Stus-List Battery test

Della Barba, Joe Joe.Della.Barba at
Wed Feb 8 08:05:34 EST 2017

We were usually testing really big banks. You are correct a lower rate will be more accurate for smaller batteries.
This is just a very rough test anyway, there are sophisticated conductance meters and meters that keep track of amp-hours used that are better. If you use what should be half the capacity and the battery ends up dead, you know it is beat.
From: CnC-List [mailto:cnc-list-bounces at] On Behalf Of Rick Brass via CnC-List
Sent: Tuesday, February 07, 2017 19:54
To: cnc-list at
Cc: Rick Brass <rickbrass at>
Subject: Re: Stus-List Battery test


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<mailto:cnc-list at>>
Cc: Della Barba, Joe <Joe.Della.Barba at<mailto: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% discharge

From: CnC-List [mailto:cnc-list-bounces at] On Behalf Of Russ & Melody via CnC-List
Sent: Tuesday, February 07, 2017 00:41
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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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