How to test working capacity of battery: not technical

[profdant139]

(If you are a highly sophisticated technical battery expert, I advise you to stop reading. This post is designed for people like me, who are not experts.)

I have two group 31 12V marine deep cycle batteries, which supposedly have a total capacity of 110 amp hours. In the real world, that means that they have a theoretical working capacity of only 55 amp hours, since I am told that it is not good to draw the battery down below a 50 percent state of charge. When the measured voltage gets down to 12.1, that's a 50 percent state of charge.

So I wanted to see if I really have 55 amp hours available. That information is useful for a couple of different reasons: first, if I do have that much power available, that tells me that my usual battery maintenance routines are adequate. If I don't, I have to do something different. And this reading will provide me with a baseline, so that I can tell when the batteries are starting to get old. Finally, if there is a material difference between my two batteries, that would be very interesting (and disturbing), since they were purchased at exactly the same time and have been used in exactly the same way.

(As you'll see below, the results were not what I was expecting. Hint: this story turns out well.)

My overall plan was to hook up a lamp and then to see how long it took to draw the battery down to roughly 12.1.

I started with a fully charged battery (which reads 12.9 V when it comes off the charger) and then let it rest for a day, so that the initial reading was 12.7 volts.

I then hooked up a 60 watt incandescent bulb, plugged into a small inverter, which was plugged into a "cigarette lighter socket" adapter, which has alligator clips that go to the battery terminals. (If you don't have one of those adapters, they are really handy when you want to hook a 12 V appliance directly to a battery.) I then used my multimeter to find out how much current the bulb and the inverter were drawing, which was 6.1 amps.

(If you already know how to measure the amount of current that a device is using, skip this paragraph. Personally, I can never remember how to do this, so I have to reinvent the wheel every time.) Put the red (positive) multimeter plug into the "10 amp" socket on the front of the multimeter. Turn the multimeter on to the 10 amp setting. It should read "zero." With the lamp still plugged into the adapter, unhook the cigarette lighter adapter's black alligator clip from the negative terminal of the battery. Touch the multimeter's black lead to the negative terminal. Touch the multimeter's red lead to the unhooked black alligator clip on the cigarette lighter adapter. The meter will display the amperage.

Anyway, I left the light on for two hours, thus consuming 12.2 amp hours. I unplugged the light and let the battery rest before taking a reading. I was told that it had to rest for two hours to settle down. But with careful measurement, I discovered that after a half hour of rest, the voltage had plateaued and did not continue to change. (Maybe that's not true of all batteries, but it certainly was true of mine.) So for the rest of the experiment, I let the light run for two hours, followed by a half hour of rest, at which point I measured the voltage and then plugged the light in again.

Here is a table of the results:

Run-time Voltage

2 hr 12.6

4hr 12.5

6 hr 12.4

8 hr 12.3

10 hr 12.2

So this means that after ten hours of actual run-time (consuming 61 amp hours), the battery got down to 12.2 volts, i.e., with 60 percent of capacity still remaining. It looks like I could have gone two more hours to get to 12.1, for a total working capacity of 67 amp hours.

That is a lot better than the 55 amp hours of working capacity than I was expecting!

I did this experiment twice, once with each battery, and got exactly the same results. This tells me that this wasn't a fluke.

I am not sure how it is possible that my batteries are outperforming their rated capacity, but I'm not complaining. This won't change my consumption patterns when we are camping – we are very careful about electricity. But this is encouraging news, and it gives me a baseline for subsequent comparisons.

[landyacht318]

If AH consumed to ~50% and measuring voltage were an accurate method of determining capacity, my 90AH rated AGM battery would actually be around 115 to 120AH capacity.

Seems the AGMs do well down to 12.2v and then quickly fall after that.

Either way it looks like you batteries are still healthy.

The real test would be comparing results after 50+ more cycles and using this same test, and if one were to have data from when the batteries were new as well