# Lithium in low temp possible to derate charging current, I have an idea.

I recently installed 28kw of lithium batteries in a bus conversion and we'll hit colder temperatures in central MN USA.

It is my understanding that it's no so much that lithium can not be absolutely charged below freezing but that it must be derated or compensated below 45f or so. This is because the negative ions stack up and can damage the cells of charged too quickly.

I have an idea... what if we set a temp compensation value intended for lead acid batteries was reversed? Example, I think a traditional value would be -36mV/C, but what if that value was reversed to be +36mV/C or higher. Wouldn't this invert the voltage compensation curve to produce a higher voltage as it got warmer and a lower voltage as it gets colder?

I'm just looking for a way to gradually derate the charge curve instead of a hard off or on switch. I'm also aware of a super secret "Low temperature charge current" setting HEX-Protocol in VE Direct. Would be nice to see that exposed behind a password or something.

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It is my understanding that the lithium becomes crystallized into a larger form and then clogs up the carbon plate causing permanent damage. Why not use a heating element or blanket to winterize your system.

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Right, but that doesn't happen instantly. There is a fall off, you absolutely should not do a full 1C charge rate at 5 degrees C. Yes I plan to add a warming system as a last resort, but I figure, why not use the batteries themselves as well. They should warm with charging, example:

Start at 5 degrees C, charge at .1C rate, as you charge internal temp rises to 7 or 8C, subsequently raise charge amps to .25C rate and so on. Now you're using charge energy to raise the temp and also not damage the cells.

I dont see any temp rise in my battery charging at .1C

It's definitely possible to charge below 5C (or 0C) without damage so long as the charging rates are reduced -- there's no "brick wall" at 0C, just that at low temperatures the ion mobility drops rapidly and the current you can charge at without causing lithium plating falls rapidly. Certainly charging at high currents below 5C (or even higher) is not a good idea.

The problem is finding this information -- IIRC I've seen figures quoted like 2C at 5C, 0.5C at 0C, 0.1C at -5C, 0.02C at -10C, but these are not authorised by battery manufacturers. Some of the LiFePO4 battery manufacturers with built-in BMS (BYD, MG) do allow charging at reduced currents below 0C but they don't disclose what the currents are -- if you buy their batteries you'll get this feature "for free" so long as the internal BMS controls the charging current from outside sources.

For example BYD specify minimum operating temperature of -10C, allow full-rate charging above 5C but say "charge derating occurs between -10C and +5C". This was done to meet repeated requests for operation at low temperatures in winter off-grid applications.

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Any links to those figures?

My bank is 1120ah@24v , so even .1c is 100amps , which I rarely charge at anyway.

Now I'm thinking maybe this could be done by changing DVCC charge current with temp. Could this be done with a script of some sort?