Monday, January 1, 2018

Warming Up The Solar Batteries With Heating Pads

Aside from this unresolved thread, I can find no further information about anyone trying to warm up their batteries using surplus generator (or solar) power in the winter time to allow them to hold more charge.

Living in Thunder Bay, we aren't really remotely concerned with shortening our battery life due to overheating issues.  It rarely seems to hit 30 degrees, let alone anything approaching a significant threat to battery health.

On the other hand, as we've experienced a few times lately, when it is -30 degrees, the batteries have a very narrow range of ability.  They charge up extremely quickly, and discharge just as quickly.  This is frustrating on multiple levels.

At lower temperatures, solar panels actually become more efficient.  I can see voltages from my panels of over 120 volts DC at this time of year, still outputting up to 60 amps if the batteries would accept that.

Unfortunately, with battery capacity at much less than half of its summer capacity, most of this output goes completely to waste.

This is compounded by the fact that at these temperatures, my charger needs to go to voltages in excess of 33.6 volts on my normally 24 volt system.  The unintended consequence of this is that my inverter shuts down at 33.6 volts - essentially feeling that there must be a problem with my system and trying to protect itself.  So my power goes out because ironically, I am generating too much... power.

Two days ago, this gave me an afternoon without power while the batteries fully charged, and then an immediately following morning where the lights went out because the batteries were depleted - and that was with us only running our lightweight stuff - no pumping water or doing laundry or anything high demand.

Mulling over solutions, one that came to mind at first was a UPS type of battery backup, at least to keep our internet online while the batteries charged on cold, but sunny days.  I may come back to this idea.

The other thing I noted was that they were quite close to the 33.6 volt mark, and once they warmed only a degree or two, the power would return.  This did indeed happen at the end of the afternoon when the sun finally got to the west side of the cabin where the batteries are housed.

This started me thinking about just warming the batteries in general.  Then they could accept and give out more charge.  Again, I'm not concerned about overheating them.   I'd be more than happy to just get them five or ten degrees above ambient temperature and at least have power in the morning until the sun is back up!

After some short thought about heating pads as I have seen for vehicles, I took the chance to use a sale in the city to purchase three of the smallest, 50w heating pads.

Just laying out the equipment.  I managed to find an outdoor extension cord with three outlets and exactly the right length!
A closeup on the important details - 50w, 120VAC.
I folded them back on themselves (directly against the instructions) and tucked them in the spaces between my batteries.

Tucked in between each battery.
At these temperatures, outgassing is very minimal, and besides, I put the plugs outside the battery box and below the lip, so there is very little chance of a spark causing any issues.  I won't need to use them in the summer anyway.

When the sun came out and I had good power, I plugged them all in and observed that they each draw about 2 amps of 26 volt power.  That makes perfect sense in terms of them being 50w apiece.

Later I powered up the generator to ensure a good charge, and kept them plugged in.

As you can see, at the start of the experiment, there was a 2 degree spread between battery temperature and outside temperature.

1pm, -16.5 degrees ambient.
1pm, -14 degrees in the battery box.
Currently there is an 8 degree spread.  I can't say for certain that some or all of this was due to internal heating due to them charging too, but it seems unlikely that it could cause that much of an improvement.

5pm, -15.3 degrees ambient.
Batteries at - 7 degrees C; so an 8 degree spread with the outside temperature.


I will see how that difference lasts through the night, and report back in the morning.

If this seems to work, I will incorporate it into the new and improved battery box I am planning for this summer - perhaps with a thermostat, and maybe trying to get the batteries tighter together and wrapping the heating pads all the way around all four.

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