RE: Battery load testers

["Roger Stockton" <rstockton@delta-q.com>]

> -----Original Message-----
> From: Rob [mailto:westco@jps.net] 
> Sent: May 31, 2002 11:49 AM
> To: ev@listproc.sjsu.edu
> Subject: Battery load testers

> Now I am looking for a battery load tester to monitor the six 
> 12-volt batteries in my Bombardier. I would like to be able 
> to evaluate the batteries, diagnose battery problems and 
> identify weak batteries in the string.  After doing some 
> reading, it appears that the conductance meters made by 
> Midtronics were easier to use and more accurate than a 
> standard load tester (I think).  So I began to check out a 
> PBT-100 ($109) and a Micro 400
> ($349) testers, rather than buy a simple Electronic 
> Specialities Digital 120 AMP Battery Load tester ($45).
> 
> Someone out there must know a lot more about battery load 
> testers.  I would appreciate any advice.

I think it depends what sort of information you hope to get about the
pack.  A traditional carbon pile load tester tells you about the ability
of the battery to sustain a peak load (i.e. measure how many amps the
battery can source when loaded such that its terminal voltage sags to
~9V).

The Midtronics tester (which I believe is what I had the opportunity to
use when pit crewing for Rod Wilde on the Maniac Mazda a few Woodburns
ago) essentially gives you the same information, but does so by
measuring the battery impedance.  When you connect it to the battery it
will advise you if the battery needs to be charged before testing.  If
it decides the battery is sufficiently charged to test, it will proceed
to provide you with an estimate of the battery's cranking amp capability
(don't recall if it is CCA or CA).  I believe you can tell the unit what
your minimum acceptable value is so it will deliver a PASS/FAIL verdict
as well; if not, you must decide if the value represents a healthy,
weak, or defective battery yourself.

I don't believe either of these tells you much about the capacity of the
battery, which may well be the most important thing for an on-road EV.

If your load tester is sufficiently beefy, you could use it to perform a
capacity test by dialing it up to a suitable load current and monitor
how long it takes for the battery voltage to fall to some level.  If you
want to be able to compare the result directly to the manufacturer's
specifications, then you are pushed into performing a relatively deep
discharge, however, if you want to monitor the relative health of the
pack, you could standardise on a shallower discharge at a convenient
rate (e.g. time to 1.90Vpc @ 50A) and keep a journal so that trends over
time would become obvious and you could immediately identify a battery
on its way out based on its differing behaviour in the periodic (e.g.
monthly) test.

Hope this helps,

Roger.