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I'm looking into the fault detection and monitoring subsystem of out unmanned vehicles (airborne and ground-based) batteries. We employ LiPo-battery packs of various configurations which are charged by cell-balancing chargers. Only properly charged and balanced battery packs are put to service. During operation, only the total pack voltage is monitored.

I figured that monitoring the individual cell voltages and temperature will help us detect battery deteriation early in order to avoid catastrophic losses in-flight.

I wonder, whether cell-balancing during discharge would do any good, or basically would only waste energy without much effect on battery health or reliability.

Edit: We build our batteries ourselves from basic unprotected cells. Cells are not specifically matched as far as I know. However, we generally use cells of a single batch for one battery only.

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Because charge efficiency of the battery and control circuitry is not 100%, you're just going to end up wasting energy and adding cost for minimal returns. As long as your batteries have the same specifications and are properly balanced prior to use, then your risk of damaging one battery before another during discharge is pretty remote. You're much more likely to damage an unbalanced cell during recharge.

Generally it's bad practice to fully discharge lithium ion and polymer batteries as it results in shorter battery lifespan. So assuming you're only running your batteries down to 50-80% depth of discharge (DoD), the risk of damaging an unbalanced battery is negligible.

To ensure quality of your packs, I would recommend putting them through an 80% discharge test as soon as they're ready. Measure the voltage at that DoD to establish a baseline. Anything below the normal average you'll know has a problem.

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Kirchoff's Current Law ensures that all cells in the battery will lose the same amount of charge from active use. As long as the cells were matched when assembling the pack, this means that the batteries will essentially remain as balanced during discharge as they were at the end of charging.

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  • \$\begingroup\$ Does this really holds true for the non-linear discharge behaviour of LiPo batteries? \$\endgroup\$ – Arne Jan 28 '16 at 10:07
  • \$\begingroup\$ @Arne: The goal of matching cells is to select cells with extremely similar characteristics, including discharge curves. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 28 '16 at 10:15
  • \$\begingroup\$ OK, I see. Well, assume our cells are not specifically matched then.. \$\endgroup\$ – Arne Jan 28 '16 at 10:19
  • \$\begingroup\$ If they're not matched then yes, you could damage one or more of the cells during discharge. This is why matching cells is so important when assembling a battery. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 28 '16 at 10:21
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    \$\begingroup\$ Keep in mind that evenly matched cells can degrade in capacity at different rates, leading to one cell discharging before the other after countless cycles. I would inplement a low voltage cutoff where the voltage is monitored on a per-cell basis to counter this, if I were to design a commercial product. Balancing during discharging is not necessary, IMO. \$\endgroup\$ – jms Mar 14 '16 at 22:46
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Don't balance during discharge. It would only waste energy and it would do more harm then good.

I answered my own question about how balancing works and maybe it would also help you.

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