Deep cycle battery capacity - how to know?

 One day Just Allan got dressed and committed to text



One method is to measure the volts under load with an accurate voltmeter
such as a digital volt-ohmeter. Most manufactures say that if it drops below
10.5v then deteriation (SP?) starts to occur. Personally I stop it at 11v to
be on the safe side. There should be graphs from the maker somewhere to
judge the remaing capacity. Google for Bill Bowdens battery faq.

unless it's a sealed or gel electrolyte type, use the classical method -
electrolyte SG and temp, and look up a chart or do some simple maths.

You can be sure a battery is fully charged - use a good voltage regulated
charger - some of these even have two or three "program" steps - initial
charge, mild current force for capacity extension, float.  You can leave
your battery on a good charger for as long as you like without damage.  Good
solar chargers also do not overcharge.

Once you know your battery is charged, you can estimate the depth of
discharge - amps x hours.

I don't recommend constant use of a hydrometer, because you lose a little
acid each time you take a reading, and after even a dozen readings, you may
need to top up again.  Unfortunately, you have lost sulphate ions, not just
water, and you would need to top up with acid to replace them.  All too hard
and I have messed up batteries doing this.

If you go to the Yuasa etc websites, you can get datasheets for sealed
types.  For deep cycle, battery manufacturer websites generally offer data
about state of charge vs voltage, providing current drain is small or has
been discontinued for some time.  Deep cycle batteries are generally not
drained rapidly compared to automotive types, and voltage will be useful
even under load.

Roger

Gee Roger - the rest of us squirt the 'acid' back into the cell after taking a
reading - I sincerely hope you do not just put it down the drain etc - that is
not a good thing to do

David

Roger Lascelles wrote:

message

taking a
that is
I reckon you'd only get a couple of readings that way !

As part of my job I sometimes do a hydrometer reading on the same battery
every few days, over a number of charge, settle, discharge cycles, recording
battery characteristics.  I notice that the electrolyte level gradually
drops, eventually requiring top-up.  If you think about it, you always lose
a drop here and there, and you have to wet the hydrometer.  So hydrometer
readings are not much good long term - say every day when you want to check
charge level on a deep cycle installation.

I have noticed that the simple act of adding distilled water to a battery
changes the battery characteristics to the extent that I can't use it any
longer for characteristics measurements, and the battery takes a long time
to recover.

On that basis, I would not want to spoil an expensive deep cycle battery by
juggling with acid  - all too clever - I would rather leave the sulphate
ions inside that battery to begin with.

Roger Lascelles

If it's a good quality deep-discharge battery then the manufacturer will
provide a table giving open-circuit voltage vs. capacity and specific
gravity, with a temperature compensation table.

Cheers.

Ken

A intelligent charge meter would be the hi-tech way to do it. They can
track how much charge goes into the battery and how much comes out,
among other things.
Don't know if you can get them for car batteries though, but chips are
available for LiIon battery packs etc.

Dave :)

One intelligent tester for all types of car batteries is the Cadex
CA-12. It looks and sounds expensive though....
http://www.cadex.com/prod_testers_ca12.asp