Stus-List Battery test

Josh Muckley muckleyj at
Tue Feb 7 17:52:35 EST 2017

There is no need to go full discharge.  The voltage and capacity properties
of a lead-acid battery have a near linear relationship so a 50% discharge
is fine for testing.  That should be 11.6v.  All of this is just a
comparison.  A brand new battery might show +/- 10% of the labeled rating.
That doesn't mean that you got a good or bad battery.  It's just an
indication of how difficult it is to accurately measure the capacity.
Typically batteries are only considered at their end of life until they
only test at 50% of original labeled capacity.  Capacity tests really are
most powerful as a trending and comparison measurement rather than a
go/no-go.  I've successfully used 100Ah batteries that measured at only
7Ah.  I know they are junk but keep them around for various workbench
projects.  I've even used them to successfully start my spare marine diesel
sitting in the garage.

Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD

On Feb 7, 2017 5:09 PM, "Michael Brown via CnC-List" <cnc-list at>

> If you have the original battery Reserve Capacity specs it is an
> acceptable quick test
> to see if the battery is good but maybe worn or at end of life.
> If possible test the battery(s) individually, Charge fully, bring battery
> to around 80F and
> time how long it will sustain a 25 amp load before reaching 10.5v. Note
> that takes the
> battery to almost complete discharge and may significantly shorten it's
> life. If the voltage
> gets to 11.0v and the time is less than half the RC spec there is not much
> to be gained
> testing further, the battery is worn out.
> The 10.5v sounds pretty bad but that is an under load voltage and is
> different from an
> at rest voltage.
> Michael Brown
> Windburn
> C&C 30-1
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