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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\$ Jan 1 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\$
    – mkeith
    Jan 1 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\$
    – mkeith
    Jan 1 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\$ Jan 1 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\$ Jan 1 at 11:17
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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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I have this setup in my Westfalia using a victron 75/15 MPPT, a renogy wanderer 10A PWM, and a 150w panel. The victron is connected to a 100ah 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 amperage. So far, it is working great. The PWM overrides until the FLA is topped off (14.6v) and goes into float mode (13.7v). Then the Victron takes over and does great.

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