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Just wondering if you can get a rough idea of battery capacity from how long it takes to re-charge a completely discharged battery. For instance if you are using a 6 amp charger and it takes 30 hours to charge a completely discharged 12 volt Lead acid battery, does that mean you have a 180 ah battery? I bought an old Lansing Bagnall platform cart a few years ago and the batteries were dead when I got it. So I took them out and left them in my field ready to go to tha scrap man one day, but never got around to taking them. Anyway, to cut a long story short, after these batteries were sat in the field, in the open, with no lids on, for well over 12 months, I decided to try to get some of them up and running. So I chose 6 cells which happened to be sitting upright and full of rain water. I emptied them and sluiced them out with tap water and EDTA, left them sitting for a couple of days with EDTA and tap water, jammed a hose pipe into the opening and forced water in so hard the plastic cases bulged out like a balloon. I then drained them and filled 3 of them with battery acid (with more EDTA), and plugged a 6 volt charger to them. Wouldnt take a charge so I wired up a good 6 volt battery to them and tried again. Amazingly they stared to charge. So I did the same with the other 3 cells, and then hooked the 6 cells up to a 12 volt charger. They were on charge for days and amazingly got up to 12.7 volts. I was still sceptical of how good they would be so I hooked up a 50 watt 12 volt light to them last friday afternoon, and then promptly forgot all about this project until monday morning. When I went down there again I noticed the extremely dim light still on, turned it off and checked the voltage which was reading 4 volts! They would not take a charge again as the voltage was so low, so I hooked up a full 12 volt battery to them to fool the charger, and am now charging them again with the 6 amp charger which has now been going for about 20 hours. My question is (if you are still reading!) if I check on the cells every hour and can note how long it takes for the battery to achieve a full charge, would this be a good indication of the Ah of the battery how it is now. Originally there were 24 x 2 volt cells, and the battery bank was 480 AH, so each cell would have been 2 volts at 480AH, and the 6 hooked together would be 12v at 480 ah. If the battery started charging from 4 volts, to get the charge up to 12.7 again would be another 9 volts almost, which should be approx 60 hours on a 6 amp charger. (if I am right in my thinking!) If it achieves full charge at say 30 hours, then this would mean the battery is only around 50% efficient and not up to much. Which brings me to my next question. If I have a 480 AH battery that is only achieving say 180 ah at full charge, would this then still be a better battery than a 100 ah brand new battery? it seems to me the more I read up on this stuff the more confusing it is.
Thanks for any help anyway,

Stuart
 

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It has been my experience that simply using "charge time" is a poor indicator of a battery's capacity. For example: I had a battery in my pack that took longer to charge than any other, and at the end of my commute had the lowest voltage.
I have been working with used batteries for over a year, and it easy to see how they can be confusing. Each battery seems to have its own personality! But with proper care, they always give you the right clues as to how they will perform; and it has been my experience that unless you have a shorted cell, most batteries can be succesfully brought back from the dead. But basically, the only real way to measure capacity is using it, not charging it.
 

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Thanks for the reply. Problem is, the 6 x 2 volt cells are only roughly wired together and have no lids, I didnt really want to put any more effort into these cells if they will not be up to much. If I measure the amount of time a 50 watt 12 volt light will run ie if it runs for say 4 hours and the voltage drops from 12.7 to 12.3, could I tell anything from that? Thanks again,
Stuart
 

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Thanks for the reply. Problem is, the 6 x 2 volt cells are only roughly wired together and have no lids, I didnt really want to put any more effort into these cells if they will not be up to much. If I measure the amount of time a 50 watt 12 volt light will run ie if it runs for say 4 hours and the voltage drops from 12.7 to 12.3, could I tell anything from that? Thanks again,
Stuart
Yes, measuring discharge is the way to do it.
 
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