Why you cannot charge LiFePO4 below 0 degrees Celsius

Sojourner1

Itinerant
I've looked at my cells for many years now and using them everyday with no notice of the cell temps rising more than a degree with a draw of .35c. I can't imagine the cells warming up from a draw on the system. Heat source is a must in known cold temps and away to see if the cells temps, going by ambient temp isn't necessary the cell temps.
 

Sojourner1

Itinerant
Here is an example and notice it's only 29F outside ambient, cells are 39F and compartment various temps throughout the day catching heat from the inverter/ heat duct.

From first reading @ 0934 cells 39F. Solar was doing very little charging but having loads on the system till I started the generator for a charge on the batteries. The cells did not warm up. Now picture the ambient temps 10f, 20f or more lower and cells even colder you're not warming them up without help from a heat source.


10_18_20 readings.PNG
 

Hedges

I See Electromagnetic Fields!
If you are able to barely able to get your battery above zero degrees C, that doesn't mean it can now accept maximum charge rate.
A posting earlier in this thread has tables from some manufacturers showing that the acceptable charge rate is about 0.1C, for a battery which can take 1C at moderate temperatures.


What that doesn't mention is the voltage required to put in such current.
Upnorthandpersonal's comment is that at normal charging voltages, very little current flowed at -20C.
It is possible the manufacturer's table showed currents that couldn't be achieved with a voltage-regulated charger. But without knowing that for sure, I think you should limit charge current according to those tables.

If you're using a separate battery charger for emergency charging in cold weather, better limit it to 0.1C (or 1/10th of whatever maximum C rate battery is specified for) when just above freezing.
For your system's PV charger, I would determine the C rate it was capable of delivering, then adjust low-temperature cutout to a temperature where that C rate is allowed. Perhaps > 7 degrees C for 0.3C charge rate, using the ETC table.
If you have multiple charge controllers, one could have zero degree cutout and the others higher, for a stepped charge rate vs. temperature.

Not an issue of PV wattage is less than 1/10th battery Wh capacity, which would provide max 0.1C. But something to consider for larger PV array.
What we really want is for battery charger to have temperature compensated charge rates.
 

Sojourner1

Itinerant
Some folks aren't grasping the top balancing of a battery or should I compress cells, you really think they're going to grasp this...
"If you're using a separate battery charger for emergency charging in cold weather, better limit it to 0.1C (or 1/10th of whatever maximum C rate battery is specified for) when just above freezing.
For your system's PV charger, I would determine the C rate it was capable of delivering, then adjust low-temperature cutout to a temperature where that C rate is allowed. Perhaps > 7 degrees C for 0.3C charge rate, using the ETC table.
If you have multiple charge controllers, one could have zero degree cutout and the others higher, for a stepped charge rate vs. temperature.

Not an issue of PV wattage is less than 1/10th battery Wh capacity, which would provide max 0.1C. But something to consider for larger PV array.
What we really want is for battery charger to have temperature compensated charge rates."

When just adding a heat source would be easier if in extreme cold temps and wanting to use or living off LFP batteries. I'm going to guess that in a couple years they'll be threads of "why haven't I got thousands of cycles out of my batteries." ;)
 
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Substrate

Solar Addict
I think upnorthandpersonal has the most workable *reliable* solution. If you don't have a heating pad, well tough - get one!

The "cold start procedure" hack from an entirely different application like motorcycles that use lfp doesn't scale well. :) Their typical measured capacities at the C/20 rate range from 4ah to 12ah. Which leads to consumer confusion when they try to equate them with made-up values like Pb/Eq (lead acid equivalents for starting purposes), but that's a different story for another forum...
 

HRTKD

Boondocker
I figure, "If you can't dazzle them with brilliance ..."

...Then give them something simple. I figure my warming system is simple. Keep the batteries at a temperature range where they can charged.

With regards to the low C charge rate at low temps, it may be that some of that is inherent in a PV charge. There isn't much juice early in the morning, so the C charge rate can't be that high.

My warming system is there for when I'm not actively using the RV (no furnace running) or if the furnace dies (has happened due to propane outage). When I'm there, with the furnace running, the batteries are happily in the 55° F to 65° F range since they are located in a compartment that has a non-insulated heat duct going through it.
 

Hedges

I See Electromagnetic Fields!
So my simple suggestion is determine what C rate your PV can deliver, look up temperature your cells have to be for that charge rate, and set BMS low-temperature cutout to that temperature. My point being that if you've got 0.3C charge capability, the cells can't take that at zero degrees C.
If you succeed in keeping the battery warm, then the low-temperature disconnect will never occur. BMS is backup protection.
 
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