I believe all batteries perform worse in the cold but NiMH suffer more than LiIon batteries do.

I really like the Light and Motion Vis 360 helmet-mounted light. I bring my helmet in to dry out the liner, and the lights/battery warm up without any further thought or extra motions. Plug it in to charge every 3 days or so. My winter ride is about an hour each way and the temps get down to 0F on occasion - with this system and in these conditions, I've never had an issue with battery life.

I've had mine for about 6 years now and really like it too. The battery life has been decreasing with time, but it's still good enough to get me through my longer rides and it doesn't bother me to charge it more often now.

Right there may be your biggest problem. I charge and store my batteries inside the house. That way they are warm before I begin and the heat generated by the discharge keeps them warm enough so that I don't have run problems. You are dipping a bit into the cold end of the pool for optimal battery range but starting cold makes the problem worse.

I also always charge my batteries after use. That way the battery is always a full charge and I don't have to guess if I have enough light to get home. Li-ion doesn't need to be cycled like NiMH or NiCd needed to be so topping up the charge isn't as much of a problem.

Finally, alkaline batteries seem to be unaffected by the cold. I use alkaline for my taillights which are stored out in the cold. Never had a problem with them. Granted they are disposable but a set of batteries will a year or more per light so I don't feel like I'm destroying the planet by using them.

Electrochemical cell potentials are temperature dependent, as is the forward voltage drop of an LED. If the current through the LED is regulated with a simple series resistor, then the output of the light will be temperature dependent. If the LED's are held at constant current, then light output won't change. So the behavior of the light at low temperature is really a design issue, probably driven by how much you want to pay.

Likewise, simplistic charging circuits could suffer from similar issues. Lithium batteries are a special case because they have a thermal runaway mode, and can asplode if they're not charged under the correct range of conditions.

First it's damned difficult to freeze a car battery. The electrolyte in the battery drops the freezing point significantly. The solution in the battery is about 25% sulfuric acid which drops the freezing point to -25°C (-4°F for the metrically challenged). Most of the stuff you find on the web says that a discharged battery freezes around the freezing point of water but I suspect that just Internet plagiarism at work. A fully charged battery doesn't freeze until around -60°C (-75°F).

For a frozen battery, I suspect that the explosion risk is due to a higher charge being forced into a solid rather than a liquid. This probably starts gas formation...water splitting...and results in increases in pressure in the battery which can probably be highly localized.

I wonder if current would even flow if the electrolyte were solid.

auto batteries aside. I wonder if a bike light battery, best practice, would be to let a very cold battery warm up inside a little while before plugging it in

Probably a good idea, since the charger may be designed to operate at its best within a certain temperature range. If it's a lithium battery, it may have a built-in temperature sensor (typically a third terminal) that the charger uses to help control the charging process and prevent thermal runaway.

I wonder if it makes sense to just have two batteries, and use them on alternate days.

I don't see why not. There are solid electrolytes and most conductors are solid. The fact that there is an explosion risk for changing a frozen battery says to me that there is current flow. It's probably not fast and resistance would be high but enough current is going in to split the water molecules.

Yes. According to Battery University:

Plating of metallic lithium is a very bad thing! Lithium metal doesn't like to be metal and will aggressively strip oxygen from water. That's not the problem, however. The hydrogen left behind combines and the oxygen stripping is hot so the whole thing goes BOOOOOOOOM!

how long would it take a .6 lb bike battery to go from 17F to 50F in a 65F home?

Your light battery is 2 pounds?:eek:

oh sorry, I was estimating & I was waaaay off. turns out it's only 0.6 pounds

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Reviving this thread since it made me think.. Last year the batteries typically lasted what I put above. This year it wasn't even lasting a day. Finally figured out why! Last year put the batteries in the panniers typically wedged with junk around it, the light I have has extensions. This year just had it attached to the stem. I moved it back last weekend and indeed it lasted 3 days this week or all week. Maybe someone needs to come up with a battery insulator!

My Nite Ridr Halogen was a cord remote battery.. 20 years ago , battery under your coat , helmet mount.. waterproof.

I store my bike inside, and usually wait until after I've changed clothes, heard the cats report on how sad she is etc... and figure that the battery has warmed up enough by then - usually 10 to 20 minutes. The batteries don't get too cold during the ride home from being stored indoors at work, and the use helps keep them warm.

Maybe you should work on redesigning the dynamo to work on Tesla wheels so they can charge themselves while driving and have infinite range.

Since OP I've started bringing in my batteries inside. It's an extra step in the morning to remount them but they are lasting much longer.

Hmmmm. I wonder if this is the reason my Bontrager taillight has been acting funny. Just before a ride I turn it on. When I get home, I find it's turned off. But I can't turn it on. So I plug it in but the charging light doesn't flash and it stays uncharged overnight.

Thinking it's reached the EOL after only three years I get another taillight at half the price but with AAA batteries.

Then I check the Bontrager and it turns on again. So I continue using it and carry my new light as a spare.

Back in my RC car racing days we used to charge batteries in a cooler because the charger would take longer and report a larger charge. But in reality it just took more because the battery wasn’t working as well. Then we’d go race and the Nicads would get label-melting hot. Some companies sold aluminum finned heatsinks. Bro science at its finest...

Lol

have multiple batteries that are good to go

These battery problems might be a reasonable argument to convert to a dynamo hub.
Also: computer USB ports do not necessarily output as much charging power as dedicated USB chargers.

There are 18650 batteries from Nitecore especially for use in low temperatures:
nitecorestore.com/SearchResults.asp?Search=low+temperature&Submit=GO
charger.nitecore.com/product/nl1829ltp
charger.nitecore.com/product/nl1829lthp
charger.nitecore.com/product/nl1829rltp

Lead acid car batteries have the same issues, and I'd imagine that car battery shops are busy this time of year, and go into hibernation in July.

The electric cars are supposed to regulate their battery temperatures (heat or cool as needed). It might help if they are on a float charge at night so they can have power heating.

My guess is the Tesla range was shortened because the guy was using electric resistance heating in the car (and perhaps for the batteries).

Ideally, though, a car will also recover heat from the motor for use.

This is one of the reasons I no longer use lights with integrated rechargeable batteries. Instead, I use lights, front and rear, that can use removable primary or rechargeable batteries. They are getting more difficult to find, but they are much more versatile if you ride in the cold or need for a light to longer than a few hours (and don't want a battery pack with cord). My front light is a Fenix BC30 (in warmer weather I use 18650s, and in cold weather I use the same and carry extras or CR123A primaries), my rear is a PDW Radbot (rechargeable AAAs in warmer weather, extra off the same when it's colder or lithium primaries).

I solved this by keeping my bike next to an electrical outlet, then only buying lights that I can leave on the bike while charging.

Pull in on the bike, connect the charging wire, done. Of course having to plug anything in is a slight amount of hassle but I find it's easier on my brain to do it every time than to mentally remember something periodically.

Yeah mine to. :-) Even better, in the winter with the snow reflecting lights you need less light, so even my older dynamo lights gets me more light than I need. Dynamo's are really ideally suited for winter riding.

Pacific
 

Sub 0C being rare here* , My studded tire bike has removable battery lights AA & AAA.. they go in my pocket when I'm off the bike..

When 27f it was dry, last year not a cloud to be seen (or retain heat from daytime) ..* I'm by the ocean.. ranges between lower 40s to mid 50s.. ...




Mountaineers headlamps have the battery pack on a cord, so you keep the batteries in a pocket under the down parka..







....

You guys are so old school. I use the power of fusion to see at night. :roflmao: