Cold weather lifepo4 charging hack

[saltydawg]

Okay as we know charging lifepo4 batteries below freezing is very bad for them, as in on time can kill 20 % or more of the capacity.

So how do you prevent this besides the obvious of dont charge them below freezing, not always an option. Also since lifepo4 dont out gas we can have them in the trailer body, where we will assume it wont be below freezing, but what if it is?

What have I done, well keep reading if you want to know.

I have 2 60 amp hour lifepo4 batteries, under each battery is a 12 watt bee hive silicone heater. So each heater pad draws 1 amp or two total, if I have 4 amp hours left in each battery I could run the heaters for 2 hours to warm them. Math says 12 watts would heat 1 gallon of water from 35 to 40 degrees in 1 hour 2 mins. Now I know my batteries are not water, but water takes a lot of energy to change temp, but it does give a base line. And my batteries are sitting in the corner of my trailer so on 3 sides they are sitting on or against a foam panel, so basically in a insulated box

I have a switch on my master switch bank labeled seat heaters ( will be battery heaters ). That switch energizes a 12 volt thermostat with a remote sensor between the batteries that I will set for 35 or so degrees to turn on and 40 degrees to turn off. That means when the switch is on the heaters will get power when ever the batteries are below 35 degrees. The out put also turns on the led in the switch, so I have a visual indication that my batteries are cold. It also energizes a 30 amp relay that interrupts the power from my shore power charger ( maybe a second or a two pole when I do solar ). The charging circuit is on the normally closed side of the relay, so under normal conditions, with the heater switch is off the charger just charges normally. But as soon as it gets cold I can just turn on the heater switch and any time the batteries are cold, I get a light telling me so, power to the heater pads as well as the relay which interrupts the charging circuit.

It is a simple turn on and forget system when it gets cold, even if it gets turned on when warm the only draw in the thermostat, so no real wasted power.

[rjgimp]

Nice!

[tony.latham]

A while back, I was curious about how one would charge a Tesla or other electric in cold weather and that's more-or-less how it's done in the industry.

But I was wondering how it could be done in a camper. Thanks.

Tony

[saltydawg]

A while back, I was curious about how one would charge a Tesla or other electric in cold weather and that's more-or-less how it's done in the industry.

But I was wondering how it could be done in a camper. Thanks.

Tony

There is a few companies who are building lifepo4 batteries with built in heaters, if you try to charge the battery when cold, it takes the charging current and sends it to heaters until the battery is warm enough to charge. Great idea but one flaw, if you have a shunt power meter, it see the power going that is being used to heat the battery as charging power. So now the power meter is wrong. Granted the flaw does not effect most users, but would or could throw off meters in campers or rvs.

The really comical one was on the first model 3's. Tesla had battery heaters and controls to shut down charging when cold, but would start heating the battery when it saw a charging current. The design flaw was the heaters where 2k watts and the max input from the small 110 volt charger was 1500 watts. So you go to your cousins house and he lets you plug in to his 110 volt plug in his garage, well the car says okay lets turn on the battery heater, which takes 2k watts, 1500 from the house and 500 from the battery. yes trying to charge in cold weather actually took more power out of the battery than what was going in. so you wound up with a warm battery with less charge in it.

no connection or anything just a video to see it works
https://youtu.be/v1txYs5dmbU