Cold Weather. Does it affect NIMH?

[nickz]

I just pulled a rechargable energizer led light from my car and used it today for about 35 minutes max. Probably closer to 25 minutes but I'll say 35 to be safe. The light started to dim fast at the end. I have had the pack on my triton. The pack is rated at 1000Mah but I got around 1300mah during charging and discharging. At 1000mah, energizer gives the light an 85 minute runtime.

I am just curious if the cold weather would reduce the charge of the cell. It has been in the car since Christmas with weather being around 25-30F most days, but the last three days or so has been around 9 degreess F.

Any help would be appreciated.

 

[FlashCrazy]

Cold weather will affect the performance of the pack, especially the amount of current it'll put out at a given moment. This really isn't a problem unless you're discharging a pack at higher current levels.

Anyway, what has more than likely happened is the batteries have self-discharged. NiMH batteries lose capacity over time, even when not in use. Some are so bad that they'll be at half capacity just a couple of days after charging. You'll notice many manufacturers touting "low self-discharge" NiMH's now (Sanyo's Eneloop, Rayovac Hybrids, Amondotech Enduros, etc.). These batteries have a different separator material in them that helps to combat the situation.

Another thing to consider is that listed runtimes are rarely accurate, and sometimes downright dishonest. If you have a multimeter, you should check the current draw of your light after charging the pack. This way you can calculate an expected runtime.

 

[Marduke]

NiMH cells follow the same principle for temperature as most cells.

Most all chemistries prefer to be stored at cold temperatures, and used in warm temperatures. If you use the cells cold, they will preform like you described. If you store the cells warm, they will self discharge much more rapidly.

The specific figures depends on the temperature and the cell chemistry.

Here is a good place to look:
http://data.energizer.com/PDFs/temperat.pdf

But it also sounds like a self-discharge issue too.

 

[nickz]

I have discharged the pack on my triton at 1 amp and it gives me just under 1300mah (pack is rated at 1000 mah and is a 3 1/2 A cell size pack). It is right at about the same when I recharge at 1amp. I'll check the current draw of the light. On the matter of self discharging, the light had been in the cold car since Christmas and used last night. It gave full output for around half an hour but then really died down. I guess I'll sit the pack out in the garage with a full charge for a week and then do a discharge run on my triton to see how much capacity is left at that point. If the pack has a large amount of self discharge then I'll just have to have another one built for the light or take it back to Wal-Mart.