# Thread: How would you measure car battery capacity?

1. ## How would you measure car battery capacity?

There are many ways to measure lead acid car battery capacity, but most methods such as open circuit voltage testing are not very precise. My hobby charger can measure capacity at a discharge rate of 2 amps, but I don't want to wait over two days for the battery to discharge. I would like to get an exact capacity measurement. How would you do the measurement that won't take days?

The battery I want to measure is listed as 115 AH.

2. ## Re: How would you measure car battery capacity?

To be honest I wouldn't. Unless you have a deep cycle lead acid battery (generally the marine type) you can do serious damage to a lead acid battery by fully discharging it.

AH on a car battery are not as important as CCA (cold cranking amps) although increases tend to go hand in hand.

3. ## Re: How would you measure car battery capacity?

Actually, it is a deep cycle marine battery. I have three of them and need to know if they are close to being replaced or not for the upcoming season. I was being general with the "car battery" statement.

4. ## Re: How would you measure car battery capacity?

If you go to an auto electrician he should be able to do the CCA (cold cranking amps) test for you and establish the condition of the battery.

I recently bought a 110Ah AGM battery which was delivered cracked after sending a picture to the battery company they sent out a new battery. I decided to see if I could rescue the cracked battery, only one end cell was cracked, I had the crack welded and went to the local battery wholesaler here in the country town where I live, he topped up the acid in that cell and then checked the CCA for me to establish the batteries condition.

Norm

5. ## Re: How would you measure car battery capacity?

car battery capacity can be measured by looking at the voltage sag from float voltage to near depleted voltage by applying a small CC load, [say around 1A] then derate accordingly for higher loads after the discharge curve is plotted. BUT! Doing so in a SLI battery will essentially take away about 1/3 of its life on the first cycle, but if you are measuring this as a general reference only terminating at 11V would be recommended.

I agree that "CCA" doesn't really mean a thing when you are trying to size a battery for a load that is not automotively related. AH measured capacity while CCA measures plate area.

6. Are you trying to measure capacity to test the battery condition ? Or are you trying to figure if the battery will meet your requirements for a specific use ?

7. ## Re: How would you measure car battery capacity?

I want to determine how long my three deep cycle batteries will power my 36V trolling motor. Last season, I was able to use the trolling motor all day without running out of capacity. If I can get a "reserve capacity" measurement, I would be happy. The manufacturer quotes a reserve capacity of 200 minutes at 25 amps. CCA is not what I need.

I have found a battery tester that states it measures conductance, which is a relative measurement of a battery's ability to meet its rated capacity. Can anyone show me how to measure the conductance of a battery? I own a good voltmeter and high precision resistors.

8. ## Re: How would you measure car battery capacity?

What if you bought a \$25 Turnigy watt meter, then hooked up a couple of 100watt automotive headlamp globes and kept an eye on the amp hours used and the voltage. The watt meter goes between the battery and the load and measures Voltage, amp hours, watt hours etc consumed. It's back ordered at HK but I'm sure you could find it for sale elsewhere.

Do not link directly to sales sites - Norm

9. ## Re: How would you measure car battery capacity?

Do you have car 12v to AC Power inverter? Then you could hook that up and try to measure the capacity at a known load over time.

10. ## Re: How would you measure car battery capacity?

too bad you are not near dayton oh.
i have an rct.
load/esr testing can tell you a lot about a battery but not everything.
had a wheelchair battery here that was bad even though it passed an esr test like new.it had very limited capacity.the chairs owner would go a few blocks and it would drop dead.dealer tested the battery and said its good.you must have a bad controller module.\$400 later same thing.
when i held the load test on it would suddenly drop about 2v and slowly climb as it reversed the bum cell.
release and watch it recover with a delayed jump back to normal.

11. Conductance testers are used to determine battery condition.
I would also test the acid with a hydrometer. I have found the old fashioned 'Acid Test' with a hydrometer to be the best to determine condition.
How about putting your motor in a 55 gal barrel and running each battery to it's end with a timer at the speed you will be using on the motor.That would be the absolute test for how long those batteries will be useful.

12. ## Re: How would you measure car battery capacity?

Ok, so your hobby charger only discharges at 2a. What does it charge at? (perhaps 10a or more?)

You could get a good idea of the usable capacity by depleting the battery only down the voltage at which you would normally stop using it. Then, measure how many Mah it takes to charge it back up.

Or...buy a better hobby charger that can discharge at 30a+. Everyone needs an excuse to buy something neat and here's yours. Just be sure not to take it down too far....probably not good to do even with a deep cycle battery. I mean, I wouldn't kill it totally dead if I were you.

13. ## Re: How would you measure car battery capacity?

Originally Posted by Lou Minescence
Conductance testers are used to determine battery condition.
I would also test the acid with a hydrometer. I have found the old fashioned 'Acid Test' with a hydrometer to be the best to determine condition.
How about putting your motor in a 55 gal barrel and running each battery to it's end with a timer at the speed you will be using on the motor.That would be the absolute test for how long those batteries will be useful.
I think this test would give the most useful data. Short of getting a CBAIII and a 1000W 'amplifier' and cycling the battery at either what the trolling motor pulls, or making a ragone plot (which would wear on the battery a lot), testing it with the load you want to use is the best way to figure out how long it will last with the load you want to use.

14. ## Re: How would you measure car battery capacity?

+1 I agree. That would 100% tell you what you really want to know.

15. ## Re: How would you measure car battery capacity?

I use the three batteries in series to power my 36V trolling motor. I can use the motor all day and not run out of juice. My dilemma is how to test my setup without doing hours of discharging. My batteries are seven years old and I want to consider replacing them. I think that I might try to measure the internal resistance of the batteries using a DVM following this thread:

http://www.candlepowerforums.com/vb/...tal-multimeter

and then relate the measurement to a known/new battery. Do you think I am on the right track?

16. ## Re: How would you measure car battery capacity?

why don't you just test them in real world, installed on your boat using your trolling motor, and see how they perform not on paper but under conditions you will be using them. cuz i really doubt you'll see real picture with the ways you've been told here, well at least most of them.

17. ## Re: How would you measure car battery capacity?

Originally Posted by alpg88
why don't you just test them in real world, installed on your boat using your trolling motor, and see how they perform not on paper but under conditions you will be using them. cuz i really doubt you'll see real picture with the ways you've been told here, well at least most of them.
i agree, its the only real way to get the results you want.

18. ## Re: How would you measure car battery capacity?

The next time I get a 12V battery it's going to be LiFePo4 chemistry.

19. A seven year old lead acid battery is a senior citizen by age.
I would recommend that you do not rely only on a conduance test to judge the condition of the batteries. I have tested batteries that pass the test, only to find they are defective.
Test your batteries by specific gravity, conduance, and load. These are all automotive type tests. You should be able to judge battery condition expected performance from the combined results.

20. ## Re: How would you measure car battery capacity?

Hello Aquanaut,

This is an interesting problem.

The ideal solution is to use something like the CBA with an amplifier that allows you to do the test in something like 5 hours. Unfortunately this set up is a little expensive.

A work around may be to use an external load. You could rig up a light bank and get an idea of how much current it draws, then add in your hobby charger to keep track of the process. The external load won't give you constant current so your numbers will be a little off, but it should be close enough for fishing...

If you do this you have to be careful. Your charger will shut off, but the other load won't. You could end up destroying the battery you are testing.

Tom

21. ## Re: How would you measure car battery capacity?

Just take it to Autozone and let them load test it. You may poo poo that idea, but there equipment is better than likely what you are likely to have at home.

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