Lithium battery heaters for cold weather.
12-12-22 lks
Leisure Solar builds custom solar generators specifically designed for the rugged, cold Colorado outdoor lifestyle. We use state-of-the-art LiFePO4 batteries to run the generator. Everyone knows that LiFePO4 batteries won’t charge below 28F, but not many people know that they will still give you 90% of their energy down to -4F.
We wanted to know exactly how much energy our 70W 12V heater uses to keep a LiFePO4 battery at an acceptable charge temperature. So we placed a LiFePO4 battery in an Igloo cooler (made in the USA) and put the whole thing inside a chest deep freeze to get real data on how much energy is need to keep the battery warm.
We used the Thornwave Battery Monitor and RV Whisper monitor system to keep track of battery voltage, current and state-of-charge and temperatures inside the freezer and inside the cooler. Our LiFePO4 battery heater kit includes a 70W 12V heater pad, a custom industrial 35F-45F thermostat and all wiring and fuses.
Most people know that LiFePO4 batteries shouldn’t be charged below 28F. Most LiFePO4 batteries have a Battery Management System (BMS) that will prevent them from charging below 32F, so that is the temperature we want to stay above while charging. What most people don’t know, is that LiFePO4 batteries will give you 90% of their stored power down to -4F, making them an excellent cold weather battery. Below -10F, you run the risk of damaging a LiFePO4 battery, so you should always prevent that from happening.
Spoiler Alert! Here is the results of our experiment, you need to read further down for the actual procedure and data.
1) With the freezer set to -5F, the heater used 18AH per day to keep the insulated (cooler) battery between 35F-45F, which is what our custom industrial thermostat does.
2) With the freezer set to -5F, the heater used 33AH per day to keep the insulated (cooler) battery between 45F-65F, which is what the heater pad built-in thermometer does.
3) With the freezer set to -5F, the heater used 72AH per day to keep the UN-INSULATED battery between 35F-45F.
The results are clear...keeping the battery at 35F-45F in an insulated battery box is easily done. Letting the heater try to keep the battery at 45F-65F uses twice as much energy! And heating an un-insulated battery is a losing proposition.
Our experiment actually heated the battery for the full 24 hour day. In most RV/Cabin installations, we really only need to heat the battery when we are trying to charge it, which is only 6 sun-hours per day. So we normally use timers or sun sensors to just turn on the heater when the sun is shining. To get the actual energy usage in the real world, you would divide our numbers above by 4! But remember, you must keep the battery above -10F to prevent damage to the battery. If you are in a situation where the battery will be exposed to -10F or lower, you need to heat it all the time, not just when charging.
Here is some of the data the RV Whisper acquired during the experiment.
Battery Temperature measured with RV Whisper BTH1 Temperature Sensor – in cooler 35-45F
Freezer Temperature measured with RV Whisper BTH1 Temperature Sensor 35-45F
Heater Current Draw measured with Thornwave BT-DCPM Battery Monitor – insulated 35-45F
Zoom in of the Heater current draw. The Heater turns on once every 35 minutes for a short time.
Also Cooler Temperature as measured by the Thornwave BT-DCPM Battery Monitor 35-45F
Cumulative Amp-Hours drawn by the heater in 24 hours is 18AH – insulated 35-45F
Battery Temperature measured with RV Whisper BTH1 Temperature Sensor – uninsulated 35-45F
Heater Current Draw measured with Thornwave BT-DCPM Battery Monitor – un-insulated 35-45F
Cumulative Amp-Hours drawn by the heater in 24 hours is 72AH – un-insulated 35-45F
Battery Temperature measured with RV Whisper BTH1 Temperature Sensor – in cooler 45-65F
Freezer Temperature measured with RV Whisper BTH1 Temperature Sensor – 45-65F
Heater Current Draw measured with Thornwave BT-DCPM Battery Monitor – insulated 45-65F
Cooler Temperature as measured by the Thornwave BT-DCPM Battery Monitor 45-65F
Cumulative Amp-Hours drawn by the heater in 24 hours is 33AH – insulated 45-65F
