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.

[MEXICOWANDERER]

To bring suit means to PAY for expert technical advice. I qualify. And I guarantee a specialty law firm that would reasearch precedent is going to charge a client a minimum of 1K per day out of court. This is not guesswork. I have more than 20 calendar days spent as expert testimony with Superior Court of Los Angeles County. Filing suit to gain punitive damages is expensive. Filing a class action suit is big-game territory that an attorney not confined to a hospital for the mentenally ill would not touch until he has a briefcase of cash. Research the effects of a cross-complaint.I suggest you reach for your cellular and find this out for yourself.

[NinerBikes]

Profdant139... try this instead.... Wire your battery clamps to the input side of the charge controller from the solar panel, on the back of that portable solar panel. That should give you 17v with sunshine.... but you won't see that at the battery. Monitor the single battery until you see 16.0V at the terminal, on the fully disconnected battery. That should equalize one of the batteries. Swap in and perform the same procedure on the second battery. a 6 amp panel for one day in the sun won't boil off your electrolyte, not this time of year.

[profdant139]

Mex and I differ on the meaning of boondocking. When I spend about 60 days a year in remote country, that is boondocking to me. If I need to spend a year at a time, that is not boondocking. That is survival off the grid. What I do is a hobby, a vacation. Off grid survival is a business. If my life depended on the life of my batteries, I would be as scientific as Mex is.

And Fubeca, I am quite sure that if the BatteryMinder Plus were a hoax, a trial lawyer would have been contacted by a tech-savvy client and would have brought suit. No one has done so. Absence of evidence is not always evidence of absence, but in this case it just might be.

[Fubeca]

Most battery charger users have no idea what is happening with the battery or the charger.

Most just know that after charging the battery works - but they have no clue as to the total capacity of the battery or the specific gravity of any of the cells. How many people actually have the expertise to question whether pulse desulfation has any real impact on the battery?

[MEXICOWANDERER]

THE REASON THE ONE AND ONLY REASON...

That pulse desulfation has never been challenged is

NOBODY CARES

Got it? The FTC used to monitor three-shells-and-a-pea hucksterism. They no longer care unless big headlines are made and everyone's goes "Awwwwwwwww"

Removal of latticed H2SO4 is almost impossible.

Once again claims are made about battery life with absolutely zero data provided about the interpretation of "Boondocking". What is "boondocking"?

It isn't spending 5 days a year without power. Boondocking is when batteries spend many weeks or months CONTINUOUSLY reliant to alternative charging. Read the word CONTINUOUSLY again.

TEN FREAKIN' years of GC220 battery life is normal even when they see THREE MONTHS PER YEAR off-grid. Properly taken care of, good quality, GC220's go 24/7/365 for 5 to 7 years before biting the dust.

L16's last 7-12 years and for example by bank of two-volt cells have been in service since the day's of Monica Lewinsky's clandestine visits.

Even a lowly car jar Trojan group 27 commonly lasts 4-7 years with 90 days boondocking per year.

Notice. No formulas. Pure hard-to-take reality instead.

[BFL13]

profdant139 wrote:
................

As to the voltage when fully charged, this is my third set of marine deep cycle batteries in eleven years of camping. In every case, it reads around 13.5 while charging. When I unhook from the charger, it reads around 12.9. After a day of sitting around, the batteries are at 12.7. This is true when they are new and when they are well-used. Maybe that is not typical of what others are experiencing, but I can tell you that based on a small sample of six batteries, that is what the numbers say.

Back when I had 27s, I did a little test on surface charge decline.

Posted: 11/29/10

Separated batteries and let sit.

Voltages by batteries No 1,2,3 where 1 and 3 are from 5/08 and 2 is from 3/10.

* Was 13ish volts next day after removing charge to 16+ *

so start with day after that by Day number:

1- 12.86, 12.87, 12.89
2- 12.83, 12.82, 12.88
3- 12.80, 12.78, 12.86
4- 12.79, 12.76, 12.85
5- 12.78, 12.74, 12.85
6- 12.78, 12.74, 12.84
7- 12.78, 12.74, 12.84
8- 12.77, 12.73, 12.83
9- 12.77, 12.73, 12.83

[profdant139]

To answer Niner, I used to equalize but no longer do. My WFCO does not go beyond 14 V. My stand-alone auto battery charger won't go up beyond 14 volts. My generator used to go up to 16 V, but the DC output died -- I think there is an internal fuse that I can't get to. Fixing it would cost a bundle. I have been using these batteries for years without equalizing, with no deterioration in performance that I can detect. I keep them fully charged at all times and never go below 12.1 volts.

If the primary purpose of equalization is to avoid sulfur on the plates, then the BatteryMinder Plus (which I use religiously) is supposed to accomplish that. I know that several experts scoff at the manufacturer's claim that the pulse really does the job. But I have faith in the American legal system: if this claim were false, someone would already have sued the pants off the company, and no such litigation has ever been brought. (I have checked on this.)

The other issue is one of convenience vs thrift. I might get better life from my batteries if I put in more work on them. But I have other things to do with my time. So I believe in rational battery maintenance: more than nothing, but not as much as some experts do. If I were running a submarine, my attitude would be very different. This is a conscious and carefully considered decision, not a product of blind ignorance.

[SoundGuy]

profdant139 wrote:
As to the voltage when fully charged, this is my third set of marine deep cycle batteries in eleven years of camping. In every case, it reads around 13.5 while charging.

Which most likely means they've never seen a bulk charge @ 14.4 volts, exactly the problem I and probably every other WFCO owner has even though the WFCOs are rated by the manufacturer to bulk charge @ 14.4 volts. I recently solved this by turning my WFCO off and replacing it with a CTEK charger that DOES bulk charge @ 14.4, confirmed with my DVM. :B

[NinerBikes]

The question remains: How often or frequently do you perform an equalization charge to 16.0V on your batteries?

[profdant139]

Some very interesting observations here. I like the idea of monitoring amp hours continuously -- if I were to do this test often, I would get that little monitor.

I also understand how a low draw of 6 amps would lead to an apparent capacity of more than 110 amp hours, if the rating is derived from a 9 amp draw. The faster the draw, the lower the total capacity, or so I have been told.

But the good news is that in real world usage, I am rarely drawing 6 amps -- usually much, much less. So that must mean that my working capacity while camping is pretty darn good.

As to the voltage when fully charged, this is my third set of marine deep cycle batteries in eleven years of camping. In every case, it reads around 13.5 while charging. When I unhook from the charger, it reads around 12.9. After a day of sitting around, the batteries are at 12.7. This is true when they are new and when they are well-used. Maybe that is not typical of what others are experiencing, but I can tell you that based on a small sample of six batteries, that is what the numbers say.

[grizzzman]

red31 wrote:

grizzzman wrote:

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.

110/20 = 5.5, where does 9ish come from?

Its simpe realy. Care to guess what 110 ÷ by 12 will get ya? Thanks for catching my screw up!

[red31]

grizzzman wrote:

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.

110/20 = 5.5, where does 9ish come from?

[westend]

MrWizard wrote:
My five batteries have a c/8 rate of 135 amp hrs
The Mfg specs don't even list the a c/20 rate
I have five of these in parallel, a total of 675 amp hrs at c/8
My typical load 4 to 20 amps is way below the listed use loads and rates
At pure dawn hours I'm showing voltage from 12.6 to 12.74
76 to 80+ percent SOC

Seems you hit the "sweet spot". 80% SOC should mean a lot of cycling left.

[MrWizard]

My five batteries have a c/8 rate of 135 amp hrs
The Mfg specs don't even list the a c/20 rate
I have five of these in parallel, a total of 675 amp hrs at c/8
My typical load 4 to 20 amps is way below the listed use loads and rates
At pre dawn hours I'm showing voltage from 12.6 to 12.74
76 to 80+ percent SOC

[grizzzman]

BFL13 wrote:
"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.

He is using marine batteries (calcium) with likely 1.265 specific gravity acid (when new) as it is a thick plated starting battery the voltage fully charged would be around 12.7 also due to the plates being more porous the surface charge will go away much faster then a true deep cycle battery will.
specific gravity acid that is higher will have higher voltages.