Battery AH Reduction in Winter

mjp24coho

I know I've seen some general statements before (can't seem to find specifics) - does anyone know the average % of battery AH capacity reduction for every XX degrees below 60 degrees F in the winter? My batteries sit in an insulated box in my detached garage at my cabin. Inside temperature in the garage can often get below 30 degrees F in the winter, at times my batteries can get down to 35-40 degrees F. I'm trying to estimate (for my calculations) the AH capacity reduction in my battery (24V 800 AH forklift battery)

vtmaps

Re: Battery AH Reduction in Winter

The reduction in capacity depends upon the rate that you are discharging.
Here is a graph from a handbook of batteries:
Attachment not found.
Note that all curves meet at the point where capacity = 100% and Temp = 25° C (standard temp).

Cold temperature is supposed to prolong life of batteries, and in some ways it does. Batteries are dieing in many ways at once: grid corrosion, cycle life, shedding, sulfation, etc. Cold temperature reduces self discharge and slows down some aging processes.

On the whole, however, my feeling is that keeping batteries cool in the winter SHORTENS their life. This is an issue for me because my batteries, like yours, spend much of the year in an ambient temp just above freezing.

The reason I feel that cool temperatures shorten battery life is because cool temperatures cause deeper discharges and therefore use up more of a battery's cycle life.

Suppose you start from 100% SOC and draw 1 kw for 4 hours (4 kwh) from your battery at standard temp. I will assume (guess) that the voltage averaged 25 volts during the draw.
1 kw ÷ 25 volts = 40 amps 40 amps for 4 hour = 160 amphours drawn from the 800 ah battery
Therefore, the load is at the C/20 rate (40 amps for a 800 ah battery).
Therefore your load left the battery at (800-160) ah ÷ 800 ah = 80% SOC.

Now do the same calculations for the battery at 0° C, with the same load.
The battery has higher internal resistance and higher Peukert factor, and will therefor operate at a lower voltage. I will assume (guess) 24.4 volts. This means that it will draw MORE amps from the battery:
1 kw ÷ 24.4 volts = 41 amps
and over the course of the 4 hour load will draw more amphours: 164 ah

At the same time, the battery has a reduced capacity therefore your 41 amp draw is at a higher than C/20 rate. I will assume that battery capacity is reduced by 15% 85% of 800 ah battery is 680 ah at the C/20 rate of discharge.
(680-164) ah ÷ 680 ah = 76% SOC

But now your 41 amp draw is at more than the C/20 rate and therefore the battery capacity has to be reduced even further. I'm done guessing... I think the real SOC might be a bit lower than I've guessed. Too many variables and unknowns.

To get back to the question: Does cooler temp shorten or prolong battery life? I think that if a battery is cycled a lot while cold, more of its cycle life is used up, and the battery dies sooner. If the battery is not being cycled very much, I think cool temperatures benefit the battery.

--vtMaps

waynefromnscanada

Re: Battery AH Reduction in Winter

I'm lucky, very lucky to have micro micro hydro helping me through these short, dark days of Winter. BH (before hydro), this time of year I had to basically shut down my system to avoid prolonged states of deep discharge and thus sulfation, except for perhaps a couple of small lights in the evening. Over the years I've found that adding thermal insulation to my battery pack over Winter kept their min temp at around +3C to +5 C. The last few years I got lazy and didn't bother with insulation (with my battery box design it's a difficult job to add and remove insulation). The result was min battery temps of perhaps minus 3C. Not a huge difference. Oh, and we often see minus 20 C temps here over winter. Batteries are in a ventilated box with lid, in an unheated outbuilding. Because I only lightly discharge my batteries, I see little difference in their apparent capacity Winter over Summer, insulated or not. And the last bank lasted 12 years.