[Oeva-list] Leaf battery

Myles Twete matwete at comcast.net
Tue May 3 09:43:57 PDT 2011


The Peukert equation was determined empirically and doesn't state the cause.
As such, it includes resistive losses in the batteries as well.  Since
that's the case, I wouldn't conclude that LiIon batteries don't have the
same non-linear Peukert or perhaps a Hammerstrom or Twete effect...in fact,
I have seen at least one reference online indicating a specific Peukert
coefficient for the LiFePo4 batteries coming from China.  The Peukert
equation doesn't distinguish between recoverable vs non-recoverable
effective capacity loss.  Hence you've had many folks point out how you can
draw power at a high rate, then when the voltage is detected as at some
apparent low SOC, switch to lower rate and draw more power out at a lesser
rate until reaching the same low SOC...etc.  Both the internal resistive and
chemical effects can arguably cause this effect.

While any empirically measured Peukert coefficient for LiFePo4 is likely
very low, non-zero internal resistance alone means that loss will be
proportional to current squared, while power delivered will largely be
directly proportional to the current.  That square loss non-linearity with
current seems analogous to the Peukert effect to me.

-Myles

-----Original Message-----
From: oeva-list-bounces at oeva.org [mailto:oeva-list-bounces at oeva.org] On
Behalf Of Dan Hammerstrom
Sent: Monday, May 02, 2011 3:32 PM
To: OEVA
Subject: Re: [Oeva-list] Leaf battery


As someone mentioned, it is probably not Peukert with Li-Ion batteries.  All
batteries have some internal resistance.  Although just a few ohms for a
large pack, if you get enough current going (in or out) you get a pretty
good voltage drop across that resistance, which results in a energy loss (as
heat).  There is then even less voltage to drive the motor, so the draw gets
increased even further.  Add to that increased rolling resistance and
aerodynamic drag (which increase as the square of the speed).

So depending on the speed you are driving, that is, how much current you are
drawing, your batteries will be significantly less efficient.

Also, it is not clear if the Leaf SOC estimation factors battery resistance
into it's calculations.  Most BMS systems do not, since measuring and
updating resistance measurements is complex.  However, the Leaf represents
some pretty sophisticated engineering and may very well do so.

-- Dan




On Apr 29, 2011, at 6:20 PM, Alan Batie wrote:
> I've been looking at the Carwings data for that trip to Portland a couple
weeks ago, and either the estimator is being awfully conservative (which
would make sense except for the reports of people running out of juice
early), or the energy consumption sensor/calculation Carwings is using is
faulty, or the battery is smaller than they say:
> 
> Battery spec: 24kwh
> 
> Carwings report for the return from McMinnville (where it got a 100%
charge that Sunday): 11.7kWh 48.6miles 4.2miles/kWh
> 
> Range estimator/battery gauge: 66% (27 miles remaining)
> 
> kwh capacity derived from usage: 11.7/.66 = ~18kwh
> 
> The 66% is consistent with 27 miles remaining after going 48 miles.
> 
> I'm guessing Peukert at work...
> _______________________________________________
> Oeva-list mailing list
> Oeva-list at oeva.org
> http://www.rdrop.com/mailman/listinfo/oeva-list



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