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Proposed circuit for battery heater

RockPaperScissors

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Joined
Oct 29, 2021
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So the SCC, solar charge controller, I'm using has a separate interface to output 12v power for a load up to 15A. It has a separate sensor attached to a LiFePo4 battery to monitor temperature and voltage.

The SCC will power the Load interface if there is enough power from the solar panel even if the battery is disconnected. It is programmed not to charge the battery when the battery temperature drops below 40F.

The circuit should allow the heat pad to keep the battery warmer than 40F as long as there is battery power. The SCC will shut down the Load interface if voltage drops to 11V.

Once there is sun on the solar panel, the SCC will wake up and check the temperature. If it is too cold, it won't try to charge the battery, but it should power up the Load interface which will start the Temperature Control which has its own temperature sensor. If it is cold enough, it will connect the relay which powers up the heat pad on the battery. I think I saw 50F as the optimal temp to try charging the battery in another discussion.

Heat pad reaches a max of 135F. Operating temp for the battery is max of 140F.

Think this will work?
Bad MSPaint circuit attached. Only showing + wires, except for relay for heat pad.
 

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Which solar charge controller? Always provide a link to your exact device, please.

135° F is awfully hot. I would never put that much heat on my LiFePO4 battery.
 
Sorry, caught up wiring it up and testing.

Victron MPPT 75 10 with the Victron Temp/voltage sensor. Using a generic Inkbird temp controller with sensor.

12volt 30AH LFP battery. Insulated with 5mm foam mylar sandwich insulation.
Generic silicon backed honey bee heating pad.

Here it is wired for testing. Wiring mess, but I got it all in there.
Box will be closed to retain heat.

Normal draw is 6 watts without the heater. 23 watts with the heater. Or 1.6 amps on the load circuit.
 

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Why do you have an in-line fuse between the solar charge controller and the circuit breaker?
Breaker is rated to 50 amp. It's too high for the SCC so the inline is 15 amp and a little insurance. Battery can put out 30 amp continuous and 60 amp surge. The other in-line is for the Victron temp sensor.

Testing the setup today in 20F temps. I want to see if that little pad can warm up the battery. Wish me luck.
 
Breaker is rated to 50 amp. It's too high for the SCC so the inline is 15 amp and a little insurance. Battery can put out 30 amp continuous and 60 amp surge. The other in-line is for the Victron temp sensor.

Testing the setup today in 20F temps. I want to see if that little pad can warm up the battery. Wish me luck.

OK, so you're using the circuit breaker as a switch. Good luck with the test.
 
Testing with the breaker open, so no battery power or charging is occurring.

Measured surface temps with a laser thermometer. Victron sensor on left side of battery. Controller sensor on right side of battery. Heating pad on back of battery. Sensors and pad are insulated from the box using 5mm foam / mylar sandwich insulation. Controller set to turn on heat at 50F and turn it off at 55F.

Outside box surface temp 20F
Victron temp 48F
Controller temp 45F
Exposed areas of battery measured 20F


After 1 hour and 23watt draw to components.

Outside box temp 9-16F
Victron temp 53F
Controller temp 48F
Back of box directly under heat pad 55-60F (Insulation seems to be working)

The 30AH battery may be too small to make it through the night at this level of draw. I think I could get 13 hours of runtime before low voltage cut off. If the low voltage cuts off, the battery will need 2-3 hours to warm back up under solar power before the charger will kick in. Which will not leave much time to charge the battery for the next night.
 
Once you get the battery into the 55° F to 60° F range, it will stay there for quite a while as long as your insulation is thorough. Mine draws so few amps to maintain 35° F to 45° F that I don't worry about it any more. But I have a 560 Ah battery, so I have more Ah to play with than you do.
 
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