I read somewhere that keeping a NiMH battery in cold temps (even the freezer) can slow down the trickle drain effect of a not-in use battery. Is this true? If so, would I be better off leaving my NiMH battery on my bike overnight when it's 10degF outside? I know at least with standard alkalines that the cold wears the battery down, so I've always brought all my batteries inside, but this new piece of news could make getting rigged in the morning and eve WAY easier if it turns out to be true.

Caveat, the article also said if it was stored in the cold there needed to be an allowance for it to warm up before it would run at full speed. True?

My experience has been they do need warmed up, and when they're as cold as what you stated it would seem to take quite a few minutes to warm them to the core again. So I'm not sure the drain is significantly more by keeping them warm in the first place to offset the chance of forgetting to warm them before a morning commute. I mean, you can't exactly pop them in the microwave if you need them warmed up quickly.

The other thing to consider is when a battery is fastened into a contact for something that draws amperage....it does that to some extent even with the switch off. So, if possible.....it would make the most sense to bring them in the night before sans the light they're attached.

Or, and this to me makes a lot more sense, get extra batteries, keep them warm overnight and take them in your coat in an inside pocket. Redundancy, redundancy.

Here is some information from an Energizer NiMH application guide. The first attachment shows that storing NiMH batteries at low temperatures significantly reduces the self-discharge effect. Here's some text from the application guide as well:

"Storage recommendations for nickel-metal hydride cells parallel those for nickel cadmium cells:
Store at the lowest feasible temperatures (-20°C to 30°C being the generally recommended storage temperatures).
Store cells/batteries open-circuit [completely disconnected from the device] to eliminate loaded storage effects"

The second attachment shows that a NiMH battery has highest capacity around room temperature. Any temps significantly above/below that will reduce the battery's capacity. I've noticed this now that the temps have really dropped. My bike light battery doesn't last nearly as long as it did during summer months.

Here's a link to the application guide itself. It has some good information; some technical, some not.

The temperature you're talking about (10F) would leave the batteries with about 40% capacity according to the graph. If it's a battery for a headlight which might get fully drained each day then definitely store it inside at night so you start off with a warm battery for longest run time. Since you need to recharge it regularly, perhaps daily, self-discharge effects are irrelevant in this case. If the batteries are for a blinky it's probably not as important since even at 40% capacity the batteries should last many days, assuming they're only used a few hours a day. You could probably get by with recharging them on the weekends. You'll have to experiment with that.

By the way, do you really leave your bike out in 10F overnight? Talk about a cold seat!

It's a Cygolite Dualcross, FYI. I'll be bringing it inside between my commutes (office and home). Supposedly it has a 4 hr. run time on hi! Since I only use my light on the 1-1.5 hr ride home I'm hoping I can get away with charging it once a week. Reason being that there are several legs of my commute I won't need the hi on, if any.

Thanks for the info, guys.

BTW, while we're on topic, what would the advantages of the Li-Ion battery be other than size?

If you are worried about nimh drain, that is the benefit of the new hybrid or eneloop nimh batteries.. They do not drain power when not in use and act more like an alkaline..

I bought some Rayovac 2100 Hybrid batteries and they have worked very well.. I think they were 9.00 for a 4 pack of AA's..

That's the same light I have.....but I purposely sought out the Li-ion version for a solution to this problem. I can leave it out in the cold with no adverse affects.....same with while on the commute, I don't have to worry about keeping batteries warm. That is the main advantage, I think.

The other advantages besides the size....are the lighter weight and the overvoltage which makes the light brighter.

About the only advantage to the nimh's for this particular light is the purported longer run times.