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I have two identical solar panels in series that are connected to an MPPT controller (EPEver Tracer 1210A), which is currently charging two SLA batteries connected in parallel meant to power some lights. The overall solar panels wattage is 200Wp, whereas the MPPT is meant to support 130W at 12V.

I bought LiFePo4 batteries as I plan to power some tools so I need larger battery capacity. This however leaves me with two perfectly fine SLA batteries that I'd much rather re-use somehow. I know I can't connect the two different battery chemistries directly in parallel, so I'm trying to work out different solutions.

I have a spare PWM charge controller (your typical random chinese brand), so I was wondering whether I can connect the series of solar panels to both the MPPT and PWM controller (assuming the latter can tolerate the voltage of two panels in series) without causing strange behaviours (on the MPPT controller in particular, given the power tracking features) or other problems on either controller. The MPPT controller would be used to charge the LiFePo4 batteries, whereas the PWM controller would charge the SLA batteries. You can assume that both chargers have embedded reverse-charging protection, so the batteries won't be drained at night.

Is this a sensible setup?

Here follows a diagram. Apologies if it's not perfect: I'm not used to drawing electric diagrams.

enter image description here

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  • \$\begingroup\$ Solar panels are just a power source to charge batteries. So by right, this configuration should work. But, both batteries will be charged slowly. \$\endgroup\$
    – David Norman
    Commented Jan 1, 2021 at 2:01
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    \$\begingroup\$ Nobody can answer this. You will just have to try it and see if it works OK. The issue is that the MPPT algorithm and the PWM controller algorithm may fight each other and cause instability, or, one of the charge controllers may end up using most/all of the solar power and starving out the other one. \$\endgroup\$
    – user57037
    Commented Jan 1, 2021 at 3:56
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    \$\begingroup\$ Another option is use the MPPT only, and charge the 12.6V battery using the 13.3V battery as a power source (with the MPPT connected to 13.3). You would use a DC-DC charge controller for this. Then only one MPPT is involved. \$\endgroup\$
    – user57037
    Commented Jan 1, 2021 at 3:57
  • \$\begingroup\$ I thought about that other option, the problem being that the higher voltage of the LiFePo4 battery would end up continuously charging the SLA battery, thus draining the LiFePo4 batteries even though no load is attached. That's not a viable option. \$\endgroup\$
    – Filippo Possenti
    Commented Jan 1, 2021 at 11:07
  • \$\begingroup\$ @DavidNorman let me update the question then, as the solar panels are significantly oversized for a number of reasons (or, rather, the MPPT is undersized and can't use all the power output of the panels). \$\endgroup\$
    – Filippo Possenti
    Commented Jan 1, 2021 at 11:17

3 Answers 3

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And after several months, given that there was no conclusive answer, I took advantage of the good weather and I tried.

As soon as the solar panel was connected to both the MPPT and the PWM controller, the MPPT controller started making strange noises. Needless to say that's a bad sign, so I disconnected the PWM controller.

It "may" work on other models, but I'm inclined to say it's not a safe option regardless.

Bottomline, the short answer is: No, it's not a sensible setup. Don't do it.

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This will work. The "strange noises" as you put it were due to the PWM charger. Use a capacitor before both chargers and that should cut down on the noise, a 400-600 μF should work. The fuses are a great idea and you should have a way to monitor the power each is using and the voltage on the bus, to make sure the voltage doesn't sag, as that may be the problem.

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I have this setup in my Westfalia using a Victron 75/15 MPPT, a renogy wanderer 10 A PWM, and a 150 W panel. The Victron is connected to a 100 Ah LiFePo4 battery, and the renogy is connected to the FLA van starting battery. Both batteries are grounded to the frame. All electronics run off the LiFePo4 and the FLA is for starting and ignition only. It is important to make sure both chargers are grounded the same - negative grounded in my case. Otherwise, they will not be able to regulate the charge current. So far, it is working great. The PWM overrides until the FLA is topped off (14.6 V) and goes into float mode (13.7 V). Then the Victron takes over and does great.

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