LiFePO4 Charging in cold weather

[Captain Larry-DH]

I have ReLion batteries in my house bank, and I want to be sure I’m correctly interpreting the charging instructions in italics below. When they say “5% or 10% of the battery capacity” below, does that refer to the total amp hour rating? E.g In example 1, if the battery is rated at 300ah, don’t charge at a rate higher than 30 amps?

The following excerpt is from the maker’s charging instructions:
Charging Temperature
LiFePO4 batteries can be safely charged between -20°C to 55°C (-4°F to 131°F). However, at temperatures below 0°C (32°F) the charge current must be reduced, until the temperature is >0oC (32oF), as follows:
1. 0°Cto-10°C(32°Fto14°F) charge at 0.1C(10% of the battery capacity)
2. -10°C to -20°C (14°F to -4°F) charge at 0.05C (5% of the battery capacity)

 

[dlochner]

Correct. Charge acceptance rates are commonly stated as a percentage, expressed as a decimal, of the battery's capacity. Thus, .1C is 10%, .2C is 20% etc.

 

[Hayden Watson]

Another way to deal with low temperatures is just to not charge when it is cold which is what I do on my boat and motorhome. On the boat, if I am using any amount of power from the battery, it will be at a time when the boat is being used and therefore heated. Given that the batteries are inside of the boat, they will also be heated so I can charge at will. If I am not on the boat, the batteries might get cold but the demand is so low that I can wait a day or 5 to charge. If I was in very cold climate with the boat on the hard, I would charge the bank to 13.5v and then turn off charge and discharging and put it to sleep.
In my motorhome, the battery bank is in an unheated compartment below the floor but so far, I have not seen battery temperatures drop below 45ºF on mornings with outside temps close to freezing. The heating of the cabin provides enough waste heat to keep the batteries in the safe charging range.
The BMS's on both boat and motorhome have low temp cutoff and I have them set so that if the battery temperature drops to 35ºF they will not charge until they warm up to 38º.

 

[Captain Larry-DH]

Thanks. I have two solar arrays, each with a dedicated controller, and a temperature sensor on the house bank. I set the larger one to shut off charging at 5 degrees C, and the smaller one to shut off at 0 degrees C. Based on the info provided by ReLion I’ll probably change the smaller one to shut off at -9 degrees C because it’s small enough to fall well within the acceptable charge rate for the house bank.
edited

 

[dlochner]

Thanks. I have two solar arrays, each with a dedicated controller, and a temperature sensor on the house bank. I set the larger one to shut off charging at 5c, and the smaller one to shut off at 0c. Based on the info provided by ReLion I’ll probably change the smaller one to shut off at -9c because it’s small enough to fall well within the acceptable charge rate for the house bank.

The decimal point is really important. .5c is 50% of the battery capacity, 5c is 500% of battery capacity.

 

[Captain Larry-DH]

The decimal point is really important. .5c is 50% of the battery capacity, 5c is 500% of battery capacity.

My bad, I meant to write 5 degrees C and 0 degrees C. I corrected my last post above.

 

[Hayden Watson]

The decimal point is really important. .5c is 50% of the battery capacity, 5c is 500% of battery capacity.

For me, the question was do I need to charge when the temperature is that cold? Unless you are using a lot of power, AND the bank is at a very low state of charge, there really isn't a need to charge. LFP's do not need or even like to be at a high state of charge for a long time so just shutting down charging when the temperature is below freezing is not a problem.
My boat will be unmonitored all winter which made me nervous given that this is my first year with the build. Given that, I charged my 560Ah bank to 13.5v, turned off the BMS and removed the class-T fuse to put it into hibernation. I switched the bilge pump duties to the AGM start battery which has its own charger for maintenance power.

 

[rwsandersii]

I'm happy to see a discussion of LiFePO4 batteries. I have a Compac 27 with an electric motor and I'm considering making the (financial) leap to LiFePO4 for power for the boat. I was watching a discussion about them with Nigel Calder and he was saying that any battery of that type should be certified to UL 1973. Are the Relions certified like this? Honestly I can't find any that are on a web search.
I'm leaning toward the Relion because my motor is 48 V and the have 48 V that can be added in parallel which would make it easy to expand the capacity.
I scorched a set of AGMs and I believe it was because I discharged below 50%. Of course 50% is not easy to measure because if you run at a high speed the meter says it is only 20%, but if you back off it comes back up.
Any tips or referrals to other sources would be helpful.
Thanks,
Richard

 

[dlochner]

I scorched a set of AGMs and I believe it was because I discharged below 50%. Of course 50% is not easy to measure because if you run at a high speed the meter says it is only 20%, but if you back off it comes back up.
Any tips or referrals to other sources would be helpful.

What kind of meter were you using and what was it measuring?

 

[Hayden Watson]

With any battery, voltage is not a very accurate way to measure the capacity. I have Columb counters on both my boat (link-10) and motorhome (Magnum BMK). Anything else is just a guess. Most Bluetooth app have will show % charge and amps remaining but they are not nearly as accurate as one based on a shunt.
If you are planning to go with a 48volt systems there are a lot of options with great reviews. Look for rack systems. Many of these are also serviceable. 48V › Basengreen