Lithium and winter storage

[markopolo]

Originally Posted by @Michael

I recall the Battleborn guy saying something to the effect that the colder the temp, the lower the acceptable charge/discharge rates. Something to do with how fast the ions can move between poles. He implied that these were not hard limits or strict cutoffs. BB's internal BMS cuts off charging at something like 25F.

I also recall one of the major battery mfg's publishing a temperature derating curve for charge/discharge rates, implying that lower charge rates == lower temps.

Trojan Battery gives a reduce current / temperature chart:


I had posted it here: https://www.classbforum.com/forums/f...tml#post113349 but you have to be logged in to see it.





Also JEITA charging recommendations:

[Davydd]

Originally Posted by booster

Davydd, you missed the point. I never said anything about if they can be used when they are warm enough, like yours were with heaters when stored and charging above the prescribed limits.


What I said was that I wondered just how hard and fast the limits are, and if being close to them is causing some, but not as much damage, and if going under them gets progressively worse. That is what, IMO, is still up in the air.

I take it as conservative numbers since you can shoot past and override battery discharging and temperatures rather easily. For instance, just about every lithium battery manufacturer says you should not charge below freezing. ARV takes it a step further and the BMS shuts down charging at about 36º F., a programmable number. Their heating system maintains batteries above 42º F.

I've read the messages here and some battery people are still confusing centigrade numbers with Fahrenheit numbers. Elite Power Solutions I think initially got them mixed up. ARV was the first upfitter I think realized this and I corroborated it directly with Elite. I got this response in August, 2016.

"Hi David,

Temperatures below -4°F (-20°C) will cause the electrolyte to freeze which will cause permanent damage to the batteries if it does not kill them entirely. If storage temperatures will go below -4°F then you will need to heat the batteries to above -4°F to prevent damage to the battery. There is no exception to this.

Regards,
Rick Suiter
Applications Engineer
Elite Power Solutions LLC"

Note it said "if it does not kill them entirely" which means the permanent damage could be just diminished battery life. You can also discharge batteries down to -4º F but not charge, though it is not as efficient to do so. Discharging heats the core temperature of the batteries. I can tell this because my battery heater often does not come on until the ambient temperature gets down to the mid 20 degree F. Also the same with charging. You can charge below freezing but it diminishes the life of the batteries, it does not kill them outright from my understanding. You have to have some memorable numbers and not be wishy washy with them so people can better understand. The numbers presented are good, possibly conservative, or absolute lab tested. It doesn't really matter.

After 5 years with monitoring of my batteries in all kinds of conditions down to -20º F. I am comfortable with my comments and as I said I am not up in the air.

[Davydd]

Originally Posted by jakegw2

The article ilmor links to is very interesting. Not comprehensive, but seems to suggest that the limits are:

32 degrees F for charging
-4 degrees F for discharging
-40 degrees F for storage NO

If I had known this I would have left my LiFEPO4 batteries installed and just turned off the disconnect to prevent any charge or discharge.

-4º F for both discharging and for storage. -40º F. is identical to -40º C. which by the way is the practical limit for AGM battery storage and if you had ever experienced that temperature (I have) you can readily understand why.

[Davydd]

Originally Posted by jakegw2

Davydd - Did the core temperature of your batteries drop into the -10 degree range at these times or was there some sort of heating applied to prevent that from happening? Booster might be onto something here - it could be that there is a time and temperature depth scale that matters. Speculation: perhaps briefly hitting -4 or -10 for a few hours causes some very minimal level of damage (on the order of storing a battery at 99% charge instead of 50%) but hitting -20 degrees ruins the pack.

My thinking so far has been along the lines of what avanti suggests - the -4 degree temperature is the freezing point of some critical material inside the LiFEPO4 batteries and the phase change will destroy their internal structure.

My batteries have never gone below freezing because, though they are in an outside the van box they have electric resistant heating pads of 10a total to keep them always above 42º F. Only once did they get below that target temperature in my first week having the van I realized the on/off switch was wired backwards when I thought the heaters would turn on but they were off. The tiny red dot idiot light on the toggle switch I assumed was on that was really off. Since then it has been corrected and there is a double fail safe on my Silverleaf monitor to actually tell you when the pads are actually heating with a flame symbol.

Heating pad between batteries.



This was my temporary solution until I figured out what was happening.

[jakegw2]

DAvydd - Thanks, I feel better about having swapped them out for AGM for the winter. Your temporary solution would work fine when there is no snow, but pretty unpleasant if I had to shovel out an opening just to get it set up.

[Davydd]

Originally Posted by jakegw2

DAvydd - Thanks, I feel better about having swapped them out for AGM for the winter. Your temporary solution would work fine when there is no snow, but pretty unpleasant if I had to shovel out an opening just to get it set up.

I couldn't remove my batteries if I wanted to. I'd have to put the van up on a lift and need to lower them with a jack. The batteries and protective steel cage and skid plate might be 500 lbs.

I have photos at ARV doing this taking 3 people.