Following discussion on a thread below, initiated by Dancr,

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=0014aq

I went looking for a concise explanation of batteries and temperature. The following is copied and pasted from the Northern Arizona Wind & Sun site. It explains the effect of temperature on both capacity and on battery life. Its good information for anyone considering batteries and an inverter.

Temperature Effects on Batteries

Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass. Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature.

Even though battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times.

-- de (delewis@inetone.net), July 16, 1999

bold off

-- me (again@my.computer), July 16, 1999.

I don't understand. I was told to put my batteries in the refrigerator. Anyone else know?

-- Mara Wayne (MaraWAyne@aol.com), July 16, 1999.

Mara ,

You keep batteries in the refrigerator before you use them because batteries have a limited shelf life, and like food they keep longer at lower temperatures. You want to use them at room temperature.

-- biker (y2kbiker@worldnet.att.net), July 16, 1999.

Also, Mara,

The material applies specifically to deep discharge batteries for renewable energy systems, most of which are lead-acid type, not nicad rechargables.

-- de (delewis@inetone.net), July 16, 1999.

Thanks De,

My solar man also warned me about simply storing solar batteries. They will sulfate if left alone. They should be hooked up even if they are not in use. I bought a sulfator that will keep them from sulfating. You can also connect a small battery charger and run a small charge through your batteries and they will keep in good shape.

-- bb (peace2u@bellatlantic.net), July 16, 1999.