Does 12.5V remaining mean I only used 10% of charge?

Barnacle Bill posted this chart awhile back on a prior battery question I posted. Today I fished for 5 hours using my 40-lb thrust motor, spending about half of that trolling on low and the other half using the trolling motor to make a lot of small changes to boat position. In addition I probably used the trolling motor on medium for about 1 mile getting to/from my fishing grounds.

I expected the trolling motor to be more depleted at the end of the trip, but I checked it with a voltmeter a half hour after leaving the water and the steady state voltage was 12.5v. Does that mean I really only used 10% of the charge? The battery had a full charge before leaving. I charge it when I get home, then give it a top-off or trickle charge if it has been more than a few days.

http://www.crappie.com/crappie/attac...voltchart1-gif

Yes it means you only used 10% of the Battery give or take a little .

The charge level will go back up while it is resting, so it came up to the 12.5V you saw. To get a true reading the battery has to be under load.....in other words, take the reading while the trolling motor is pulling the boat through the water.

It depends on a lot of things. The main consideration is the particular battery. That is a general chart, but for some batteries it is wildly off. For example, an Optima at 12.5V is discharged 35%. Different battery technologies and different manufacturers have different design voltages.

When estimating state of charge from voltage, you need to use the Open Circuit Voltage. That is the voltage at the terminals when the battery hasn't had a load on it for an hour or so.

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Originally Posted by CatFan

When estimating state of charge from voltage, you need to use the Open Circuit Voltage. That is the voltage at the terminals when the battery hasn't had a load on it for an hour or so.

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That is how I did it. The battery in question is an Walmart 27DC, 1 year old.