Should I heat a SLA battery in very cold operating environments?

[corrado33]

Apologies for the double thread post on SLA batteries.

If you read the other thread then you'll know I want to put a (relatively small) SLA battery on my pedal bike so I can power heated gloves and a few lights. Unfortunately, this is mostly going to be useful in the winter (short, cold days). We often reach temperatures of -20F that's ~-30C for the rest of the world (or 243K for the chemists!), and I was wondering if it'd be worth it to run a strip of insulated NiCr around the battery and in the enclosure to keep it say, above 5C. I know drawing current will produce heat and therefore warm things up, but I'm not drawing that much current.

It shouldn't be hard to put a comparator with a voltage divider and a temp sensor that outputs voltage to turn on a circuit around 5C. I could use some high value resistance NiCr so that it doesn't draw a ton of current and doesn't get hot enough to melt anything. So, is it worth it? Imagine I wasn't using free batteries and I wanted to preserve the battery, would I do it then? Would it be worth it to keep the enclosure warm in terms of battery capacity saved vs. capacity used to heat?

[DanielS]

At -25C and under, I would start to worry more about the battery's electrolyte freezing and causing a structural failure so you definitely want to keep it above that if you can. The freezing point of lead batteries rises as they discharge so you also want to keep them as close to fully charged as possible.

[mzzj]

I wouldn't bother  heating the battery with the battery energy.
I would consider extra insulation layer and some sort of quick-mounting for the battery so that you can keep it indoors warm and cosy, or run the heaters  when the battery is charging so that you wont drain your battery with heating.
½ to 1" insulation layer should keep your battery warm for a while, probably longer than I would spend at -30C 

Up here battery heaters are quite common in military, law enforcement and rescue vehicles but they are mostly powered from grid along with the engine block/oil pan heaters and battery chargers.

[SeanB]

Probably would be best to use a well insulated box for the battery. Think of a small cooler box that will fit the battery and still close, and a connector to provide power. Then a charger to charge it in the night and during the day when at work so it will be both fully charged and warm when you want it. That will likely fit on the bike frame in a carrier, and then simply use a strap to hold it down during driving and then undo at home and work to take inside. Cabling can stay on the bike and have a short flylead to the box.

[sunnyhighway]

I guess you will be be wearing some serious protective clothing with those temperatures.
Why not simply wear the battery as close to your body as possible underneath that protective clothing to keep it warm and run some wires down your sleeve to where where the power needed.
This way, if you don't freeze to death, neither does the battery.

[saturation]

I wear heated clothing and gloves with smaller batteries inside the clothing, to keep it warm, this allows one to use lower cost batteries, easily changed, and optimize its mAh and lifespan.  Its best not to run batteries outside their optimal temperature, if you must it will be suboptimal even if it worked.  In auto applications, the batteries normally have more than enough CCA to just start a car even with cold exposure, thereafter engine warmth will bring the surrounding environment to its normal operating temp, and an improved discharge/charge condition.

http://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures

If you had to in a bike, I'd wear the battery underneath my clothing too so choose a chemistry, size and form optimal for wearing, then draw wires out to power your lights, if need be.   Thus the batteries need not be left exposed to a harsh environment.  The batteries keep you warm, and you keep the batteries warm, symbiosis.