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Old 11-01-2016, 11:52 AM   #1
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Default How to test working capacity of battery: not technical

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.
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Old 11-03-2016, 02:26 PM   #2
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Hi Profdant139,
First, I believe you have done a lot of research and have prompted me to do better... I am adding the following to stimulate discussion...
Now... I don't know if this provides anyone an answer, will help, or just adds BS that everyone already knows...

Like you and many others, I also have 2, 12 vdc, group 31, deep cycle batteries, connected in parallel... thus, supplying 12 volts at twice the amperage rating of a single battery; in my case the reserve amps is 195 amps per battery and in parallel, I should have 390 reserve amps. The article below defines Reserve Amps as "the number of minutes a battery can maintain a useful voltage under a 25 ampere discharge."
In my case that should calculate out to 6.5 hours of discharge at 25 amps... If my math is correct (195x2)/60=6.5... (Ass-U-Ming that each battery still produces 195 reserve amps).

In monitoring flooded cell, lead acid batteries, measuring the specific gravity of the electrolyte (IN EACH CELL) measured directly with a common bulb-type hydrometer with a glass float is more accurate than voltage... please refer to the article for more detail here. HOWEVER, unless one has flooded cell batteries AND can access each individual cell, you will need a different method... e.g., monitoring charge and discharge voltage as pointed out by Profdant139. In my own practice, I simply monitor the voltage charge indicator, but it has never dropped below 1/2... Not very scientific

Courtesy of Frequently Asked Questions (FAQ) - DC Battery Specialists

FAQ List

3) RESERVE CAPACITY
What does the Reserve Capacity rating mean and how does it apply to deep cycle batteries?


ANSWER:
Reserve capacity is the number of minutes a battery can maintain a useful voltage under a 25 ampere discharge. The higher the minute rating, the greater the battery's ability to run lights, pumps, inverters, and electronics for a longer period before recharging is necessary. The 25 Amp. Reserve Capacity Rating is more realistic than Amp-Hour or CCA as a measurement of capacity for deep cycle service. Batteries promoted on their high Cold Cranking Ratings are easy and inexpensive to build. The market is flooded with them, however their Reserve Capacity, Cycle Life (the number of discharges and charges the battery can deliver) and Service life are poor. Reserve Capacity is difficult and costly to engineer into a battery and requires higher quality cell materials....


6) INCREASING CAPACITY THROUGH SERIES AND PARALLEL CONNECTIONS
What is the difference between series battery connections and parallel battery connections and how do they increase battery capacity and voltage?


ANSWER:
In the SERIES CONNECTION, batteries of like voltage and Amp-Hour capacity are connected to increase the Voltage of the bank. The positive terminal of the first battery is connected to the negative terminal of the second battery and so on, until the desired voltage is reached. The final Voltage is the sum of all battery voltages added together while the final Amp-Hours remains unchanged. The bank's Voltage increases while its Amp-Hours, Cranking Performance and Reserve Capacity remain unchanged.

In the PARALLEL CONNECTION, batteries of like voltages and capacities are connected to increase the capacity of the bank. The positive terminals of all batteries are connected together, or to a common conductor, and all negative terminals are connected in the same manner. The final voltage remains unchanged while the capacity of the bank is the sum of the capacities of the individual batteries of this connection. Amp-Hours Cranking Performance and Reserve Capacity increases while Voltage does not.


How can I evaluate the health and charge state of a battery?

ANSWER:
Routine battery examinations divulge irregularities in the charging system as well as in the batteries. The principle method is to examine the electrochemistry of the battery through hydrometric electrolyte inspection. As previously discussed, this important examination cannot be accomplished with sealed absorption or gel batteries. Voltage readings alone require experience to interpret. Hydrometric readings will uncover early warnings of overcharging or overdischarging before batteries are damaged. The state-of-charge and reliability of a lead acid battery can best be determined by the specific gravity of the electrolyte measured directly with a common bulb-type hydrometer with a glass float. We do not recommend the ball float type hydrometer. Specific gravity is a unit of measurement for determining the sulfuric acid content of the electrolyte. The recommended fully charged specific gravity of marine batteries is 1.255 to 1.265 taken at 80°F. More than .025 spread in readings between fully charged cells indicates that the battery may need an equalization charge. If this condition persists, the cell is failing and the battery should be replaced. Since water has a value of 1.000, electrolyte with a specific gravity of 1.260 means it is 1.260 times heavier than pure water while pure concentrated sulfuric acid has a specific gravity of 1.835


Pics Below ---
My Battery label on one Battery --- Series vs. Parallel Battery connections
Attached Thumbnails
Batery Label.jpg   Battery Series vs Parallel.jpg  
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Old 11-03-2016, 02:45 PM   #3
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Quote:
Originally Posted by profdant139 View Post
... 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....
A quick question... Is this supposed to be 6.1 hours in lieu of 61 and if so, will it change your calcs... or just a simple typo???

See, I read the whole thing and think you did a great job and have pushed me to do a better job of monitoring my bats... I just bought a small DC volt meter to install at the wall panel with a temporary push-button switch/or use the one already there!!!
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Old 11-03-2016, 05:38 PM   #4
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Very insightful comments! I think my arithmetic was right, but let's see -- I was drawing 6.1 amps, it ran for ten hours, so I consumed 61 amp/hours. My battery is supposed to hold a total of 110 amp hours. So 61 should have drawn it down by more than half, below 12.1 volts. But it didn't -- after ten hours, the voltage read 12.2 hours. So I have more capacity than I thought I did.

Am I missing something here? That is entirely possible -- when we go out to eat, I have my wife calculate the tip. Math has never been my favorite subject, unfortunately.
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Old 11-05-2016, 07:09 PM   #5
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Quote:
Originally Posted by profdant139 View Post
Very insightful comments! I think my arithmetic was right, but let's see -- I was drawing 6.1 amps, it ran for ten hours, so I consumed 61 amp/hours. My battery is supposed to hold a total of 110 amp hours. So 61 should have drawn it down by more than half, below 12.1 volts. But it didn't -- after ten hours, the voltage read 12.2 hours. So I have more capacity than I thought I did.

Am I missing something here? That is entirely possible -- when we go out to eat, I have my wife calculate the tip. Math has never been my favorite subject, unfortunately.
Hey Profdant139,
In your earlier post, you said that your batteries seemed to run longer than your calcs, did you double the battery amps due to dual batteries?? Whoops, I am assuming again ... I assumed you were running them parallel, but something drags in the back of my old brain that someone in an older post was running the batteries separately...
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Old 11-05-2016, 10:40 PM   #6
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No -- they are not in parallel. Each is rated at 110 amp hours, which means I can only use 55 amp hours (half the total) without recharging -- otherwise I might damage the battery. But it looks like I really have over 60 amp hours, not 55, based on my test.
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Old 11-06-2016, 07:28 PM   #7
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Quote:
Originally Posted by profdant139 View Post
No -- they are not in parallel. Each is rated at 110 amp hours, which means I can only use 55 amp hours (half the total) without recharging -- otherwise I might damage the battery. But it looks like I really have over 60 amp hours, not 55, based on my test.
That's great and will easily give you several days of cold camping if you watch it... My biggest elec hog is the heater and when camping by myself, I have been known to leave the heater off and use the stove to heat the trailer while making breakfast and coffee... I also carry a small elec heater that will keep the trailer warm if the furnace goes haywire, but I have only taken the FF on a couple dry campouts and previously used my 6X10 cargo conversion...

Check the bike rack thread and if you like, I can hang a bike over the area and get some measurements, but that's probably useless on my trailer when the rack needs to go on yours...
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Old 11-06-2016, 07:56 PM   #8
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My furnace draws three amps, so we use it very sparingly -- just at shower time. In very cold weather, we dress like Siberian peasants -- sweaters, thick wooly hats, gloves without fingers, etc. Plus a heavy down comforter at night -- no problem.

As we often say, "After all, it is camping!"
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Old 11-07-2016, 11:54 AM   #9
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my furnace draws three amps, so we use it very sparingly -- just at shower time. In very cold weather, we dress like siberian peasants -- sweaters, thick wooly hats, gloves without fingers, etc. Plus a heavy down comforter at night -- no problem.

As we often say, "after all, it is camping!"

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