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I am currently building a robot that has one 12V battery for each drive motor. The system was originally setup so that the two motor/motorcontroller/battery systems were completely separate. After testing, I discovered that one motor is used significantly more resulting in the batteries discharging at different rates.

I would like to leave the batteries on opposite sides of the robot for weight reasons. Would it be possible to connect the batteries so they would recharge each other when one battery was low?

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    \$\begingroup\$ I think you mean "drain at the same rate". \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Feb 19, 2015 at 17:48
  • \$\begingroup\$ Why don't you just put them in parallel? Attach the positive terminals together and then attach the negative terminals together. This will create one virtual battery. \$\endgroup\$
    – horta
    Feb 19, 2015 at 18:13

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Given that you mean (as Ignacio Vazquez-Abrams commented) "drain at the same rate", then it's easy. Easiest, of course, is just to short the batteries. If that doesn't appeal to you

schematic

simulate this circuit – Schematic created using CircuitLab

will do the trick. I've specified Schottky diodes just because they dissipate less power than regular power diodes for the same current. Note that you'll have to provide a proper heat sink for them, while you don't need any such thing if you just short the batteries.

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