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.

[grizzzman]

profdant139 wrote:

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.

Well the reason is simple. Your 110 AH rated battery is likely rated at the C20 rate. This means that the battery amps pulled were in the 9ish rate. The more amps you pull from a battery the more heat (waste) happens. So the lower the amps pulled, the more amp hours you will get.

[BFL13]

"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 don't understand this part. When I finish charging, the batts are still over 13v and it takes a couple days for the surface charge to get below 13v.

I would be worried if I got the quoted numbers.

[2112]

BFL13 wrote:
When you use an inverter and a 120v appliance such as the 60w bulb he used, the inverter will draw more amps as time goes on as the battery voltage falls. Called "inverter creep" as it tries to maintain output voltage.

DC lamps dim when battery voltage falls so they draw fewer amps as time goes on.

This is true. The battery will see the inverter as a constant power load.

Regardless, without measuring current throughout the entire test he doesn't know what his true Ah depletion is. I'm not saying it invalidates this as being his baseline.

[MrWizard]

this is an excellent situation for using a 'watts up' , Turnigy, or other power meter
that has a watt hr and amphr counter

something like this one, from Hobby King



or this one from Amazon

[BFL13]

When you use an inverter and a 120v appliance such as the 60w bulb he used, the inverter will draw more amps as time goes on as the battery voltage falls. Called "inverter creep" as it tries to maintain output voltage.

DC lamps dim when battery voltage falls so they draw fewer amps as time goes on.

[profdant139]

2112, I did not know that the draw decreases over time -- d'oh! I will have to re-check the draw, to see if this variance messes up my data.

And I am revealing my electrical ignorance here -- this was an incandescent bulb. The filament heats up. The resistance increases, right? So shouldn't that increase the draw, rather than decrease it?

And now I am revealing my level of technological incompetence -- I do not know how to use Excel. When someone sends me an Excel spreadsheet, I can open it and look at it. I may not know what I am looking at, but at least it's open.

[2112]

Did you measure the current periodically during this test?
As the voltage decreases the current will decrease. As the bulb gets hot the resistance will increase, causing the current to decrease as well. I do not believe you were drawing 6.1A throughout the whole 10 hour test. It would have been slightly decreasing with the voltage.

If you have the time, repeat this test taking simultaneous voltage and current readings at a periodic interval noting the time of each data point. Maybe every 1/2 hour or once and hour for the data record rate. From this data you can use Excel to calculate and graph actual depletion vs time.

[SoundGuy]

profdant139 wrote:
SoundGuy, you are exactly right -- and even more so, there is the mysterious Peukert effect to think about, which would more than double my capacity.

But I am one of these belt and suspenders guys -- I use one battery at a time. The other is a spare in case the first one dies.

I wouldn't disagree at all but I didn't see in your original post where you made it clear you use your batteries sequentially, not paralleled together. Now we're on the same page. 🙂

[profdant139]

SoundGuy, you are exactly right -- and even more so, there is the mysterious Peukert effect to think about, which would more than double my capacity.

But I am one of these belt and suspenders guys -- I use one battery at a time. The other is a spare in case the first one dies. If they are in parallel and one dies, I could be left without power. So they are independent. If and when the first one gets too low, I swap them. Due to my solar panel, I may never be forced to swap them again. This last month of boondocking, I swapped them every week, whether they needed it or not.

And red31, I take your point -- the resting periods of about half an hour may have extended the "run time." But this is sort of like the Heisenberg Uncertainty Principle -- the act of measuring something can alter the object being measured. (Didn't see that coming, did we??)

[SoundGuy]

profdant139 wrote:
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.

Each, yes (at the 20 AH rating) ... but paralleled together you'd have twice that, say 200 to 220 AH. A pair of 12 volt G31s paralleled together would typically average similar capacity to a pair of 6 volt GC-2s wired in series - say ~ 220 AH, half of which would be considered usable, say 100 to 110 AH, not 55 AH. What am I missing in what you're saying? :h

[red31]

up for doing it again with no rest between loads?

good baseline for the future ya got there!

[profdant139]

red31, the temp was a pretty constant 70 degrees during both tests. The climate in coastal Southern Calif is usually very dull.

And BFL13, that is an interesting point about using 12.2 as my 50 percent level. I like that idea - it is a cautious approach to battery maintenance.

[korbe]

From one 12v battery nerd to another 12v battery nerd: 🙂 I have been playing with my batteries for quit a few years and here is my situation; I have 2 6-volt batteries that are rated at 225ah. When fully charged, rested and tested, they read 6.37v. In reading from the 12 volt side of life, I learned that a volt reading determined a percentage of state of charge. Now here is an assumption that I know is not correct, but I will use a straight across association of percentage of state of charge with amps used. So for me a 50-percent state of charge means that I have used 225/2 = 112a.

So I tested all my 12 volt things for amp usage to get a general understanding of what uses a lot and what doesn't.

We went dry camping in conserve mode with LED lights and most all of our time was spent out side. Took 2 navy showers per day and it takes 6 days to get our batteries down to 50-percent.

So now, after all that testing and assumptions, I will now check my batteries with my simple volt meter once every couple of days to see if the numbers still make sense. And if so, I go back to enjoying my 6 or 7 days of dry camping.

[red31]

what was the temperature during these tests?

[BFL13]

IMO you should take that as proof your particular batteries have 12.2v as their 50%. You should not stick with that 12.1v idea. Actually some Wet batts do use 12.2. Trojan uses 12.1, AGMs have another.

In any case the "morning voltage" (with nothing much on and before solar kicks in) would be used to decide if you need a recharge that day or can wait till next day. You might well have to recharge at 12.3

You note your voltage rate of decline morning by morning from arrival. Starts at 12.7 but first morning it is (example) 12.5 Next morning it is 12.3 So you say "Hmmm, tomorrow it will be 12.1"

Now you have to decide to either recharge today from 12.3 or if you are due to be away today, how much harm can you do by waiting till it is 12.1? You know it is ok to go below 50% a few times, just not every time. So it is down to how many times have you gone below so far.

Also you can say you are here to have fun so you are going to keep to your planned outing and too bad if the batteries take a small hit. Who is running this place anyway, me or the batteries? 🙂