lifepo4 low temperature charging idea

[John Frum]

Scenario:
I get up before dawn and I notice that its dropped below -2 Celsius.
Fortunately I have a low temperature cutoff capable BMS which has tripped to protect the batteries.
I'm in the middle of nowhere so noise is not a problem.
The sun will soon be up and harvest-able.
My batteries are ~50% discharged .
I want to be ready to charge from the sun.
I fire up my tea kettle(need the coffee anyway) and whatever else I need to create a load approaching 1c or whatever my BMS will comfortably allow.
The high rate discharge should warm the batteries enough to re-enable the charging fets.
If my battery bank can't sustain the discharge I can start charging via generator and ac_charger at between .5c and 1c to keep the batteries warm.
Sound feasible?

 

[snoobler]

NiMH based Hybrid vehicles do this all the time. When it's below the safe temp for their 70-90A charging rate, they discharge them at high current to warm them and also apply low charge currents to generate internal heat.

Watch the video interview Will did with the Battleborn guy. He talked about low current charging being safe when cold. That would also be an option if you can limit your charge controller's charge current.

Also important to know the temperature of the batteries themselves. They still have a decent mass, and that mass reacts more slowly to temperature change than the air.

If you're going to run the genny, might as well attach a 1000W space heater to speed it up.

 

[HRTKD]

Are your batteries stored outside? On my RV, the batteries are normally stored on the tongue (dual 6v GC). When I transition to LiFePO4, I'll put those inside the trailer. When I'm using the trailer, the battery location should never go below 32° F. When I'm not using the trailer, it's going to get cold. But I won't need the batteries charged to 100% in that situation.

 

[snoobler]

Bet something like these isn't a horrifying idea in some situations:

Robot Check

12V/78W isn't horrific, and they come on at 45°F and run to 68°F. I have a few acquaintances that swear by these for cold weather RVing.

The batteries would be keeping themselves all toasty warm....

 

[John Frum]

Are your batteries stored outside? On my RV, the batteries are normally stored on the tongue (dual 6v GC). When I transition to LiFePO4, I'll put those inside the trailer. When I'm using the trailer, the battery location should never go below 32° F. When I'm not using the trailer, it's going to get cold. But I won't need the batteries charged to 100% in that situation.

Should have been more clear.
Its a hypothetical scenario.

 

[HRTKD]

Bet something like these isn't a horrifying idea in some situations:

Robot Check

12V/78W isn't horrific, and they come on at 45°F and run to 68°F. I have a few acquaintances that swear by these for cold weather RVing.

The batteries would be keeping themselves all toasty warm....

I've been thinking about something like that for warming the batteries when the trailer isn't in use. I'm not sure it's necessary in the first place. I'm also worried that slapping a heating pad on one side of the battery (set of cells) could warm one side while leaving the other side very cold. So two pads may be necessary. Providing power to the pads during the day shouldn't be a problem as long as the SCC is allowing power through (due to ambient temperature). At night, the pad(s) could suck up a fair amount of juice. I would want to see some real world results on how effective the pads were and how much power they really used.

I have pad heaters on my water tanks. I turned them on once on a return trip from camping when it was cold (< 32° F). Even with the moderate charge from the tow vehicle and the charge from the solar, the batteries were discharged much lower than I expected.

 

[snoobler]

78W overnight is a big draw. 12 hours is a 0.94kWh drain or 78Ah. That can put the smackdown on some batteries.

78W isn't too much of a concentration of heat to be horribly worried about uneven heating to warrant a second pad. Simply wrap the battery in something insulative... Like the blanked Will used in his solar shed.

Ah... I see now trailer. Ignore me in the other thread.

 

[HRTKD]

Getting back to SmoothJoey's original question, if people aren't worried about the batteries getting hot during discharge in summer weather, how warm will the batteries actually get in the winter from a moderate (normal?) discharge?

I'll go back to what I said in my first post of the thread: If you're living there and the batteries are located indoors, they won't see freezing temperatures. If you're not there and you're not trying to power the cabin, do the batteries need to be charged at all?

 

[John Frum]

Discharging/charging should heat the batteries relatively evenly.

 

[snoobler]

Discharging/charging will heat healthy batteries relatively evenly.

Fixed and qualified. A cell with notably higher IR will definitely buck the trend.

 

[snoobler]

smoothJoey Dude... how often do you change your avatar, or is is some random thing?

 

[ianganderton]

I think the problem here would be if there was an error.

The basic premise of if it’s too cold don’t charge is easy as it’s simple decision making and very safe for the (expensive) battery

If you wanted to dump a load of power to warm the battery up you would need to know

a) how cold the battery actually was
b) how many watts of energy would be produced by any given high current drain
c) how many watts are needed to warm the battery a given amount (on here folks regularly quote data on energy needed to warm given amount of water by 1 deg)
d) what effect would the surrounding have to counter act (cool) the battery.

If you are in an RV and getting up on a cold morning to make your coffee looking at the sun rise expectantly to start charging energising your solar panels isn’t the sun going to be hitting your vehicles straight sides and pretty much 90deg and warming it up anyway.

first couple of hours of winter sun is pretty useless on solar panels most places in the world but does warm houses through windows very well.

 

[JoeHam]

Should have been more clear.
Its a hypothetical scenario.

Also make a cup of coffee for your battery.

Place it underneath and let the steam warm your battery before charging.

 

[A.Justice]

Bet something like these isn't a horrifying idea in some situations:

Robot Check

12V/78W isn't horrific, and they come on at 45°F and run to 68°F. I have a few acquaintances that swear by these for cold weather RVing.

The batteries would be keeping themselves all toasty warm....

I (haven't HAD to yet) use a plant seedling warmer to keep my betteries warm, it draws about 20 watts (AC side), and heats up to 85f. You can even get a temperature regulator for it. They run about $40 on Amazon. If my batteries weren't kept inside, I would have sandwiched the mat between the cells.

 

[snoobler]

I (haven't HAD to yet) use a plant seedling warmer to keep my betteries warm, it draws about 20 watts (AC side), and heats up to 85f. You can even get a temperature regulator for it. They run about $40 on Amazon. If my batteries weren't kept inside, I would have sandwiched the mat between the cells.

Link?

 

[A.Justice]

Link?

Amazon.com