Lead acid battery temperature tests

[bobc]

Having failed to find credible data on this subject I have conducted some discharge tests at the 2hr rate.
If I look at total energy supplied by a battery while being discharged to 9.9V (23C) or 9.5V(50C) I get the forllowing general results:
charge and discharge cold - call this 100%
charging at 50C gives +15% energy c.f. charging at 23C
discharging at 50C gives +35% energy c.f. charging at 23C
charging and discharging at 50C nets 49% more energy output c.f. 23C
Tests were done in this order
1) charge cold discharge cold
2) charge cold, heat up, discharge hot
3) charge hot, discharge hot
4) charge hot, allow to cool, discharge cool
5) charge cool discharge cool again
The battery was an old car battery which has endured several deep discharges. Over the tests outlined above it appeared to lose a further 8% capacity.
I was astonished by how much extra energy is available when the battery is warm.

[ChrisB]

Having failed to find credible data on this subject I have conducted some discharge tests at the 2hr rate.

C2 is pretty harsh on a Pb really especially on a old starter battery and to sub 10v

The battery was an old car battery which has endured several deep discharges. Over the tests outlined above it appeared to lose a further 8% capacity.

Just shows why Starter batteries just arnt up to deep cycle work and why they fail soooo quickly

I was astonished by how much extra energy is available when the battery is warm.

Yep good old fashioned chemical reactions work much better when warmed

Great set of results and pretty much what I would expect, always good to do stuff like this and thanks for posting the results for others to see first hand .

ChrisB

[bobc]

Yes all quite unsuitable - 2hr discharge - lead acid car battery, deep discharge - none choices you would make if they weren't mandated by the "rules of the formula"....
Anyway - I was expecting the hot results to give more volts out, due to reduced equivalent series resistance but was shocked to get so many more amp hours out as well; I guess it's down to the old peukert thing - discharging hot being equivalent to discharging cold much slower. But 50% more juice...wow.... our car will certainly be going faster in next years races thankyou very much!!! (and a load of heating & thermal insulating gear going in)
Incidentally - thermal time constants for a 19kg battery in a cardboard box is 9hrs, for a battery out on the bench it's 2 hours. So now you know!

[MalcolmB]

Very useful results there Bob. I've heard about EV racers preheating their batteries, but didn't realise it could make such a difference. I do know from personal experience that the range of my bike is almost halved at 0ºC

It raises a couple of questions though:
Will frequent heating of batteries shorten their life significantly?
How do you practically preheat batteries above say 30ºC?

[clnbrtltt]

Yes, many thanks for all that information.
We knew temperature was important but obviously it really matters this time of the year!

[GregsGarage]

Very useful results there Bob. I've heard about EV racers preheating their batteries, but didn't realise it could make such a difference. I do know from personal experience that the range of my bike is almost halved at 0ºC

It raises a couple of questions though:
Will frequent heating of batteries shorten their life significantly?
How do you practically preheat batteries above say 30ºC?

The Killacycle's batteries are preheated before a race. I can't remember the figure, read it once and can't find it now but 70ºC sticks in my mind, although maybe it was only 50ºC! As far as effect on life time, well the Killacycle's batteries are supplied and sponsored by A123 so I expect they aren't too concerned about that aspect.

[GregsGarage]

SEC battery has a good chart showing battery capacity vs temp along with a note stating that operating above 20 degrees will shorten the life of the battery. Look on page 7, link is about a 1M download.
http://www.secbattery.com/content/images/articles/Red-Top-2006-11.pdf

[ChrisB]

One thing thats not mentioned or taken into account, is the energy consumed to heat the batteries and how that is off set against the extra capacity that is gained.

Not that it really matters, unless you are concerned about the eco side of things or the price per mile side of things, and its the latter I'd be interested in, no point in putting 3kwh of energy in to heat the battery for only an extra 10miles range say

I must admit I by trying to use my Blingo as soon as it comes off charge as the batteries are naturally warm from the charge

ChrisB

[bobc]

OK energy needed to keep batteries hot is zero if they're well insulated! But keeping them permanently hot apparently increases self discharge & reduces life. But we are talking about a 50% increase in range, so not insignificant!
As I suggested above, the peukert effect gives a very appealing explanation of the phenomenon: if we assume whatever reactions in the battery are 1st order reactions, then their rate will double every 10C rise in temperature (thankyou Mr Boltzman) So a 2hr discharge at 50C would effectively seem like a 16hr discharge at 20C. And mr peukert says you get 40% more amp hours at the 16hr rate than you do at the 2 hr rate. Then - double whammy - the effective series resistance of the cells is reduced at higher temp. netting another 10% power.
I like this explanation - it fits known facts very nicely & answers "where does the extra juice magically come from?". Furthermore, it tells me that a similar power boost is available from new cells as well as old ones!
Frankly this is brilliant for the racer but probably not much use to the average EV driver - though he could put an electric blanket round his cells on the coldest nights, or warm them up if a slightly longer journey were in prospect. I guess not many would permanently heat them.
PS lifetime would be increased because of the reduced depth of discharge in normal use - I suspect this would more than cancel the faster ageing due to higher temp.
PPS I suspect most EVs discharge at the 1 hr rate or even faster. At this rate you will get considerably more than the +50% in my test.

[ChrisB]

OK energy needed to keep batteries hot is zero if they're well insulated! .

Now I'm confused, yeah I know its not hard , but how did you heat them in the first place ??
OK if they are well insulated then they will stay hot but you need to heat them first which unless this is done during charging then surely this is an added cost, and the heat generated while charging isnt that huge ?

ChrisB