Unequal size Li-Ion batteries in series?

I think by mA you mean mAH throughout.

If you put unmatched batteries in series, one will discharge first, and then the other one will reverse-charge it by continuing to drive current through it. With NiCd cells this causes immediate severe damage. I'm not sure about Li-ion, but it sounds from your description as though the results were equally fatal.

BTW, if you needed 4 volts, why did you put them in series? Was 3.7 V not enough?

In parallel, batteries of matched *voltage* are OK, regardless of whether the capacities match. The strong ones recharge the weak ones, rather than reverse-charging them. But if one battery shorts or becomes electrically leaky, it will discharge all the others.

The 3.7-volt, 750 mAH (if it's mAH) battery will supply 3.7 V to your 30-mA load for about 24 hours.

If you need technical assistance with your design, please drop me a line (mc at uga dot edu). I'm in speech science myself.

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You've already found out the hard way that the small battery won't work. If you must have a certain voltage, you should use a V converter to boost the single large cell up to what you need, and skip the multiple oddball cells.

No - in general, it is a Very Bad Idea to use batteries of different capacities in series, as the lower-capacitiy battery will become fully discharged before the higher capacity one, then the higher capacity battery will start to reverse charge the low capacity one..

What was the current drain? If it was 30 mA, your 170 mAH batteries would be fully discharged in about 5.6 hours, while the 750 mAH cell would be good for about 25 hours.

The order of the batteries doesn't matter - the 170 mAH cell is only good for 5 hours, regardless of where you put it.

Do NOT charge or discharge equal lithium ion batteries in series without some monitoring circuitry to keep 'em from doing what yours did. NEVER, EVER charge or discharge unequal lithium ion batteries in series. Equal means same part number, same history, same batch.

You don't want to do that with ANY battery technology, but at least other technologies are LESS likely to explode when you do. I said LESS LIKELY...

If you intend using lithium cells, you need to do a LOT of reading on the subject. The Cadex site is a place to start. mike

Others answered about the setup you have - don't do it.

But say you had a series setup with identical batteries. To measure the current draining from each battery, just measure the current draining from the series combination. The current flowing in each battery is the same.

Yes - since you are designing the circuit, why not design it to work from 3.7 volts instead of 4 volts? Otherwise, a DC-DC converter, as Watson suggested. These days they are unbelievaby simple - a chip and a few components.

Ed