Unequal size Li-Ion batteries in series?

Is it possible to use two rechargeable Li-Ion batteries of unequal sizes in series? I'm designing a device that will run at 4V, 25-30mA. The device is pager size so a 9-volt battery is too big. I thought I'd found the solution in RC car batteries. These are flat so they can lie on top of the circuitboard. They are polymer li-ion, 3.7V. They are available in two sizes: 750mA and

170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15 hours. The case can fit one large battery and one small batteries, or two small batteries. Possibly I could fit one large and two small, or three small batteries. I set up one large battery and two small batteries in series. The voltmeter read 11.9V. I set up the device and came back ten hours later. The large battery was at 4.1V - fresh - and the small batteries were at -0.1 volts. They had puffed up. I charged them but they never went over 0.1 volts. How could the large battery be fresh and the two small batteries be destroyed? I can't figure out a way to measure current draining from each battery. Now I have two test devices running. Each has one large and one small battery, in series. The difference is that one device has the large battery connected to the positive terminal and the small battery to the negative terminal; the other device has the small battery connected to the positive terminal and the large battery connected to the negative terminal. Any other suggestions?
Electronic Anti-Stuttering Devices       http://www.fluencydevices.com
Thomas David Kehoe       Casa Futura Technologies       (303) 417-9752
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Thomas David Kehoe
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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.

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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.

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Watson A.Name - "Watt Sun, th

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.

Peter Bennett, VE7CEI  
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Peter Bennett

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

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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.


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