# How will heat affect my idle battery bank?

Solar Expert Posts: 1,572 ✭✭✭✭
We all know heat is bad for batteries and shortens their usable lifespan. I was wondering how my battery bank in Baja will be affected during the worst of the summer heat while they are sitting idle and on float only. For 2 1/2 to 3 months the daily temps. will regularly exceed 100 degrees and never go below the 80's overnight. Is the fact that the batteries aren't being cycled for this period of time helping to keep the aging to a minimum?
I have spoken to some folks in the area who have constructed underground battery bunkers within their homes to help regulate the heat.
Any and all thoughts welcome.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Super Moderators, Administrators Posts: 31,176 admin
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

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,572 ✭✭✭✭
BB, thanks for your informative and speedy reply. Yes, doing an underground Battery bank could help regulate temps. My  question though is to what extent does heat affect an idle battery bank compared to an actively cycling one.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Super Moderators, Administrators Posts: 31,176 admin
edited July 2016 #4
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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,046 ✭✭✭✭

Will just add the fairly obvious,  in addition to BB Bill's great information;

Cycling a battery will add some heating,  particularly the Absorb stage,  and,  especially when EQing Flooded batteries.   This heating is not trivial when recharging a battery that has been fairly deeply-discharged.

Generally,  Bulking the battery is efficient,  until the Gassing voltage for the particular battery is reached.

For batteries that are being cycled,  this is another good reason to NOT fully recharge every day.

IMO,   Vic

Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 1,572 ✭✭✭✭
Got it. It's (heat) going to have an equal effect regardless of whether they are cycling or not. This starts to make the underground bunker make sense.
Thanks guys for sharing your knowledge.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Super Moderators, Administrators Posts: 31,176 admin
Sorry, 1,000x for example above on cycle life math. 2,000 was not right.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 835 ✭✭✭✭
Heat really is the enemy!  We see battery cellars a LOT here in South Central Texas. (We've already had 58 days of 100 F d higher) Some applications put them in a partially cooled insulated building.

Concorde publishes a 50% reduction in life for every 10 degrees C increase in temperature. Keep in mind that it's the average temperature, not the peak.

I always have more questions than answers. That's the nature of life.