Peukert's exponent effect on performance - DIY Electric Car Forums

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#1
02-05-2009, 09:32 AM
 m38mike Senior Member Join Date: Dec 2008 Location: Colorado high country Posts: 333
Peukert's exponent effect on performance

What exactly does the Peukert's exponent value have to do with battery performance? I think that from what I've read, that essentially the higher the Peukert's value, the more time it takes to get the full potential engergy from a battery. I think it also means that if the Peukert exponent is high (i.e. 1.4+) then trying to get a high amperage draw from the batteries will greatly shorten the time that you can draw that level of amps from the battery. Does that sound right?

In practical terms that would mean that if I drove at 60mph for 20 minutes, I would be using the same amperage I would need to drive at 30 mph for an hour.

I just discovered that the Deka GC8V batteries in my pack have a Peukert's exponent of 1.55. When I fed the battery characteristics into a performance spreadsheet, the amps available dropped like a rock very quickly. Keeping every thing else the same, but changing the Peukert's exponent to 1.2, I got a much higher amps available value over time.

#2
02-05-2009, 01:46 PM
 bblocher Senior Member Join Date: Jul 2008 Location: Oakdale, CA Posts: 519
Re: Peukert's exponent effect on performance

Yeah that's basic principal to keep in mind. Here is a site that explains the math and gives you the ability to get the peukert's exp for any battery as long as you have enough data points.

Sadly even the best LA batteries will only deliver 60% or so of the rated 20Hr Ah. This really makes them not an ideal selection for EVs since they like to be drained slowly over time and we want to drain them quickly over a short time.

Peukert's principals don't go away with lithium but the number is very low and you get, if I'm not mistaken, high 90's percent of the capacity with short time discharges.
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http://s2kev.blogspot.com
#3
02-05-2009, 02:04 PM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 7,337
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by m38mike In practical terms that would mean that if I drove at 60mph for 20 minutes, I would be using the same amperage I would need to drive at 30 mph for an hour.
Hi mike,

You confuse current (amps or amperage) with charge (amp hours). Your statement is incorrect using amperage, but sounds about right if you meant amp hours. Driving faster requires more power, so more current (amps) from the battery. The higher current draw from the battery means it will deliver less charge (amp hours). And besides that, the higher vehicle speed means more wasted energy lost to aerodynamics. So going fast is a double whammy on the range for battery power vehicles.

Regards,

major

#4
02-07-2009, 07:28 PM
 dtbaker Senior Member Join Date: Jan 2008 Location: santa fe, nm Posts: 4,146
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by m38mike What exactly does the Peukert's exponent value have to do with battery performance?... the Deka GC8V batteries in my pack have a Peukert's exponent of 1.55. ... Keeping every thing else the same, but changing the Peukert's exponent to 1.2, I got a much higher amps available value over time.

hence, some batteries are 'better' than others all else being equal. Either internal construction for the same style may lower resistance, or different designs (FLA verus glass-mat for instance) have pros and cons for peukerts, charge time, discharge rates, voltage sag, etc.

if you KNOW your average drive is extended highway speed versus urban stop and go, you might make different choices on batteries.

D
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Dan
http://www.envirokarma.org/ev - '97 Suzuki Swift
http://www.envirokarma.org/ev2_mx5e - '94 Mazda MX5 Miata
#5
02-08-2009, 07:57 AM
 m38mike Senior Member Join Date: Dec 2008 Location: Colorado high country Posts: 333
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by dtbaker hence, some batteries are 'better' than others all else being equal. Either internal construction for the same style may lower resistance, or different designs (FLA verus glass-mat for instance) have pros and cons for peukerts, charge time, discharge rates, voltage sag, etc. if you KNOW your average drive is extended highway speed versus urban stop and go, you might make different choices on batteries. D
Do you have a recommendation for good highway batteries? It's obvious to me that the GC8V batteries are not good choices for extended highway use.
#6
02-08-2009, 08:11 AM
 dtbaker Senior Member Join Date: Jan 2008 Location: santa fe, nm Posts: 4,146
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by m38mike Do you have a recommendation for good highway batteries? It's obvious to me that the GC8V batteries are not good choices for extended highway use.
well.... I have no actual experience to back me up on this, so take it for what it's worth. That being said... Here are a couple things to think about.

The higher system voltage you can squeeze in, the lower amps will be required, will give less issues with Peukert and voltage sag. So, design in 120, 144, or 156 v system is you can. Except, don't use 12v batteries as people say they die much quicker than 8v batteries.

In looking for 8v batteries, try to find out the Peukerts exp, or 'guess' from the amp-hour ratings they may give. What you are after is what batteries look like they will have the highest 1 hour rate... which the OEMs never publish. I settled on the us battery (interstate) 8vgchcx . These are FLA.

Glass-mat is supposed to have lower resistance, less voltage sag, but lower range and more expensive....

The best choice will eventually be Li.... and there are people giving them a shot, but as far as I can tell the charging and Battery Management Systems (BMS) are not quite ready for off-the-shelf DIY people. The initial investment is very high, and the life cycles are not proven.
__________________
Dan
http://www.envirokarma.org/ev - '97 Suzuki Swift
http://www.envirokarma.org/ev2_mx5e - '94 Mazda MX5 Miata
#7
02-08-2009, 08:22 AM
 Qer Senior Member Join Date: May 2008 Location: Sweden Posts: 1,760
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by dtbaker The best choice will eventually be Li.... and there are people giving them a shot, but as far as I can tell the charging and battery management systems (BMS) are not quite ready for off-the-shelf DIY people. The initial investment is very high, and the life cycles are not proven.
http://www.metricmind.com/ac_honda/main2.htm

This car was converted back in 2001 and uses LiIon-batteries. I don't think he has updated his pages since 2004 or so, but you could always mail him and ask about how the batteries worked out. Of course, LiIon isn't the same as LiFePO4 so it's hard to translate his result to the latter (but then, two different brands of lead acid batteries aren't quite the same as well), but it can still give some kind of indication.

There's also a table over different kinds of rechargable batteries on Wikipedia:

http://en.wikipedia.org/wiki/Recharg...y_technologies

Of course, everything written on Wikipedia isn't immediately to be considered as truth, but if that article is right, LiFePO4 is supposed to handle about twice as many cycles (>2000) as LiIon (~1200) and if they do I'd say it's a bargain despite the initial price.

But yeah, it's easier to go with lead acid, they're much more proven and there's no lack of good chargers etc on the market. But the range will be less and they can't handle as many cycles as well, especially not if you run them down to 80% DoD...
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Swedish Programming Dude for Evnetics, LLC.

Quote:
 Originally Posted by toddshotrods I like the Soliton's hi-tech build and ability to deliver whopping doses of current until someone screams "Uncle!"
#8
02-23-2009, 03:15 PM
 yarross Senior Member Join Date: Jan 2009 Posts: 122
Re: Peukert's exponent effect on performance

Quote:
 Originally Posted by m38mike What exactly does the Peukert's exponent value have to do with battery performance? I think that from what I've read, that essentially the higher the Peukert's value, the more time it takes to get the full potential engergy from a battery. I think it also means that if the Peukert exponent is high (i.e. 1.4+) then trying to get a high amperage draw from the batteries will greatly shorten the time that you can draw that level of amps from the battery. Does that sound right?
Amps describe the rate you remove charge from your battery, but plates see apparent current that is higher than load current. How much higher depends on Peukert's exponent and load current, and can be calculated from Peukert's law.
Main conclusion is that you draw your batts down faster than load current. At high currents that are typical in EV drives you loose about 40-50% of batt charge, and that implies you loose even more energy than that amount (because of voltage sag). This effect does not only decrease range, but also renders system efficiency below 50%, counting outlet-to-wheels.
Apparently, Peukert's exponent depends on current-to-plate-area ratio. High power AGMs should suffer less.

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