I have about 10KW of lithium ion 36V hoverboard batteries that I want to connect to my 12V AGM battery in my Prius.

I am placing several 18V solar panels in parallel instead of series on the roof of my Prius due to varied shading conditions across each panel.

I cannot find a good charge controller that will allow a step-up charge into 36V at the expected parallel currents, so I must charge the 12V battery directly from the solar panels using a typical charge controller and then use that to power an inverter that will be connected to a normal 36V li-ion charger to use on the lithium battery cluster, which will then be looped back into the AGM battery for extra capacity.

I am not sure the best way to charge the AGM pack via the lithium pack.

Should I get another charge controller and use the lithium output as the 'solar' voltage or is there a better (cheaper) way?

As a side note, the purpose of this set-up is not to add power to the motive portion of the car, it's to power the inverter and add the capacity of the 36v pack to the 12v battery.

  • \$\begingroup\$ 1. "Looping it back" doesn't make sense (and has a good chance of starting a car fire). I started writing an answer about a way that would make sense, but while doing some research I discovered — 2. The 12V battery system in the Prius isn't what actually provides motive power, so this seems like a lot of work for almost no benefit. \$\endgroup\$
    – hobbs
    Commented Sep 20, 2021 at 14:58
  • \$\begingroup\$ The purpose isn't to move the car, it's to power the inverter. and add the 36v cluster to the capacity of the 12v pack. Why would it start a fire? \$\endgroup\$ Commented Sep 20, 2021 at 15:02

2 Answers 2


"Looping it back" is a bad idea. If you have A charging B and B charging A at the same time, you'll end up with all of your power going into losses in the chargers, and probably a car fire. You need to pick one direction, and presumably that's solar -> 36V -> 12V, since everything else in the car runs off of the 12V system and that's where you need to deliver power.

You might be able to do as you say — just add a second charge controller with the 36V bank as its input. There isn't really a simpler way; every set of batteries needs an appropriate charger. I see four things that could very well go wrong there:

  1. Constant drain on the 36V batteries from the charge controller. I think it might be a good idea to make this ignition-switched, so that the 36V only tries to charge the 12V when the car is running. Yes, you lose a little bit of potential here, but if you manage to keep the 12V nearly topped up, it won't be much, and it buys some peace of mind.

  2. Something in the Prius's battery management getting unhappy if an unknown source is charging the battery without its knowledge. Do your research to find out if this is going to work at all, and the best place to make a connection, before going any further.

  3. Likewise, your charge controller might freak out when the Prius starts charging the 12V battery in parallel with it. Make sure to get something "stupid" enough to keep working in the face of this, and go back to charging when appropriate.

  4. The obvious risk of screwing up and creating a big lithium fire in a confined space with you in it. If you're not doing this in a meticulous and well-researched way, please don't do it.

  • \$\begingroup\$ Ideally I'd like to do the solar > 36v > 12v > inverter option, but since the panels are 18v it makes this challenging because it's not the best idea to wire them in series due to varied shading conditions upon each panel. Since I am forced to use parallel, the currents will be too high for a buck charge controller (that I've been able to find) So you are saying to use an additional charge controller for the 36v bank to the 12v, but not to charge the charge the 36v bank via the 12v battery. Correct? \$\endgroup\$ Commented Sep 20, 2021 at 15:47
  • \$\begingroup\$ So a parallel connection from the solar panels to an additional charge controller to the 36v pack perhaps? \$\endgroup\$ Commented Sep 20, 2021 at 15:50

The logical solution is to buy a PWM Solar Controller for Solar Power 12V 24V 36V 48V Lithium/AGM/Gel/Flooded Battery with your 300 W or so Solar Panel+MPPT controller charging the same 12V battery in parallel.

Not all PV chargers use the same MPPT algorithms to optimize the peak power which is typically 82% (full sun) to 72% (dim) of open circuit voltage Voc. So your efficiency will vary with quality and matching of controller to your inputs and output.

The site Mods do not allow shopping for you.

You must specify voltage tolerances and expectations of user interface, current range of Solar charger specifications and measured pulse frequency for possible interference.

  • \$\begingroup\$ I bought a MPPT for the 18v panels to charge the 12v battery. My problem is that I now need to add the 10kwh capacity of the 36v battery pack to the 500wh-ish of the 12v pack in such a way that both of them function into a single inverter. No worries on the delamination. I bought panels that will not delaminate. \$\endgroup\$ Commented Sep 20, 2021 at 15:18
  • \$\begingroup\$ Again, the same charger will use what power is available to charge the desired battery at a default value e.g. 5A or user selected and not exceed the CV for the chemistry selected. Interaction with PV charger ought to be suppressed by a low ESR of a good 12V battery. A dead battery, interference is possible. But you must determine the max input voltage allowed. \$\endgroup\$ Commented Sep 20, 2021 at 15:25
  • \$\begingroup\$ I don't understand what you are trying to suggest. Are you saying to use the PWM as a charger for the 36v pack in-addition to the mppt solar charger I already have? The way you worded it seemed to suggest that you wanted me to buy a PWM controller for the panel. Also why PWM? \$\endgroup\$ Commented Sep 20, 2021 at 15:30
  • \$\begingroup\$ PWM is the only efficient way to regulate current with an inductor inside to transfer the charge. Basically you only need a Buck or Step down converter with CC/CV for your chemistry but some are buck/boost \$\endgroup\$ Commented Sep 20, 2021 at 15:33
  • \$\begingroup\$ A good MPPT is 30% more efficient than PWM, also I still don't understand your solution. It seems to me that you are suggesting to simply "buy a solar charge controller for the solar panels" which I have already done and is not a solution to my problem. \$\endgroup\$ Commented Sep 20, 2021 at 15:36

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