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Connecting a pair of 6s 3700mah lipos in parallel, how close does the voltage need to be?

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I generally make sure they are within 0.1v when parallel charging or discharging, but 0.2V is still probably ok given the fairly meaty connectors we use. One key point when charging in parallel using a charge board is always to connect all the main connectors first before the balance leads, that way any momentary current spikes will not pass through (and potentially damage) the balance wires which are not suitable for anything over about 1A.

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Consider the resistance of the circuit. You have two batteries, what is their internal resistance? With 6 cells then several milliohms (say 5). Then each ground and power connection goes through a connector. Unlikely to be less than 1 milliohm each, so another 4 milliohms. This gives a total of, at least 14 milliohms. (I'm being generous here, the resistances are likely to be higher, and I haven't included any wire resistance).

Your voltage difference is 40 milliohms, so the higher voltage battery will charge the other at around 3 amps (40/14). As soon as this starts the higher voltage battery will drop in voltage and the other will rise in voltage, so the current will rapidly reduce.

I regularly use 2, 4-cell batteries in parallel with no problem. Other than they are fully charged, I don't check their exact voltages. I normally charge them using a dual charger, so their voltage is measured by two, different circuits.



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The Spektrum website actually recommends using several batteries in parallel as a best practice: "DO USE A SECOND BATTERY ON YOUR MOST VALUABLE AIRCRAFT. While all Spektrum 9-channel-and-above receivers have multiple battery ports, you can actually put multiple batteries into any Spektrum receiver. Simply choose two batteries of the same size and capacity with two switches."

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