How to determine a batteries AH rating

[JohnnyF]

Being new to all this I am getting a lot of good information from the Forum and appreciate everyones participation.

My one question is, how do you know the AH rating of a battery if it is not written on the battery or available from a manufacturer's web site?  If I know its a 12 volt battery, can I make an educated guess from the size?

Where I work I can get anywhere from two to six used 12 volt gel cell batteries a month.  They replace pairs of batteries and only one is usually shot.  The other tests fine so why let the recycle guy get the good ones, right?  At least 1/2 of them don't state on the label the AH.  They only say 12 volt and some warning info.

So I am going to start by running some garage lights and stuff off the batteries and charging them via solar and then add wind in the coming months as time and money allow.

I have 4 - 52AH batteries now and 6 more that I believe are 30AH but I have no way to tell.  They are all gel cell and they are free so I figure I should do something constructive and cost saving with them.

Thanks,

[johnlm]

The actual AH capability of used batteries is likely no longer the original rating.  To get a quick estimate of the original AH rating of your Gel Cells (comparing Gel to Gel) you might try measuring the physical volume (L X W X H) of the known AH rated battery and then measuing the volume of the unknown battery and comparing the two.  IE if the volume of the known 52AH batter was say 500 cubic inches and the volume of the unknown battery was 300 cubic inches then the unknown would be about 300/500 X 52AH= 31.2 AH.

The following is to describe a proceedure to find out the actual AH rating and the numbers are open to correction.

The only sure way to find out what they are now (being old and used) would be to first fully charge a battery and then discharge it at a known discharge current  rate of around 1/10th the estimated AH rate while monitoring the output voltage, and the amount of time until the voltage reaches a certain point.  Im not sure what that voltage point is for Gel Cells maybe around 11.6 V for a 12 V battery.  Some one who knows for sure can correct me on this proceedure.

So if the Battery was 50 AH then discharge it at 5 Amps until it reaches 11.6V and note how many hours it took.  If it took 3 hours then the actual AH capability would be 5 Amps X 3 Hrs = 15 AH.

Hope that helps get you in the ballpark.

Johnlm

[Opera House]

The AH rating is not all that usefull as a result of the Peukert effect. As the current increases, the effective AH rating of the battery decreases.  That can be as low as 40% of the listed AH rating, do a search for charts.  Generally it is specified at a very low current to give the largest number.  As said before, it is best to test the battery at a current that represents its expectd use.