When LifePO4 batteries are located in an unheated outdoor equipment shed in climates like Colorado their winter temperature can fall below freezing quite often. LifePO4 batteries will be damaged if they are charged when they are colder than freezing. A couple of uninviting options exist. First, the shed can be insulated and heated, which could be a lot of work and expensive. Second, the batteries could just not be charged until the temperature warms up. Even on a sunny day that typically means around noon and leaves only time for a partial charge on short winter days. A third option appears to be the least painful and that is to provide some external source of heat directly to the batteries through the use of heaters. There doesn’t seem to be any product marketed specifically as a LifePO4 battery heater. Researching alternatives, one possibility is the silicone heaters used to warm the bed of 3D printers. It is flat, comes in various shapes, voltages, power ratings, and it is inexpensive. A sampling was ordered and tried out. Finally selected is a 20 watt 12 volt heater shown below.
These pads fit nicely between alternating cells so each cell is adjacent to one heater. Leads are brought out and connected in parallel with wire nuts. Each heater draws 1.5 amps and in the lineup below 4 heaters draw 6 amps.
Getting this far was the easy part. Figuring out how to power the heaters is the next challenge. It was quickly learned that using the batteries themselves was a net negative. Heaters use too much power. The batteries don’t get fully charged before the sun goes down. An external set of batteries was tried but that just shifted the problem. After a few days the external batteries don’t have enough charge to run the heaters. Another failure was the use of timers to only turn on the heaters right before the sun came up. A new idea was needed. Time for …
While the batteries are too cold to charge and the heaters are running, the solar cells are sitting idle wasting generated power. Duh. Why not use the solar power to run the heaters? This idea was tried and has been working successfully for several cold winter months. Power was tapped where the solar panels go into the solar controllers. The tap is the small red and black wires in the picture below.
Raw voltage is typically 20 volts and that would burn out the heaters. A 10 amp buck converter was inserted in the line to keep the voltage at 12 volts, one buck converter for each of the battery banks. A metal box limits the rfi emitted from the digital buck converters.
A W1711 thermostat rounds out the installation. This little guy is set for 5 degrees Celsius which allows some margin to make sure the batteries are kept above freezing when the sun is up. When the sun isn’t up there is no concern because there is no solar power available to damage the batteries. What happens when there is solar power but the batteries haven’t warmed up above freezing? The Morningstar controllers were specifically chosen because of their feature called “low temperature fold back”. Even when there is solar generation, if the batteries are below freezing the Morningstar controller will refuse to charge the battery.