Cold winter use, self battery heating?

[Zeropointbug]

Hey guys I have been pondering this issue for awhile now and was thinking.... I have a 16s 15AH flat A123 pack (actually my brother, but I have built it for him), he goes to university and now that it is winter it can get cold (very cold) down to -35 Celcius (-31 F). It hasn't been near this temp yet (seasonally warm right now, but the battery won't like those temps for very long, I'm sure it will work but voltage drop will be very big issue. I was thinking, a vehicle starter battery warmers for a small car are a 25Watt heater, and that is heating a poorly insulated chunk of lead in very cold temps, up to room temperatures. So, with a bit of insulation and electrical work, could one power the heat blanket FROM the batteries own power to keep it's self warm for:

Pack Wh's ~750Wh, heat blanket say ~ 25watts, that 30 hours of what seems like plenty of heat for sustaining a good pack temp. The bike is sitting in the cold for maybe 7 hours AT THE MOST, usually 6 hours or less he says... along with a round trip of 300Wh...

750Wh pack - 300Wh trip consumption = 350Wh left for heating, that's still 14 hours of heating!

Now this is just an idea I just thought of, I never would have thought it was electrically feasible, but it APPEARS like it might be an option, just would like to hear what any of you have to say.

Cheers

 

[dogman dan]

I'd say better off to use a reptile heater or similar device, and plug it in. If insulated, a warmish battery won't cool off that much during the ride. Best of all, bring the battery inside when not in use to stay warmer.

You'll need and or want all the capacity since you will lose some in the cold. Plus, you will be discharging deeper, which costs cycles.

 

[Zeropointbug]

Those reptile heaters look decent. The ride to the university isn't a problem, it should stay warm during the ride yes, but it's the cooling off for 5 hours that is the problem, and he can't take the battery off either. The pack won't loose any 'extra' in the cold if it is kept warm, and the pack should only be discharged to maybe 450Wh including heat blanket. Total pack Wh is 750+Wh, 450 used, that's 60% of pack capacity, hardly stressing the A123's i think. I just don't know if it's possible to modify a heat blanket to run on 48 volts. This might be more work than it's worth, but it seems to work in theory.

 

[BikeFanatic]

I think you have a creative solution, I wonder if there are car battery heating pads that run off of 12 volts, like the battery itself. then you could carry a spare 12 volt, like a lipo or NIMH, and power the heater off of that.

I also leave my battery out in the cold for hours, but never below freezing, who wants to lug a battery around all day.
Good luck with your effort and Please post your solution when you figure it out.

 

[auraslip]

there are 12v NOS warmers for NOS tanks. that's what is recommended. Running your battery hot actually gives a really nice performance boost (less sag, and more capacity).... although too hot gives a marked decrease in life span.... but not as much as you'd think...

 

[dogman dan]

Ahh, that is a problem then. The battery would be quite cold, even if insulated, and isn't like a nicad that would warm itself back up. The risk is that you start your ride with a nice warm battery, then run out and end up pedaling home in bad weather.

I'm just not convinced your reserve is going to be big enough for the job, and leave the 20% you'd like to have to extend the pack life. Say you budgeted 150 wh for the heating, that really isn't that much heat if spread out all day, like 25w heater to last 6 hours. It might work better, if a 100w heater ran for an hour before you left. Possible to return to the bike an hour before he leaves?

I would think most resistive type heaters would work fine on DC. People use heat elements and light bulbs to do load tests. Perhaps one of the better heat sources could be a light bulb, mounted in a housing that allowed the heat to reach the bottom of the pack. Sort of an easy bake oven for the battery housing, using a low wattage bulb. Maybe a halogen?

Another altenative, maybe some kind of hot water container under the battery? Something you could bring a quart of hot water from the tap to fill just before you ride off? You'd fill it, ride, drain it. Then it would not be a chunk of ice when you want to ride next time.

 

[dogman dan]

One more thought, The 12v block heater or Nos heater would run great on a seperate 12v pack. Possibly carry a seperate battery for the heater? Again, maybe turn it on an hour before departure if possible. Then it wouldn't kill itself before you returned if sized properly.