarm monitors, charging bags, smoke detectors, and buckets of sand and such!)  

 

:D

 

Mike


Edited by zilch0md - 12/3/13 at 6:32am
post #1704 of 3116
Absolutely sir
post #1705 of 3116
Quote:
Originally Posted by Chris J View Post

Scenario 4:
you're over thinking it, mate.
Scenario 2 does all that already.
in Scenario 2, a 2 or 3 or 4 Ohm resistor shunt across the headphone will dominate the headphone impedance and create a "stable" load for the amp to drive even if the 'phone has wild impedance swings.
in scenario 2 it is easy to set up the vvalues such that the load on the amp is CLOSE ENOUGH to 8 Ohms if you are OCD.
example: 10 + 2 = 12 Ohms, close enough to 8.
amps ain't that fussy, audiophiles are.

+1

KISS returns!
post #1706 of 3116

Armaegis, Thanks for the great feedback. I have some questions:

 

Quote:
 I don't know why the java page has both load impedance and output impedance.

I have amp output impedance (default value of 0.1) there so you can set the value if you know it. I'm using it in the Effective Headphone Output Impedance calculation.

 

Quote:

 it would also be nice to express it as a change in dB (equation is 20logA where A is the voltage divider ratio)

Is that log base 20?

Is that dBi or dBl?

 

Quote:
 if the equations are set up correctly, you should be able to input values of zero and get valid answers

Great suggestion, I will add that feature. Right now a 0 value gives a DIV 0 Error.

 

Armaegis, could you take a look at the equations and verify you agree with how I'm calculating the Effective Speaker Load and Attenuation?


Edited by robrob - 12/3/13 at 9:04am
post #1707 of 3116
Quote:
Originally Posted by potterma View Post


+1

KISS returns!

It was here before? :D

post #1708 of 3116
Quote:
Originally Posted by robrob View Post
 

I have amp output impedance (default value of 0.1) there so you can set the value if you know it. I'm using it in the Effective Headphone Output Impedance calculation.

 

on the Java page I don't understand what the "amplifier speaker load" is there for. The only parameters you should have are headphone/speaker load (which should read "nominal impedance") and amp output impedance

 

Quote:
Originally Posted by robrob View Post
 

Is that log base 20?

Is that dBi or dBl?

 

Great suggestion, I will add that feature. Right now a 0 value gives a DIV 0 Error.

 

log base 10

 

just dB

 

oh DIV0... I'm thinking algebraically, not like a computer... when we say R2=0, that's fine because a straight wire has basically zero impedance. When we say R3=0, technically we're wrong because we're not short circuiting the output, we're putting nothing there so R3=infinitely high.

 

to make your life easier in that spreadsheet, do some intermediate calculations in a hidden protected cell somewhere (I'm thinking mostly for the parallel calculations)

post #1709 of 3116

Here are the formulas I'm using to calculate Effective Speaker Load, Effective Headphone Output Impedance, and Attenuation:

 

 

Quote:
 oh DIV0... I'm thinking algebraically, not like a computer... when we say R2=0, that's fine because a straight wire has basically zero impedance. When we say R3=0, technically we're wrong because we're not short circuiting the output, we're putting nothing there so R3=infinitely high.

 

This is now incorporated into the spreadsheet formulas but not the JavaScript page.


Edited by robrob - 12/3/13 at 1:09pm
post #1710 of 3116
Quote:
Originally Posted by robrob View Post
 

I threw up a quick JavaScript Headphone Resistor Network Calculator featuring only the preferred 2 resistor network. It's still pretty basic but it's here: http://robrobinette.com/HeadphoneResistorNetworkCalculator.htm

 

So you guys that don't do spreadsheets should be able to plug in the numbers and play around.

 

I think you may have hosed up one concept.    :o

 

If resistor 2 = 6 Ohms and you assume the amplifiers output impedance is 0.1 Ohms, then the amplifier's output impedance is ACTUALLY 6 + 0.1 = 6.1 Ohms.

 

But you want to calculate "Effective Headphone Output Impedance", i.e from the headphone's point of view, what is the apparent output impedance?

 

This number will be 2 Ohms in parallel with 6.1 Ohms, i.e. 1. 506 Ohms.

If you neglect the amplifier's assumed output impedance (i.e. 0.1 Ohms), then you get 1.5 Ohms.

Hence, the amplifier's output impedance may be neglected in a SS amp in this configuration, especially since the output impedance will probably be much less than 0.1 Ohms.

 

But if it's a tube power amp AND the output impedance is as high as an Ohm or two, then it starts to get more significant.

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