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Old 11-16-2020, 12:53 PM   #1
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Temperature Controlled Heater for Lithium Batteries

I'm looking for a way to provide heat to keep my lithium batteries from freezing. I'm thinking of using an 'enclosure heater' in the battery compartment (well insulated), controlled by a mechanical thermostat to switch on at around 35 - 40 degrees, and switch off at 45 - 50 degrees. The issue is that I cannot find a thermostat to carry more than 30 W DC, which at 12 V means 2.5 A (if I understand correctly). The heater needs to be 60 - 100 W, which would require up to 9 A. I believe I could use a relay so that the thermostat would switch the relay on and off, and the relay would carry the load to the heater. All three devices would be attached directly to the battery using 14 AWG wire.


As someone with limited electrical expertise, I'm looking for confirmation that this is a valid solution - or if there is a better alternative. I know from this forum that there are experts out there, so I if anyone can provide some advice, it would be much appreciated.



Thanks
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Old 11-16-2020, 01:44 PM   #2
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Correct, you'd want to use a relay just like you describe. Beware that most mechanical thermostats doesn't like vibration. Get on a washboard road, and it might flutter on and off.

Is your intention full-time heating? Or only heating during storage? Do you have other heat sources in the van? 9-amps full time is not a trivial amount of battery consumption....
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Old 11-16-2020, 02:55 PM   #3
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Thanks for the quick reply. And an interesting observation about the effect of vibration. I guess this does not need to be a mechanical thermostat - I just thought having something that worked independently of the battery could be helpful. But on second thoughts I don't know why that would be needed. Everything else needs to be driven from the battery, so the thermostat could be as well.

To your question about use - this is only for the battery compartment and when the van is not being used. The van is heated with an Espar diesel heater to keep us warm. But when we're not using it, if temperature drops below freezing, the batteries will be damaged during charging by the solar system. So I can turn off the solar, but I would prefer to have an automated system that keeps the batteries warm enough so they can remain on solar charge. Not only is it simpler and means the batteries stay fully charged, but avoids the inevitable day when I forget or we have an unexpected freeze, and my $3500 worth of lithium batteries gets damaged. I would hope the ‘on’ time for the heater would be limited, and use less energy overall than we regain from keeping the solar on.
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Old 11-16-2020, 03:18 PM   #4
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Have you though about heat trace? It's intended for pipes, but it's still a basic resistance heat-srouce, so it'll still heat the space. And potentially distribute the heat more evenly.

These guys sell 12v heat trace by the foot, and have a corresponding solid-state thermostat option.

https://www.oemheaters.com/product/6...le-5-wattsfoot

There's lots of options for adhesive tank-heaters too, but I think that would be over-kill for your application.
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Old 11-16-2020, 06:35 PM   #5
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Thanks again. They do have a number of options for 'flexible' heaters - cables and pads. And as you note, they have thermostats that can handle up to 30 Amps DC. They are called SoliStat. They are pricey (approx $90), but as they are solid state they avoid the vibration concern you raised, and also avoid the need for a separate relay. See this link:
https://www.oemheaters.com/category/...dc-thermostats


I am communicating with them about options and will post what I find out.
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Old 11-16-2020, 06:35 PM   #6
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I would expect that the Lithium batteries have a BMS that would disconnect and prevent battery from being charged. In any event the Inkbird ITC 1000-12V is a good unit. It can handle up to 10 amps.

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Old 11-16-2020, 06:48 PM   #7
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Thanks Greg - I would also have expected the controller to cut off before freezing, but it doesn't have that capability. But the link you sent looks promising. I'm checking it out. They also have a small 150W heater. As always, the the reviews are very mixed (and contradictory). Do you have personal experience of it?
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Old 11-16-2020, 07:49 PM   #8
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We put our lithium house batteries inside the van under the bed, no cold issues.
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Old 11-16-2020, 08:04 PM   #9
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Quote:
Originally Posted by PhilH View Post
Thanks Greg - I would also have expected the controller to cut off before freezing, but it doesn't have that capability. But the link you sent looks promising. I'm checking it out. They also have a small 150W heater. As always, the the reviews are very mixed (and contradictory). Do you have personal experience of it?
I have used the Thermostat it worked well, never used the heater.

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Old 11-17-2020, 09:03 AM   #10
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I went with simple 120VAC heat tape, bought at WalMart and wrapped around the batteries in the battery box (which has a small amount of insulation). It has no problems keeping the batteries at 50F even in howling wind at sub-freezing temps. Even when I am boondocking, and thus running the tape off the inverter, the load is really not worth worrying about, and of course when on shore power there is no problem at all.
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