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I am working on a solar setup that is powered by a Li-ion batteries. However, Li-Ion batteries can't be charged below 0 deg C, and thus if it dies in the winter, it will need to be warm before it can begin charging. I will therefore need a heater for the batteries to bring them back up to temperature so that it can begin charging again.

I had 2 questions based on this -

  1. I had an idea run a Silicone heating pad (SKU TBD) that's directly connected to the solar panels (4x Renogy RSP100D-BK in series) so that the batteries can warm up and begin charging. Would the following make sense? - diagram
  2. Assuming the above works, most commonly available silicone heaters are either 12VDC or 120VAC. Otherwise I can go a custom route. It's my understanding that these heaters don't actually care if the voltage is AC or DC, what's important is RMS? Do these heaters just work as P=V^2/R and if the panel voltage fluctuates, I should be okay as long as I'm under the max voltage? If not, what voltage should I design the heater around - the combined open voltage of the panels is 97.2V or the optimal voltage of 81.2V? Power wise, should I spec something close to the max power of my panels @400W?

For reference the charge controller (SKU Renogy RNG-CTRL-RVR60).

Edit: I believe the following thermostat should allow me to control the heater, to turn on when too cold, and turn off when too hot.

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  • \$\begingroup\$ You'll obviously have some circuitry to control the heating elements so they don't heat more than they should or when they're not supposed to. Either do a load-line analysis of your panels and use a regulated supply to power the control circuitry or use the same MPP tracker I assume you're using to charge your batteries to power it. Do not have an unregulated heater next to your batteries. \$\endgroup\$
    – vir
    Nov 5, 2021 at 20:18
  • \$\begingroup\$ @vir Thank you for the reply. I didn't specify, but I will be controlling the heating element to both stop charging from happening to the battery and to turn off the heater. In-line PSU or DC/DC converter could work, thanks. I can't run the heater off the load inputs of the charge controller due to a limitation of the charge controller requiring panels connected. \$\endgroup\$
    – fullyjosh
    Nov 5, 2021 at 20:31

1 Answer 1

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I suppose one could put 120V heater pads with redundant thermal switches on the panels directly. They're just resistors.
For example a thermal switch that is on below 5 Celsius.

They will not run full power, but they may not need to, any energy going into the pad will become heat, and if the batteries are well insulated, you may not need much.

Thermal pads may also become too hot at their surface, so you may need additional control circuitry to cycle them if you want to make optimal use of them.
For example, thermal switch that turns off at 40 Celsius.

Careful thermal design is required, evaluating the safety.
Such as redundancy, overheating, fuses.

However, wiring them on the panels will interfere with the MPPT of the charger. I am not sure what it will do. You would have to try.

This charger also has RS485, so you could make it more "smart" if you want to.

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  • \$\begingroup\$ Thank you for the reply. I'm going to edit my post to include the thermostat I intend on using which will do this for me. \$\endgroup\$
    – fullyjosh
    Nov 5, 2021 at 21:27
  • \$\begingroup\$ @fullyjosh I would not use one like you linked now. That's for ambient temperature. I would use a few of these or this formfactors that you can actually attach to the battery and the pads. If you can find them with suitable electrical specs. \$\endgroup\$
    – Jeroen3
    Nov 6, 2021 at 12:53

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