Background - what I’m working with:
Vacation cabin in the mountains, 4,000 foot elevation.
2x 5 kWh, 48V Server rack LFP batteries (without heaters)
Victron power system with CerboGX
Power system and battery and installed in an uninsulated shed, it can get very hot in the summer and very cold in the winter
My summer solution:
Installed batteries in home made metal rack with generous spacing for convective cooling. Shed has thermostatically controlled attic ventilation fan that turns on when hot and draws air across the battery rack (on sunny days I have a lot of excess solar power so fan is not a problem). This largely avoids degradation associated with operation at high temperature (>30°C).
My winter solution:
Part 1: I wrap the battery rack in TWO layers of 2” thick rigid insulation (stuff is cheap), and keep it all together with elastic straps (no taping seams or anything elaborate). It is pretty easy to remove the insulation in late spring and put it back on in late fall. Because of the thermal mass of the batteries (they are 106 lbs each) and the thick insulation, this works incredibly well at keeping battery temps above 7°C (my heater set point). In fact, last winter was mild and my heaters never even cycled on once.
Part 2: I still need heaters to keep everything safe and operational in the event of an extended bitter cold period. I typically “shut down” my AC inverter when I leave in the winter so I wanted something that would work on the DC battery voltage. The Cerbo GX comes in incredibly handy as it can read temperatures, and has auxiliary relays (rated at 2A) that can be easily programmed to control the battery heaters. I wanted heaters with a broad surface area to heat the batteries uniformly, after an extensive search, I found robust 12” x 12” heaters on E-bay used to heat 3D printer beds. They were a bit pricey @ $60 each, but I found them to be very rugged and well built. For my application, the “110V 300W” heater was the perfect solution. These have a resistance of ~40 Ohms, two of them connected in series is ~80 Ohms. My battery voltage is ~52V so when the heater is powered, the current draw is 0.64A (well under the Cerbo limit) and the dissipation is 33.5 Watts. This is a very low heater power but it is more than adequate because the battery is so well insulated.
We finally had an extended frigid period and I got to collect some heater performance data. My heater set points were on at 7°C/off at 9°C. Even though the ambient temp was below freezing and the heater power was small, the heaters were only on for about two, one hour periods per day, and able to maintain temperature for a frugal ~70 Wh of energy per day!
Final parting pro-tip: I learned (thank you internet!!) that the way to get nice clean cuts in rigid foam insulation (plus not making a lot of micro-plastic dust and waste) is to make multiple shallow passes, at a shallow angle, with a long, very sharp, razor knife. I got a utility knife with long 1” wide Japanese tool steel blades and never looked back. You can extend the blade all the way and it is not too flimsy. Made cutting up the foam boards a breeze.
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