Re: Emeter Percent Charge Question

[Mike Thompson <mikt@pair.com>]

Try to pick two data points close to your usage and calculate your own
Peukert's for the battery. The manufacturer's typically don't post the
data we want. For an EV since you drain the pack in under an hour of
conservative driving, you want data for
(Full capacity dishcarge - C) / (time in hours = 1) aka. C/1 = C

A modest acceleration will be around 5 times that load / discharge rate:
5 * (C/1) = 5 * C = 5C
This fully discharges the battery in 12 minutes.

A hard accelleration will be around 15 times that... = 15C
This fully discharges the battery in 4 minutes.

And EV Drag Racers? 30-100+ times that!! >>30C !!
That corresponds to draining the battery in two minutes!

So you can see how absurdly far away the C/20 rates discharge rates are
from where we operate EV batteries. Our most gentle rates discharge them
20 times faster! And Aggressive acceleration discharges the battery at
a rate 300 times the C/20 data!

So from what you know about equations, extrapolation and the
eccentricities of batteries, how well do you think the C/20 data allows us
to predict the amount of battery charge used during a 15C hard
accelaration - 300 times the rate of the C/20 manufacturer's data?

Well, even armed with Peukert's equation.... the answer is not very well!

The higher rate data sometimes quoted like the 75A reserve rate is around
C/4 - C/2 discharge rate depending on the capacity of the battery. It's
much closer to the rates we use vs. C/20, but it's still over a factor of
30 away from the data points during accelleration.

What to do?

Well, if could get 75A data with the Ah capacity at 75A AND Peukert's
constant, that would be the best of sometimes available data. Of coure it
would be much better to get this information at C, 5C and 20C but that's
virtually impossible data to get.

If they don't give you the Peukert's constant (exponent) then you have to
calculate it. That requires two data points.

So you use the C/20 data data and the 75A data and calculate the Peukert's
constant. Or whatever two data points provided that are closest to EV
discharge rates. (25A and 75A?)

>From the E-meter manual:
http://www.cruisingequip.com/ftp/e-mtrpdf.pdf

  Making two discharge tests, one at a high discharge rate and one at a
  low rate, that bracket your normal range of operation, allows you to
  calculate an "n" that will describe this varying effect. Your Meter uses
  an "n" equal to 1.25 which is typical for many batteries.

Notice that for EV applications, we never get battery discharge data that
"brackets" our discharge rates during normal operation. I.e. a discharge
rate slightly lower than our typical minimum draw and a discharge rate
slightly higher than typical maximum our discharge rates during normal
operation.

Ideally, EV's would probably use C and 10C data to calculate Peukert's
exponent.

Page 39 of the E-meter manual goes into some detail and tells how to
calculate the Peukert's Ah capacity, etc...

On Sat, 29 Sep 2001, Paul G wrote:

> >Cruising equipment's guidelines are based on the assumption that you will
> >get the information for programming the emeter from the battery
> >manufacturers specs.  Battery manufacturers don't list peukerts capacity so
> >they use the next best thing, C/20 capacity which (for flooded batteries at
> >least) is fairly close.
>
> Actually, as capacity and peukert's exponant increase the C/20 capacity and
> the Peukert's capacity of a battery get further apart. I just checked for a
> Trojan T-125 battery using the C/20 and 75 amp discharge data provided off
> the Trojan web site. The Peukert's capacity is 376 but the C/20 only 235
> amp hours.
>
> Now which one does the E-meter want?
>
> Neon
>
>

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