[nutt7]

Hello! I have 4 ipod mini batteries that I would like to run in series to power my cmoy. They are 3.7v and claim 1300mah each (all new, too). That should get me 14.8v at 1300mah. Each has it's own protection circuit built in (I think).

Is this a safe set-up? I could only find warnings on parallel set ups.

To charge, I have an old 7.4v li-ion Nikon charger from my old camera. I planned to series charge a pair of batteries at a time. I also thought about doing a 2p2s for charging as if it were a 7.4v 2600mah battery, but wasn't sure if that would be wise.

Any advice would be appreciated. I don't want to simply try this one without a little more advice. Don't wanna start a fire...

 

[wakibaki]

4 lithium cells sounds good to me.

How to organize them?

I'd go for 2 lots of 2 in series, to give you a dual rail supply, +/-7.4V, with a switch to put them in parallel for charging (2p2s).

This leaves you with a couple problems. You've got no equalization of cells when charging. Well, this is probably not of too much significance when they are new, and if you balance them every so often, you will get away with it. The other thing is that you won't be able to run the device while charging, if this is important to you.

You do get to use all your parts in hand, tho', and you get a supply with a proper ground and freedom from the crosstalk which is a potential problem with a rail-splitter.

w

 

[2leftears]

Follow wakibaki's advice, and put them in parallel when charging.  Never series-charge Lithium chemistry batteries.
 
Charging has to stop at a reasonably precise cutoff voltage level (usually 4.20V), and both Lithium polymer and Lithium ion batteries cannot take any trickle charging at all.  Overcharging can cause the formation of metallic lithium on the anode, which can grow christal-like structure that may eventually pierce the isolator membrane.  This can cause thermal runaway and a fire.
 
The following is a good source of information:
 
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

 

[nutt7]

I don't know how to balance the cells. Is this a big deal? I also thought about a real ground but was unsure of any unevenness of batteries, and I'd prefer not to hassle with figuring out the balancing thing. I may stick to the splitter unless you convince me it's worth the hassle of balancing batteries and redoing my PS .

So it seems the 2p2s charge will be ok? 2leftears, you said to avoid series charging...does this include how wakibaki said to do it? Some articles seem to say its fine if I can balance the cells...but my charger doesn't have this. I also don't care about charging while listening. I plan to use 4 pin plugs that put them in series on the amp and parallel on the charger.

 

[tangent]

Lithium chemistries are literally flammable. It is foolish to DIY electronics powered by lithium chemistry batteries if you aren't willing to get down in the weeds and take the time to understand what you're doing, and do it right.
 
If you want a rechargeable battery you can beat up and not worry much about if you screw it up, stick with nickel chemistries.

 

[wakibaki]

Nickel batteries have been known to catch fire, too.

Most major IC manufacturers offer products aimed at building chargers for lithium cells, including chargers intended for cells in series. Often these have no provision for the monitoring of individual cells' voltages, or for balancing the cells in a pack. A charger built by Nikon, intended for charging to 7.4V nominal, should cope with parallel charging 2 series strings of (new) identical batteries without problem. You can of course opt to charge the 2 strings one-at-a-time.

Be aware, however, of the warning issued by tangent, which would probably be echoed by other individuals more cautious than I. You might like to charge the batteries in, say, a garage, on a concrete floor, at a distance from other flammables. My experience of lithium battery packs, which includes multi-cell packs used for electric flight, tends to suggest, however, that although fires are not unknown, their frequency has been exaggerated by the not unreasonable concern on the part of manufacturers not to expose themselves to a lawsuit.

w

 

[tangent]

Nickel batteries have been known to catch fire, too.

 
Sure, nickel battery plastic casings can catch fire if you badly abuse the battery. With NiMH, you also have the possibility of hydrogen buildup, if the battery itself doesn't have a vent or is placed into a sealed container. Plastics used for electronics enclosures don't continue to burn long once the heat source is removed — ever try to burn a plastic fork while camping? — and the danger of a bit of hydrogen is fairly small in the typical case.
 
With lithium, though, you have a flammable electrolyte. It can exhibit thermal runaway, burning until the fuel is nearly exhausted. There's simply more to burn in the lithium case, and more potential for it.

 

[2leftears]

I don't know how to balance the cells. Is this a big deal? I also thought about a real ground but was unsure of any unevenness of batteries, and I'd prefer not to hassle with figuring out the balancing thing. I may stick to the splitter unless you convince me it's worth the hassle of balancing batteries and redoing my PS

.

So it seems the 2p2s charge will be ok? 2leftears, you said to avoid series charging...does this include how wakibaki said to do it? Some articles seem to say its fine if I can balance the cells...but my charger doesn't have this. I also don't care about charging while listening. I plan to use 4 pin plugs that put them in series on the amp and parallel on the charger.

I must admit that I am not familiar with the 2p2s terminology, but in any case I would recommend the batteries should be charged all in parallel, i.e. each with it's own charger & monitor.  If you use 4 batteries in total that would mean 4 individual charging circuits.
 
I could be wrong but I would not be surprised if the Nikon battery & charger you are referring to have more than two electrical contacts, which will allow for individual charging & montoring of each cell in the Nikon battery (and possibly temperature monitoring, some litium-ion batteries have dedicated contacts for that as well).
 
Having said that, I have seen many well known manufacturers produce seriously poor chargers almost by default, especially for NiMH and NiCd batteries most of them are absolute garbage.  You know the drill:
 
"with care these batteries can be recharged up to 1000x * "
 
*but definitely not by using the charger provided (usually omitted bit of small print)
 
 
As the other posters have said, litium chemistry batteries are inherently dangerous.  If you are not sure about correct charging and discharging methods (and it doesn't sound like you are), the improved energy/weight ratio of using lithium based batteries over Nickel based ones is really not worth the risk.  It is not straigtforward (ask Boeing).
 
Even the aviation authorities are now starting to get nervous about the quantity of Li-ion consumer batteries carried onto the average aeroplane by the passengers...

 

[nutt7]

Tangent, I am in process of learning. I do appreciate the stern warning. I mainly want li ion due to its punch per size benefit. I have a small tin for my cmoy, and find myself wanting more voltage and current for some super low impedance headphones. 2 9v won't work.

Also, through what I've learned, it seems that charging multiple batteries could be problematic. Is there as much problem in using the batteries in series or parallel? 2leftears, I think you warned this by stressing proper discharging too? Also 2p2s means 2cells in parallel, then a pair of those in series. I guess my situation would be better described as 2s2p

 

[tangent]

I mainly want li ion due to its punch per size benefit.

 
Nothing comes for free.
 
If we had fusion batteries, we'd have DIYers jumping up and down to use them, and isn't that cool yellow and black triangle logo on the side awesome?
 

find myself wanting more voltage and current for some super low impedance headphones.

 
Have you read my Op Amp Working Voltage Considerations article?
 

2 9v won't work. 

 
Really?
 
As for voltage, here's a short exercise from the article:
 
18V runs down to about 14V in alkaline. Subtract 6V for a non-RRIO op-amp, and another 1V for virtual ground shift, which leaves 7V. Dividing that by 2.828 gives ~2.5Vrms into your cans. Even if they're really, really inefficient, that's still plenty of volume.
 
As for current, there are lithium rechargeables in 9V packaging.
 
This solves the danger a couple of ways. First, someone else designed the battery, presumably correctly. Second, there also exist 9V lithium chargers, so you get that professionally designed for you. Third, battery charging happens outside the amp enclosure, so you can do it somewhere safer than on your desk, or wherever.
 
That said, a stock CMoy isn't going to put out a lot of current, no matter how studly your battery. You need a strong output stage if you are going to make much use of a low ESR battery.
 
("Low ESR" is a more accurate way of saying "high output current ability". A 9V alkaline can put out over 1A for a short time. The reason we describe 9V alkalines as not being good for high current is the ~3Ω ESR, which means at 100 mA, they're dropping 0.3V.)
 
If you also choose a more stable virtual ground, you may find that 1x9V is plenty. Again, see the "working voltage" article, and check your headphones' sensitivity spec. I think you'll find that 1 or 1.2 Vrms into the cans is sufficient to raise your audiologist's eyebrows.

 

[wakibaki]

If we had fusion batteries, we'd have DIYers jumping up and down to use them, and isn't that cool yellow and black triangle logo on the side awesome?

DIYers. I like 'em too.

w

 

[nutt7]

Not sure if I read that one...I will give it a read, thanks...and while you're here, thanks for all the other articles I did read.
 
By 2 two 9v "not working" I meant that they won't fit in my tin. It's not an altoid box. It's shorter and more narrow. I did retrofit two 9v outside the tin just to see, and I got better sound in both my m50s and my vintage pioneers (8ohm) at higher volumes (couldn't tell at lower volumes). If I could stuff a pair of 9v in there I would be a happy camper.
 
I will also do some more googling on using multiple li-ion batteries when charged individually. I have tons of cell phones and chargers that I could likely rig.
 
2leftears, I forgot to reply about the Nikon charger I have. It is a single battery charger and only has 2 prongs and a safety switch (I assume that's what it is) and a charging/full-charge indicator LED. No thermal stuff.

 

[tangent]

DIYers. I like 'em too.

 
I am one. I've been one long enough to recognize that as a group, we've got...foibles.
 
If you think the crack about the "logo" was inaccurate, you haven't seen the threads about which capacitor to use, based on color.
 

  By 2 two 9v "not working" I meant that they won't fit in my tin.

 
Okay, in that case:
 
1x9V lithium ion or NiMH will put out plenty of current. Amperes, continuously, if need be, until they melt or catch fire.
 
An 8-cell NiMH 9.6V battery will run down to about 7V before it's kaput. Use a near-RRIO op-amp like the AD8610 and buffer the outputs and virtual ground, and you're still left with about 1.75 Vrms into your cans when the battery is dead, which is more than sufficient for every pair of headphones I've ever owned, or seriously considered owning.

 

[2leftears]

  If you think the crack about the "logo" was inaccurate, you haven't seen the threads about which capacitor to use, based on color.
 

 Seriously?
 
Next time I'll just paint them then, cheaper than buying new ones! 

 

[vixr]

I always used 9v, so in a pinch I could swipe the batteries out of the smoke detectors...