Running 2 'identical' batteries in parallel

Just found a Deans Y connector for connecting 2 batteries in parallel.  I have slight concerns whether this is safe.  I have 2 identical make, identical type (3 cell 11.1v 2200mAh 40C), same age, same usage.  But they must be charged separately due to charge sensor cable, and they are bound to charge to slightly different levels due to minor internal differences in cells etc.

Connecting them in parallel in the Deans connector could cause a huge current surge as they balance out, and depending on the internal safety circuitry of the battery, damage to cells.

Should I be worried, or are these parallel connectors routinely used for longer flights?

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  • same topic at this DIYDrones page!

    Dual System Redundant Supply Parallel Question
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  • Parallel connection of power sources, in electrical industrial plants, is strictly forbidden. A separation devices is mandatory to avoid reverse current flow and to protect the load against a single power failure. Use of silicon or shottky diodes is not suggested because of the too high voltage dropout. We developed an ideal diode device to safely connect two or more batteries without any power loss


  • I'd like to bump this issue.

    I can't see a high current transient being very healthy for the battery no matter how short lived if applied with each charge session.

    I don't know what the current would spike to for voltage differences in the region of +/- 0.01v, but without assessing it I wouldn't want to just assume it is small.

    Saying people do this all the time is not a good argument. I am computer engineer myself, there is such a thing as the right way and the wrong way. People have batteries that explode all the time as well, so...

    I would think that placing a large value resistor between the parallel terminal until equilibrium is reached would be a good idea? This plus an in-series ammeter would reveal when equilibrium has been reached?

    • Moderator

      I do this all the time but with a twist, I have a Y lead with XT60 connectors and charge the batteries together, they remain together and then plug into a single XT60 on the Quad. no voltage difference exists and the benefits are very real by increasing the C rating and reducing the voltage dip when climbing. 

      • Yes, just treating them a single unit at all times would keep them in equilibrium.

        What did you do about the other battery leads for cell measurement? Did you have to do something with the wiring to be able to connect them to your charger?

  • Thanks for the extra advise - I'll bear it all in mind.  Not sure whether I'll need parallel batteries yet - my quad is still in testing, but I have 2 pairs of batteries so the parallel may give me the extra power I need for video + lighting + longer flight

  • I run parallel all of the time. Besides giving longer flight times it doubles the C rating which cuts down on those pesky low battery warnings when you thrust up quickly. As long as your batteries are balanced and similar you should be ok. That said, if one has a significant different level of charge things could heat up as one tries to charge the other. I have sets that I run together. I never mix them with the others. This way they age and stress similarly. If one gets out of balance with its mate then they both become solos. As far as longer flights go; make sure the power distribution wires and the motors can take the longer runs. I have melted solder connections mid air (resulting in an abrupt return to earth. ie crash) pushing things longer than they should have been pushed. I hope this helps.

  • Thanks Guys - the fact it's done all the time is what I wanted to hear - I know just enough electronics to see a possible problem but not enough about LiPo batteries to know it's safe.

  • I'm not an EE, so can't give you an engineer's answer, but I can assure you this is done all the time. That said, it bears repeating that one should observe the conditions you enumerate, plus charge the batteries on a quality digital charger to ensure all cells are charged as closely to the same value as possible. Hope that helps, Stan

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