Fidocat

Hey folks,

I have a basic question regarding the balancing of cells arranged in series while discharging. I'm hoping someone here has experience with this. I have the need for several different voltages in my project but I can't afford the bulk or hassle of too many battery packs. Nor can I afford DC/DC converters. What would do the trick is to put two packs in series for a total of 3 rails: one across pack1, one across pack2 and one across 1 and 2). It's the third that poses a problem. My research so far hasn't found a great deal of theory on the topic. Conventional wisdom would discourage creating a series stack from cells with differing voltages or discharge rates to avoid damage or failure due to voltage imbalances. However, I'm still unclear on exactly why this is. And what battery chemistries, if any, could tolerate this and to what extent.

All packs would be charged in separate, dedicated chargers, so there's no danger of charging imbalanced cells. The only issue would be with discharging. In my case, I'll employ power management circuitry to prevent each cell from being over charged or discharged below 80%. So no risk of having one cell (or pack) go dead and being reverse charged by adjacent packs. I just need to derive a mere 5-10mA from a high voltage stack of packs (packs 1 and 2). Packs 1 and 2 are discharged at different rates - around 50mA (1/4C). That doesn't seem too extreme. But what, if anything, can I safely get away with? Do I even need active cell balancing circuits? As far as the battery technology, since I need max power (especially voltage) in the minimum space and weight, I'd prefer Li-ion. But NiMH may prove more forgiving. Thanks for reading and I'd be greatful for any info on series cell balancing, circuits and techniques.

Flying Geezer

My Feedback: (14)

Without knowing what your project is, it's difficult to make suggestions. However, since you are needing low power to all rails, and if the duty cycle in not too long, you might consider a resistor divider network powered from a single battery pack.

This will eliminate the serious cell imbalance on discharge. This will allow you to use li-po for the small size you mentioned.

Fidocat

Thanks, Geezer. To give you more background...I have the need for +/- 7.4V at roughly 50mA, plus +14.4V at 10mA. Space and weight are big constraints so I don't feel I can afford switchers. This thing really needs to be really, really tiny. I also need 3.6V but I figure I'll have to LDO that from +7.4, hence the imbalance between the pos and neg packs. There's no problem with the +/- 7.4 packs assuming I don't draw the 10mA across both. This is because they're totally independent packs and will be charged separately. So the question is, are there any battery technologies (ie. NiMH or Li-ion) which will let me get away with tapping 10mA from both packs without complications. I'm not 100% sure of what you're suggesting. But one consideration is that my current draw varies on all 3 rails and thus is not predictable. So I don't think resistors are the answer, assuming I understand your suggestion, that is.

- Fidocat