[VAC] Re: Measuring battery charge with accurate voltmeter

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Oliver,
 I can't find the chart of battery voltage vs. state-of-charge at the
moment, but in general, it's recommended that you never let the battery
get below 50% state of charge if you want maximum life from it. I've
seen 2 voltages for 50%, 12.1 and 12.3. A fully-charged battery will
read 12.6, if it doesn't, it's junk.

You can tell when a lead-acid battery is fully charged by either
checking with a hydrometer (the readings will quit rising) or watching
the electrolyte in the cells. When ALL CELLS are gassing freely (not
'boiling', just a good steady stream of bubbles), the battery is fully
charged. Leave it sit for 24 hours with nothing causing a drain, then
measure the voltage.

All readings must be taken after AT LEAST several hours of 'rest', no
charging or discharging taking place. A period of 24 hours is best. Note
that because of the very small difference between voltage readings, an
analog meter or the bank of lights in a typical RV is totally useless, a
good DVM is mandatory.

One of the places I checked in trying to find that chart was
http://www.amplepower.com. They have "Killing Batteries" in their
technical section, and I took the liberty of copying it here. They sell
2 books, "Wiring 12 Volts for Ample Power" and "Living on 12 Volts with
Ample Power", full descriptions and ordering info is on their web site.
Both are somewhat general, aimed at not only RVers, but boaters and
owners of remote cabins.

They have extensive info on their web site on batteries, chargers, and
design considerations. Where it says, "ferroresonant charger" in the
list below, read that as "UniVolt".

                                           <<Jim>>
 
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Killing Batteries 

There are many ways to kill batteries ...even very expensive batteries.
Below are a few ways to treat batteries ...NOT! 

     Overcharge the battery by applying a voltage above 13.8 Volts for
extended period 
     Undercharge the battery by never charging it beyond 13.8 Volts. 
     Discharge the battery and leave it that way for a few days or
weeks. 
     Let the battery sit unattended without charging for 3 weeks or
longer. 
     Repeatedly discharge the battery beyond the optimum 50%. 
     Slosh the battery around when it is deeply discharged. 
     Boil enough electrolyte from the battery that the plates are
exposed to air. 
     Periodically add more acid, or unpure water. 
     Use a ferroresonant charger in a liveaboard situation. 
     Sock the battery with a high output alternator that produces more
than 40% of the Ah capacity of the battery. 
     Mount the battery where it regularly gets above  F. 
     Charge it hot until you can't even touch the case anymore. 
     Use a big inverter on a small battery and run the inverter until it
cuts out from low voltage. 
     Freeze the battery in a discharged state. 
     Use a starter battery in a deep cycle application. 

Ample Power products are manufactured by Ample Technology, 2442 NW
Market St., #43, Seattle, WA 98107 - USA 
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