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I am working on a project that involves running a go-kart off of solar and batteries. Recently, I tested it out using just the batteries themselves and found out that my battery array (4 12 volts 20 amp/hour batteries with 2 in series 2 in series and both of them combined together in parallel) started to get quite warm only one of the series. Why is this? My guess would be the fact that I connect my motor controller leads (Plus and Minus) directly to only one set of the series banks.

schematic

simulate this circuit – Schematic created using CircuitLab

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If you connect two batteries in parallel they can 'exchange' current. The side which has a sightly lower voltage starts getting current from the higher side. The additional problems is that the lower voltage may not become apparent until the batteries are under (heavy) load.

schematic

simulate this circuit – Schematic created using CircuitLab

What you might try is to connect each battery with its own cable to one point. All cables should be the same type and length. These cables function as a series resistor. If one battery starts delivering more current then another it will cause a higher voltage drop over the cable and this 'compensates' somewhat the voltage difference.

It is not ideal but it has the advantage of being simple!

schematic

simulate this circuit

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  • \$\begingroup\$ So this would fix my problem with the overheating and also increase the run time of my go-kart? \$\endgroup\$ Feb 17, 2018 at 14:38
  • \$\begingroup\$ It will only work if the heat was caused by 'unbalanced' batteries and then only of your batteries are not too much unbalanced. But it is definitely the correct way to connect them. \$\endgroup\$
    – Oldfart
    Feb 17, 2018 at 17:56
  • \$\begingroup\$ so my batteries were a bit unbalanced, so going about using your method would fix my balancing problems? \$\endgroup\$ Feb 18, 2018 at 18:46
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One obvious possibility is that there is a bad connection or broken wire somewhere in the loop to the left set of batteries.

Another possibility is that the state of charge was different between the two sets, and most of the current ended up being provided by the right set.

To diagnose this, measure the voltages of each battery individually. If that doesn't tell you anything, then measure the currents to see how equally the two sets of batteries are sharing the current load.

In general, this kind of arrangement is a bad idea. Instead of paralleling sets of batteries, use larger batteries in the first place to get the same capacity and current capability.

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  • \$\begingroup\$ I have no choice in this as we are not really allowed a higher capacity battery. \$\endgroup\$ Feb 16, 2018 at 12:19

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