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A Must Read Subject

PostPosted: Fri Jul 03, 2009 3:45 pm
by Coulomb
When is a battery full?

A battery is full when the voltage is at the absorption setpoint and battery current is at a very low percentage of the battery capacity. For a 100 Amp-hour lead-acid battery at 25 C, a current less than 2 Amps while voltage is about 14.4 Volts indicates that the battery is full. NOTE: Both voltage and current are needed to determine a full battery! Old batteries may never fall this low in current.

If an engine is running just to charge the battery, it is usually a good idea to shut off the engine at a current equal to 10% of the battery capacity, because current acceptance is so slow that lots of fuel will be consumed for little gain in capacity. A longer description can be found here:

How NOT to tell when a battery is full. Use an Amp-hour meter that indicates the battery is minus Amp-hours.

True story from some years back: A woman calls from the Caribbean and she's hysterical. Hurricane season is fast approaching, and they can't leave because our Next Step regulator isn't charging their batteries. It could run for hours and only replace a few Amp-hours. After finally getting her to quit screaming about the dire circumstances I asked her how she knew the batteries weren't charged.

" Because" she said, "the meter reads -536".

"How big is the battery bank?", I asked.

"It's an 8D, 230 ...", her voice trailed off for a second or two and then came back even stronger, "this meter's no good!"

At last she had something right!


There are lots of Amp-hour meters out there that fail to track Amp-hours even remotely close to reality. You can tell whether you have one because it will display Amp-hours consumed preceded with a minus sign.

Re: A Must Read Subject

PostPosted: Sun Jul 04, 2010 10:41 pm
by HansR
Interesting, but my Emon also shows -Ahs when the the batteries are nearly full. No good either?


Re: A Must Read Subject

PostPosted: Mon Jul 05, 2010 3:32 pm
by Coulomb
It shows negative Ahc when the batteries are being overcharged. Amp-hours consumed is a actual quantity that displays as a positive number. Overcharge Amp-hours are imaginary, that is, not stored by the battery. Overcharge is required for a full charge, but too much is not good for the batteries - a negative condition.

Re: A Must Read Subject

PostPosted: Wed Sep 28, 2011 4:09 pm
by CIrons
I am using a loaned Xantrex LinkLITE battery monitor for testing the battery consumption and recovery rates with a new fridge/freezer cabinet I have just built. You have to enter the parameters of the battery and when it is full the monitor flashes FULL until the current starts flowing out.

The monitor shows total battery amps consumed with a minus sign, the amps remaining with a graphic display and as a percentage of the rated amps at 20 hrs at 25ยบ. The amps flowing out of the battery are shown with a minus sign to distinguish them from the current flowing in. I have checked the voltage displayed with my multimeter and it seems very accurate. When 'FULL', the monitor shows set point FLOAT voltage of 13.8, a current of 0.4 amps and a state of charge of 100%. The battery is still quite new in cycle terms although I bought it six years ago!

I have not purchased the Xantrex yet because I am considering the purchase of a new EMON (when available) and a SARv3 when I've got the answers to some questions (forthcoming on this site). Should I buy the LinkLITE at a cost of $350, in view of your comments about battery output being shown as a minus value by a battery monitor?