I have a simple question which drives me crazy and I'm really hesitating to test it, this is why I ask for your opinion.

At home I am using a solar system, using solar panels and a battery. The panels are connected to a charge controller and the charge controller takes care of the battery. So far so good. It's working perfectly and nice since years.

I have still two charge controllers left im my toolbox and there is still a car battery left. Now I was thinking I could utilize that stuff to use abundant power. On sunny days the panels create more electrical current than what I could store in my battery.

My Idea was now to setup a second system with the new battery and the second charge controller. My naive idea is to connect the first battery to the solar input of the second load controller. If the Voltage of the first battery is high enough, the second controller should start loading the second battery. In Theory the (PWM) charge controller should "think" there is a solar panel installed and should load the battery.

After thinking about that for a while I thought that maybe the current of the big battery is a problem. Solar panels are "naturally" limited in electrical current while the battery could maybe kill the charge controller!?

Does someone of you know if charge controllers limit the current inside? I know that there is lot's of ready made battery ballancers out there on the market. But I just want to make use of the spare parts and the second battery.

Thanks in advance


  • \$\begingroup\$ You want to charge a 12V battery from the main battery bank, through a solar charge controller? What voltage is the main battery bank, and how do you intend to use the power from the 12V battery? Is the controller a PWM type, or MPPT? What is the controller make/model? \$\endgroup\$ Commented Apr 6, 2017 at 12:12
  • \$\begingroup\$ In general, terminals that are intended to have batteries attached expect to have batteries attached and nothing else. Taking a load from them while they're trying to charge the battery and manage it correctly will only confuse their operation. Can you connect a second charge controller to the same solar panels as the first? \$\endgroup\$
    – Finbarr
    Commented Apr 6, 2017 at 12:18
  • \$\begingroup\$ Hi. The battery(bank) is 12V lead acid. It's controlled by a Victron MPPT. The panels are 80V. The second controller is a cheap chinese PWM controller. The cheap controller is certified for max. panel voltage of 40V. This is why I wanted to try this esoteric configuration. I don't really need that setup. I was just curious if this would work. But I don't want to try before asking someone. \$\endgroup\$
    – JackPearse
    Commented Apr 6, 2017 at 13:31
  • \$\begingroup\$ However, your comment that taking power may confuse the first battery controller is interesting. i haven't thought about this before. \$\endgroup\$
    – JackPearse
    Commented Apr 6, 2017 at 13:32
  • \$\begingroup\$ Why not simply connect your "extra" battery in parallel with your main battery bank? - no additional equipment required. \$\endgroup\$ Commented Apr 6, 2017 at 15:45

1 Answer 1


Generally battery charge controllers have a minimum voltage drop between the input and the charge output to the battery this should be found in the specifications of the charge controller. It is unlikely that you will meet this condition by connecting the input to the pwm charger to the output of the Victron mppt.

  • \$\begingroup\$ Do you mean that the voltage for loading the second battery will not be reached? \$\endgroup\$
    – JackPearse
    Commented Apr 6, 2017 at 15:00
  • 1
    \$\begingroup\$ I mean that the voltage from the first battery and charger will not be enough to charge the second battery through a charge controller. \$\endgroup\$
    – RoyC
    Commented Apr 6, 2017 at 15:03
  • \$\begingroup\$ This not answering my initial question, however it's an argument not ding this setup. \$\endgroup\$
    – JackPearse
    Commented Apr 7, 2017 at 10:34
  • \$\begingroup\$ In general - just to know: Would the higher current of the primary battery destroy the second charge controller or not? I am nit sure which parts are built into this kind of charge controllers. As far as I know the Victron MPPT controllers don't like overvoltage but if panels could provide more current then the controller is limiting it. I assume pwm controllers would burn their mosfets. \$\endgroup\$
    – JackPearse
    Commented Apr 7, 2017 at 10:37

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