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I have solar panels to charge controller to batteries to DC loads. The charge controllers can handle twice the power at 24 VDC as they can at 12 VDC. The loads are 12 VDC (mainly LED lights).

Can I put two car batteries in series so they are 24 VDC to the charge controller and then connect about half the loads to one 12 VDC car battery and about half of the other loads to the other 12 VDC car battery?

What happens if I put a 1000 W, 12 VDC load on one battery and no load on the other 12 VDC battery (large load on one battery - but not the other)?

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They will become unbalanced if you do so. The light load one will be overcharged by the solar panels and gas out (destroyed if sealed, otherwise you need to top up with water) and the high load one will be undercharged, sulfate and be destroyed.

Two solutions:

  1. Get a 24 V inverter and a 24->12 V buck converter for your 12 V needs.

  2. Alternatively a (different) MPPT solar controller which can accept your 24 V panels as input and output 12 V and run your batteries in parallel instead.

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  • \$\begingroup\$ Option 3 is to manually balance the batteries using the charger in 12V mode. Not fun. \$\endgroup\$
    – user253751
    Jul 27, 2022 at 20:22
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    \$\begingroup\$ Option 4 is to discharge the batteries with a parallel connection. Not sure if that's a good idea, as they may get unbalanced voltage-wise when charging and then SoC-wise when discharging with a big inrush current - but at least they'll be rebalanced each cycle. \$\endgroup\$
    – user253751
    Jul 27, 2022 at 20:23
  • \$\begingroup\$ You could use a DPDT switch to connect the batteries in series for charging at 24V, and in parallel for 12V. AFAIK, lead acid batteries are fairly tolerant to parallel connection, unless their state of charge is quite different, or especially if one has a shorted or damaged cell. It's probably OK to keep similar loads on each battery if neither one is heavily discharged \$\endgroup\$
    – PStechPaul
    Jul 27, 2022 at 23:53
  • \$\begingroup\$ @PStechPaul Messy, and prone to failure if the switch is left in 12 V mode and the solar panel isn’t disconnected. But I see your point. \$\endgroup\$
    – winny
    Jul 28, 2022 at 6:45
  • \$\begingroup\$ @winny - thank you! \$\endgroup\$
    – Bubba
    Jul 28, 2022 at 23:04

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