Heater Pad with MPPT, how to make it work?

[alevchuk]

In

Will explains how to install a heater pad for LiFePO4 batteries. I'm running a system:
1. outside climate drops to -40 degrees C (about the same as -40 degrees F)
2. my load will drain the batteries throughout the day and at night everything will freeze
3. in the morning I need the solar panels to power the hating pads to get batteries ready for charging

MPPT charge controller is managing the batteries, yet when the batteries are on low temperature charge cutoff I'm concerned that the heating pads will not behave in a way for the MPPT to work. How can I make my MPPT work with the heating pads?

 

[alevchuk]

Sorry, I asked a similar question here https://diysolarforum.com/threads/will-mppt-work-with-internally-heated-batteries.41143/#post-520449
But now I'm trying to see if I can make it work without internally heated batteries. Only low temperature cutoff.

 

[time2roll]

Get more battery or limit the discharge to keep the battery/BMS active with enough power to run the heater all night.
The solution to me is to keep it running and ready for solar.

Keep in mind even if charging is halted for low temperature there can still be some discharge a bit lower to keep the controller active and ready.

 

[HRTKD]

My thoughts are the same as what time2roll posted. I prefer to keep the battery warm and ready for a charge all the time. The time needed to warm a cold battery is wasted daylight.

 

[efficientPV]

We are talking small wattages here. Depending on array voltages, you could just connect a small switching power supply to the array DC voltage to power the control and heater.

 

[efficientPV]

I just tried this with a 120W 12V 10A LED SWITCHING SUPPLY that can bed found on ebay for $12 US shipped. Recently I bought a couple at auction for $8, 9 and 10 and if you got the time they are easy to get at those prices. Don't try to het fancy and get a higher wattage one because it likely won't work on DC. With a load of 30W, 5 ohm resistor it easily fired up at 60VDC. As the sun comes up it will bump on and off till the voltage is high enough. A high ohm resistor charges up the switching chips power capacitor. At enough voltage it starts to work and then it feeds its own power from the output. If the panel can't supply enough power it shuts off for a second or two. This won't damage anything. Unless the usual array voltage os over 100V, I wouldn't go over 40W with the heater element or higher currents on the transformer primary may cause excess heating. So when the sun rises the batteries will be heated automatically. Just make sure any thermal delay doesn't start charging before every section of battery is up to temperature.

 

[rickst29]

You might take a look at my new "solution", which prevents a heater circuit from burning down it's own batteries until some level of "charging voltage" is found to be present on a test lead. In my own case, I use external heater pads with the temperature-based "heater pad control" circuit present in a new JK BMS. But, it would work with an external temp controller as well. https://diysolarforum.com/threads/bms-with-circuit-to-heat-the-batteries.40138/post-575086

In boondocking, this would prevent the heater pads from consuming battery power from low-charged batteries at night. In my case, with the JK BMS having shut off charging due to low temperature, the MPPT controller will attempt to send current "to the batteries" at charging voltage - but with the BMS charging circuit disabled, that "charging voltage" will not be be getting dragged downwards (towards battery voltage) by a closed charging circuit. That somewhat high voltage reading can be used as an additional ON/OFF control for the heater circuit.

When temps are too low but "charging voltage" is not present, the heater circuit stays off. When temps are high enough for charging, the heater circuit stays off. The heater circuit is only enabled when when temps are too low AND charging voltage is present.