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Max safe Amp Hr draw for Battery longevity

 
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Jacob



Joined: 26 Sep 2005
Posts: 35
Location: Sebastopol, CA

PostPosted: Tue Oct 25, 2005 4:49 pm    Post subject: Max safe Amp Hr draw for Battery longevity Reply with quote

I've now driven just over 40Ahr on a single drive (yes. all on a single motor, I still can get there). I'm trying to figure out how many Ahrs would be considered 50% and 80% discharge on my new pack. I have 23 group 27 gels in a single string, rated at 86.4Ahr (20hr rate, which seems to be the standard). Do I just simply multiply 86.4 * .5 for 50% or is it more compicated than that?

Thanks for your input

Jacob
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ThomasHudson
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Joined: 26 Sep 2005
Posts: 130
Location: Port Washington WI

PostPosted: Fri Oct 28, 2005 2:31 pm    Post subject: Reply with quote

This is a good question for the battery experts over on the EV Mailing List. The real issues are the Peukert constant for these batteries, and the rate at which you are draining them. As you point out, your batteries are rated at 84.4Ah at the 20hr rate. The 20 hour rate would be something like 4.2 amps. Your truck, however, is drawing much more current than that when driving. When you use more current than 4.2A from your battery, the actual number of amp-hours you can draw from it will fall, in a curve that depends on the battery's peukert number. For an explanation of this effect, see http://www.vonwentzel.net/Battery/00.Glossary/.

The bottom line is, if you can find the peukert number for your batteries and assume, say, 60A average draw, you'll be able to compute the battery capacity at that rate. Then you can compute 80% of that.

Until then, I believe that the EV Listers figure on a battery voltage of ~10V under load to be discharged, so you can use that as a ballpark figure.

I'll go post a question over on the EV List and see if I can get some more info. You're using Deka Dominators, right?

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Jacob



Joined: 26 Sep 2005
Posts: 35
Location: Sebastopol, CA

PostPosted: Fri Oct 28, 2005 11:19 pm    Post subject: Battery DOD Reply with quote

Tom, you wrote, "Until then, I believe that the EV Listers figure on a battery voltage of ~10V under load to be discharged, so you can use that as a ballpark figure. "

So does that mean, with 23 batteries, discharge voltage is 230V? And with full charge being about 320V, and 320-230=90V fullrange, so half that (45V) subtracted from full charge of 320V = 275V for 50% discharge as a rough estimate?

I looked at my Deka chart, and it says the 1 hour rate for Ahr capacity is 57. Does that mean anything related to my question?
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ThomasHudson
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PostPosted: Sat Oct 29, 2005 10:36 am    Post subject: Reply with quote

Yeah, the 57Ah is the C1 rating. You can figure that is pretty close to what I'd consider the 100% capacity, because I figure my average draw is 60A (real close to the 57A draw to get you the C1 rating). That gives you an 80% draw at around 45 Ah.

You might see what your voltage is sagging to under load as you near 45Ah and see if it's in fact sagging to around 230V. Check while you're doing light acceleration.

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Jacob



Joined: 26 Sep 2005
Posts: 35
Location: Sebastopol, CA

PostPosted: Sun Oct 30, 2005 1:26 am    Post subject: Puekert Reply with quote

Tom, David Roden wrote in response to your EV Disc. post regarding Peukert exponent for Deka Dominator sealed gel batteries:


"Several years ago, I came up with 1.32 for both (using Uve's calculator)."

And what is the customary low-end per-battery voltage for 12V lead-acid
batteries under load? 10V?

"I think it's usually 10.5 volts," said David.

So Tom, is this the info you needed for calculating further the DOD question?
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ThomasHudson
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PostPosted: Sun Oct 30, 2005 10:41 am    Post subject: Reply with quote

OK, I found a writeup on the Peukert effect and built an Excel spreadsheet that does the computation.

Using David's numbers, and the following formula:

T = C/(I/(C/R))n X (R/C)

Where:

I = the discharge current
T = the time
C = capacity of the battery
n = Peukert's exponent for that particular battery type
R = the battery hour rating, i.e. 100 hour rating, 20 hour rating, 10 hour rating etc.

If you plug in I=60A, C=100Ah, n=1.32, R=20 you get T=0.75. This equates to a 100% discharge capacity of 45.15Ah, or an 80% level of 36.12Ah.

I'm surprised it drops that much.

I've attached the spreadsheet calculator.



Peukert calculator.xls
 Description:
Excel Spreadsheet -- Computes battery capacity. Plug in your average current draw, Battery capacity, the hour rating for the battery (i.e. 100Ah C/20 rate -> Capacity=20, Hour Rating=20). Set Peukert exponent for your battery type.

Download
 Filename:  Peukert calculator.xls
 Filesize:  15 KB
 Downloaded:  1099 Time(s)


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Tom Hudson
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ThomasHudson
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PostPosted: Mon Oct 31, 2005 2:16 pm    Post subject: Reply with quote

Update -- Jeff says he's come up with a Peukert number of 1.11 for the Group 27 batteries you have, Jacob. Run that through my calculator and you get an 80% capacity of just over 60Ah.

If you want to be conservative though, you might stick somewhere between the 1.32 and 1.11 numbers to play it safe for maximum battery life.

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Jacob



Joined: 26 Sep 2005
Posts: 35
Location: Sebastopol, CA

PostPosted: Mon Oct 31, 2005 8:17 pm    Post subject: amp hrs Reply with quote

I got a bit less when I used your calculator, because the 20hr rate needs to be 86.4 instead of 100 for that battery. When you plug in the exponent of 1.11, you get 51.75 Ahrs for 80% DOD. I can live with that. And then using your other method, you got about 45. So I think we're close now. And considering 50% DOD is probably healthier for the batteries, I think staying in the just below 30Ahr most of the time should be a safe bet, and occasionally going to 45 or 50 might be fine. Am I thinking correctly here?

Thanks for your help and calculator Tom.
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ThomasHudson
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PostPosted: Mon Oct 31, 2005 11:41 pm    Post subject: Reply with quote

Yeah, you're on the right track. Keeping it under 50% for most trips definitely extends battery life.
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