Stus-List Battery test

Rick Brass rickbrass at
Tue Feb 7 19:53:46 EST 2017



Not to be argumentative, but the Amp Hour rating for a battery is by
definition based on the current the battery will provide that results in it
being fully discharged over 20 hours. For a typical 12 volt group 27 or 31
battery in the 100 AH range, that's about 5 amps of current. For a 4D or 8D
you might be in the in the 10A range. For a set of 4 GC5 golf cart batteries
wired to be a 12v bank at 480AH, maybe 24A.


Deep cycle batteries typically have the AH rating on the label. So you could
take the total of the AH ratings for the bank you want to test (My house
bank happens to be 4x115=460AH when new. ) divide by 20 and get the current
flow that you want to use as to measure your AH capacity. Then discharge the
battery to 50% as you suggest (about 10 hours, give or take a bit) and you
can calculate the AH capacity you have.


Regarding your suggestion to use a 20A discharge rate, by definition the
Reserve Capacity of a battery is the number of minutes that a battery will
deliver 25 amps of current before being fully discharged. That's because a
typical automobile uses about 25A of current to run the ignition, lights,
wipers, and other essential systems (not the air conditioner, GPS, or
stereo). The reserve Capacity is intended to tell you how long you have to
get to the mechanic after you have a failed alternator or a broken
alternator belt. A typical BCI 27 automotive battery with about 100AH rating
would typically have something like 160 minutes reserve capacity.
25Ax2.67hr=66.75AH - far less than the AH rating of the battery because of
Peukert's Law and the impact of discharging at the higher rate. AmpHour
capacity and Reserve Capacity are not the same thing


The real AH capacity of a battery bank decreases over time; with each
discharge/charge cycle of the battery the capacity decreases slightly. I
already indicated in a earlier post that my house bank (460AH when installed
in 2010) is down to about 400AH based on the information from my LinkLite.


Len, you indicated that your boat already has a Link battery monitor. If you
fully charge the battery, "synchronize" the monitor, open the circuit from
the solar panels to take them out of the system, and then wait a few hours
you can get the information needed to calculate the total AH capacity of the
bank when you "synchronized" the meter. You just need the total for amp
hours used and the percentage of capacity remaining to do the arithmetic.


I wonder why you are looking for another method?


Rick Brass

Washington, NC



From: CnC-List [mailto:cnc-list-bounces at] On Behalf Of Della
Barba, Joe via CnC-List
Sent: Tuesday, February 07, 2017 10:15 AM
To: 'cnc-list at' <cnc-list at>
Cc: Della Barba, Joe <Joe.Della.Barba at>
Subject: Re: Stus-List Battery test


Easy cheap way.
If you have accurate volt and amp meters,  a 20 amp (or near enough) load
applied for enough time to drain the batteries 50%*. You should see 12.2 for
wet cells and 12.3 or so for gel/agm.

Light loads like 1-5 amps and heavy loads like 50-100 amps both will be
inaccurate because of Peukert's law. This law deals with the fact that  a
100 AH battery can supply 100 amps for 1 hour or 1 amp for 100 hours in
theory, but in practice 1 amp will last longer than 100 hours and 100 amps
won't make the full hour. 20 amps is a good value for these tests.


* (AH capacity of batteries/load in amps) /2 = time in hours for 50%


From: CnC-List [mailto:cnc-list-bounces at] On Behalf Of Russ &
Melody via CnC-List
Sent: Tuesday, February 07, 2017 00:41
To: cnc-list at <mailto:cnc-list at> 
Cc: Russ & Melody <russmel at <mailto:russmel at> >
Subject: Re: Stus-List Battery test


Hi Len,

If you're 100 percent full and have a coffee in your hand, may I suggest
some advice from Bobby... and words I live by.

Don't warry. Be Happy.

        Cheers, Russ
        Sweet 35 mk-1

At 07:28 PM 06/02/2017, you wrote:

I think the CBA would work well so far from the little reading I have done.
I have four 6 volt deep cycle batteries for my house bank and a Link
monitor. I also have a simple analogue load tester. The problem is the solar
panel masks any shortfall in the bank by fully charging usually before I
make coffee in the morning. The house bank isn't quite as full on a rainy
day and everything works but I would like to know how well. I probably
should just leave it alone but if the bank is 100% charged but at 50% amp
hour capacity I want to know. Len

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