NiMH battery in cold weather

[661-Pete]

The answer to this one is probably self-evident, but I thought I'd check on others' experience. Because the NiMH battery for my front lamp is fiddly to remove and install, I generally leave it on the bike when parked at work, so it gets extremely cold - below zero recently. Obviously it seems to have less life between charges as a result. But I've now noticed that the 'low battery' warning lamp, which normally gives you a leeway of about 15 minutes or so at 10W, until the battery dies, now gives me less than 5 minutes - not enough time to get home. I've been caught out a couple of times, so I've taken to carrying a spare lamp.

Is this just the cold weather or is my battery kaput? It's several years old now.

 

[Cardiac]

Is this just the cold weather or is my battery kaput? It's several years old now.

It's a bit of both. The age of the battery will limit its ability to hold a charge and to deal with cold weather. Age may also accelerate self discharge. Low temperature results in lower discharge efficiency - in other words if you charge it at a normal temperature, at a low discharge temperature you won't get out as much as you would have if you had also discharged it at a normal temperature. IT can also lower the effective voltage of the battery due to an increase in its internal resistance.

Keep in mind that a new battery is only going to give (say) 80% out from what ever goes in. At low discharge temperatures, that could be down to 50 - 60%. With age as well, you may only be seeing 20% - 30%. If you combine that with the fact that some appliances will switch off below a certain voltage in order to avoid a damaging excessive discharge of the battery, and you can end up with a very short run time indeed.

If you have got several years out of it, i would say it has served you well, and it's time to get a new battery.

 

[Wobblers]

A NiMH battery will deliver about 80% of its rated capacity at 0[sup]o[/sup]C - and very much less at lower temperatures. Added to this is the effect of lowered battery voltage in the cold so you'd expect the low battery warning to come on sooner.

That you're getting less time between the warning and a totally flat battery is most likely due it being old, and having less capacity than a new one, unfortunately. Conventional NiMH batteries only seem to last 4-6 years or so it sounds like its at the end of its life. You might be able to condition the battery by cycling it a few times (charge it up, drain it, charge it up again, discharge it... no pedalling involved!) but I wouldn't hold out much hope tbh.

 

[HJ]

NiMH batteries don't like the cold at all, not good for winters like this one.

 

[661-Pete]

Thanks for the tips - more or less as I thought . I'll stick it out with this one for as long as it lasts out - I don't have a long commute.

 

[DrSquirrel]

Buy a new set.
Carry spare alks.
Wrap something around your battery case to try and insulate it a bit more.
Look hacking up an external battery pack for the winter and remove it as and when.

Which light are you talking about?