How will heat affect my idle battery bank?

One of the really neat/handy rules of thumb I learned in Engineering Math class (was one of the first "real world" Calculus problems that was not related to mass/volume/moment arms) was this rule of thumb. For every 10C increase in temperature, the "thing" will age 2x faster. For every 10C decrease in temperature, the thing will last 2x longer (i.e., 20C cooler, 2x2=4 times longer).

We really see this with our Canadian friends. They sub freezing winters and cabins unused, their "cheap batteries" can last 8+ years (compared to 3-5 years typical in warmer climates).

Assume life of battery is based on 25C--Then at 35C (95F), the batteries will "age" 2x faster that that temperature.

For example, here is a nice graphic (for Virginia) that shows soil temperature swings vs depth:

Ground Temperatures as a Function of Location, Season, and Depth


If you need a "root cellar" for storing foods--That might be a good second justification for going deep.

-Bill

Aging happens regardless of cycling. Hot battery (10C hotter) will age 2x faster than one at "room temperature".

If your battery has 5 year life and 1,000 cycles... And cycle every day--That is about a 3 year cycle life. (Fixed example, 1,000x not 2,000x cycle life for example/math to work out)

And if the battery sits at +10C for 1/2 year and at room temp the other half, the battery will "age" 1.5 years per year--Or age 4.5 years in 3 years... So, in the above example, the battery would cycle out just about the same time it "ages" out.

However, if you only cycle 100 days a year, you have a 20 year "cycle" life and a ~3.x year "age" life.... So keeping the battery cool (at room temperature or below) for 365 days a year, should give you ~5 year life on the battery bank vs 3.x years.

Is that worth digging a cellar? Only you can answer that question (and should you use "cheap golf cart" batteries, or a very nice fork lift/expensive set of deep cycle batteries will also play in the questions&answers).

-Bill