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Lead Acid and Cold Weather

Posted: Sun Oct 26, 2008 9:17 am
by geekygrilli
Blimin' cold weather has reduced my range to just 14 miles or so (from 25 miles). :( Its not too bad really, as I have to drive my van everyday, so the EV only gets used for short runs in the evenings and weekends.

S'pose one bonus is that the controller stays nice and cool :D

Couple of questions:-

1. Are there any off-the-shelf heat mats available to buy (smaller than electric bed blankets)?

2. What is the optimum charging temperature for wet lead acid, roughly?

Posted: Sun Oct 26, 2008 9:36 am
by mattcarr
the cold weather will hit lead acid batteries the hardest and reduce their capacity by quite alot. I am not sure of off the shelf heater mats, but I know that in my Gwiz the lead acid batteries have got a heaters wrapped around them and this is designed to keep the batteries arounf the 25C mark. I have noticed that the manufactures figures for batteries are always quoted with battery temperatures at 25 degree Celcius. They always advide the finishing charging voltage to be adjusted depending on the temperature of the batteries. It's would be best to check with your battery supplier to see what how much this should be.

Posted: Sun Oct 26, 2008 9:55 am
by geekygrilli
Cheers Matt

Do you now if the battery warmers work whilst charging, or whilst driving too?

Would the energy consummed to keep the batteries warm whilst on the go reduce the range to the same as when the batteries are cold?

If the warmers only work when on charge:- If you drive to work in the morning, leave you car outside in the cold and then go to drive home 9 hours later the range will be rubbish?

Posted: Sun Oct 26, 2008 9:56 am
by Jeremy
I don't know whether it's any help, but when I was reading the book on the development history of the Toyota Prius, I was intrigued by the way they dealt with the battery temperature problem. They were faced with the need to retain battery performance over a wide range of ambient temperatures, so opted to take the battery "cooling" air from the passenger compartment (it's the grill on the offside rear seat back).

They found that this was the most energy efficient way of doing it, as in summer, with the air con on, the cabin air was cool, whilst in winter, with the heater on, it was warm. The Prius system effectively recycles wasted warm/cool air from the passenger compartment, air that would normally be expelled via external vents.

Obviously they didn't have to contend with the fumes from wet cells, but the idea might still work for other battery types.

Insulating the outside of the battery compartment might help, too, although a good source of cooling/heating air would be needed when the car is running or being charged.


Posted: Sun Oct 26, 2008 10:06 am
by nino500

How about something like the ceramic hair dryers mentioned in the Vidal Sasoon heater thread.

That could blow warm or cool air through an insulated battery box to keep battery temp constant when charging or driving.

The insulation should retain reasonable temp for a while when parked up.


Posted: Sun Oct 26, 2008 10:30 am
by geekygrilli
Cheers for the replies chaps

I think budget and time constraints will only allow me to warm whilst charging. I assume the specific heat capacity of lead is quite high, so the batteries will retain their heat for quite some time.

I've done a bit of searching, and maybe something like reptile thermal heaters will do the trick but might not be powerful enough, or under floor heating, and some thermal blankets. I'll try to do some claculations based to 0c ambient, and 25c working temp.

I don't really like the idea of exposed heater elements near the flamable gases.

Posted: Sun Oct 26, 2008 12:17 pm
by mattcarr
the battery warmers in the car only work when the car is plugged in so I can only assume they use the 240 volt supply.
Have you considered using an electric blanket at all? This could be wired so that it is on all the time that the car is plugged in, and it could also double up as insulation for the batteries as well.

Posted: Sun Oct 26, 2008 8:26 pm
by JonSpence
I've come across warming pads on ebay and googled suppliers in the past. If you're having difficulty it's probably because you're looking for battery warming pads. They do exist, but they are less advertised than the same pads used to warm oil sumps.

Here is a link to one supplier.

Note that you might need to fit some form of thermostat. The heaters are expected to be used for normal car batteries which are fairly exposed. Used in a battery box they could overheat your batteries.

Posted: Sun Oct 26, 2008 8:41 pm
by geekygrilli
Wow - thats it exactly.

However, for a fully exposed battery they recommend a 1000W heater and on that assumption I'd need 8kW!

4 of my batteries are in a box in the boot, and the other 4 are sort of exposed in the engine bay.

Think I'm going to order some heat pads this week, and work on some sort of thermal blankets to wrap around the batteries.

Thanks for all your help guys

Posted: Mon Oct 27, 2008 9:06 am
by bobc
I'm interested in any information or links to any information on this subject. I hear repeated claims that you can reach a higher state of charge in a lead acid cell if you charge it HOT (like 70C+) - but I have no hard data. (and it does sound rather risky......)
Anybody know any URLs or research work on this subject?