[djwkjp]

hi. i am trying to find a circuit i can build to charge 2 or 3 9 volt batteries that i would be using to power a portable amp like a pimeta. i had thought of using the ppa battery board, but size is a concern and that would probably be a bit too big. so i was wondering if there were parts of the circuit i could implement. or could i use a resistor inline with a dc power jack to limit the current? if that would work, would they have to be charged in parallel or series? i'm sure there are better ways to do this, so if anyone knows, please share some insights. any help is appreciated.

thanks
dan

 

[intlplby]

i have a related question

if i want to supply a pimeta with 18 volts (2x9v) but i wanted the longest battery life possible is there anything wrong with making 2-4 sets of 2x9v in series and then stringing those 2-4 sets in parallel?


Code:

[left]+ <------------------- 9v 9v 9v 9v | | | | 9v 9v 9v 9v - <-------------------[/left]

 

[tangent]

Quote:

i am trying to find a circuit i can build to charge 2 or 3 9 volt batteries

You should only trickle-charge 9V batteries, since the cells are so small. (Fast-charging produces too much heat for such small cells.)

Therefore, the charger can be quite simple, even just a simple resistor inline with the power supply. Better is to use an LM317 like in the PPA battery board's tricle configuration. Using the LM317 gives constant current no matter what the battery status is. Read the recommended links pointed to by the battery board docs, in particular the Energizer and Panasonic ones.

Quote:

would they have to be charged in parallel or series?

Series only. Parallel charging or discharging of rechargeables is very problematic.

Quote:

is there anything wrong with making 2-4 sets of 2x9v in series and then stringing those 2-4 sets in parallel?

If you're using alkalines, no, there isn't a major problem. With rechargeables, as I've said, parallel configurations have several risks.

 

[intlplby]

then i guess you could add a switch that changes it from series to parallel (switch that connects and disconnects all two-four parallel connections

 

[djwkjp]

tangent-
so you say to charge the batteries in series. what voltage do they need to be charged at? should it be higher than the total voltage of all the batteries or does that not matter?

 

[tangent]

Quote:

what voltage do they need to be charged at?

Each cell will have approximately 1.55V across it at the peak of the charge, in my experience. Your supply will need to be that plus the drop across the current source (1.25V in the case of the LM317) plus any diodes and such in the way.

Really, this is all covered in the Energizer and Panasonic documents. Read, then come back if you have questions.

 

[djwkjp]

cool. i just scanned over those articles and didnt find what i needed. i guess i will take a closer look.
thanks

 

[djwkjp]

tangent-
taking a closer look at those articles, it seems as if everything pertains to A and AA batteries, but i am planning on using 9 volt batts. is there a quick way to figure out the required current and voltage to charge a certain number of batteries? also, looking at some of the numbers for charge current, they specify it as 0.5Cma or 1Cma. what exactly is the C in Cma?
thanks

 

[Gariver]

Thomas Distribuiting has some chargers that can charge AAA, AA and 9v batteries. Great! Three in one! The one I like is the MAHA.

http://www.thomas-distributing.com/n...y_chargers.htm

I realize you want to build the circuit. However, you might decide to go for something quick and easy.

 

[tangent]

Quote:

it seems as if everything pertains to A and AA batteries

You're not seeing the forest for the trees. What the articles talk about are cells. AAs and AAAs are single cells, not "batteries". A battery is a collection of cells, as in a 9V battery, which is 6-8 cells. (7.2, 8.4 and 9.6V nominal, for rechargeables). Treat each 9V as a collection of small cells to be charged only at 0.1C or lower, and I believe you will find those documents relevant.

Quote:

is there a quick way to figure out the required current and voltage to charge a certain number of batteries.

I just gave you the voltage rule. The current rule is 0.1 times the mAh rating of the cells in question -- or below -- for trickle charging. Also called 0.1C.

So for a 170mAh Plainview 8-cell "9V" NiMH battery with an LM317 current source and one input diode for protection, you need at least 14V; the LM317 should be configured for 17mA or less. Do you see this?

The LM317 datasheet tells you how to calculate the series resistor to get a desired current level. It's just Ohm's law, not rocket surgery.

Quote:

what exactly is the C in Cma?

I don't know what C is supposed to stand for, but 1C means the amount of mA it would take to charge an ideal cell/battery in 1 hour. An ideal 100mAh cell would take 100mA for one hour to charge fully. In practice, inefficiencies mean it won't be fully charged in one hour, but you still say 0.1C to mean 10mA in this example.

As I recall, the Panasonic book uses a different notation scheme, but you should readily suss out how to convert between the two. The old C scheme has some conceptual flaws, but everyone knows how it works, so no competing scheme has really superceded it.

 

[drewd]

Quote:

Originally Posted by djwkjp tangent- taking a closer look at those articles, it seems as if everything pertains to A and AA batteries, but i am planning on using 9 volt batts. is there a quick way to figure out the required current and voltage to charge a certain number of batteries? also, looking at some of the numbers for charge current, they specify it as 0.5Cma or 1Cma. what exactly is the C in Cma? thanks

A 9 volt battery is made up of 6 small 1.5v cells (1.2v for NiMH). The "C" in Cma is the "C-rate", or the amount of current that the battery can deliver in one hour. So, when the forumlae talk about some fraction of Cma, they're referring to the battery's mAh rating.

Since you'll have a rough idea of the mAh rating of your batteries, the documents that Tangent referred to earlier should give you an idea of the current and voltage required to charge your batteries and help you determine whether to use a fast or trickle charging scheme.

-Drew

 

[djwkjp]

so would you recommend a proper size resistor inline with a dc jack for charging? i will probably take a look at a data sheet for a lm317 and see if i can make sense out of anything there. thanks for the help guys.

 

[tangent]

Quote:

would you recommend a proper size resistor

No, I wouldn't. Spend the 50 cents for an LM317 and do it right.

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see if i can make sense out of anything there

Again, this is a simple application of Ohm's law. If you're going to keep at this DIY stuff, you need to get a handle on that anyway.