Battery size/type

[aandncaravan]

 

 

Reading of other's experiences, and wanting to know if it had been damaged, I decided to test it last Saturday. I connected a 60W headlamp bulb across the terminals to give a 5A load, which was as close to C20 as I could get. 95Ah @ C20 = 19 hours, so I aimed at 9.5 hours for 50% DOD. I checked the voltage hourly during the test with a multi-meter, and in the event had the battery on load for 9 hrs 40 minutes. Commencing voltage, after resting and removal of surface charge, was 12.91V. Final voltage was 12.09, rising to 12.28 after 5 minutes rest, so almost exactly 50% DOD. I suspect that was the deepest discharge the battery had ever experienced!

 

 

Nevertheless, I'm still puzzled that after three years of over-enthusiastic maintenance charging, it hasn't apparently suffered any damage. Can anyone answer that?

 

 

 

Another thread has pointed to this one, which I re-read out of curiosity, and picked up on the above test of an AGM battery.

I noticed the very low discharge voltage that was used to calculate an AGM batteries "50% DOD point".

 

12.28v is way below 50%, it is more like 75% and so a good AGM should have delivered far more than the tests 9.5 hours 'run time'.

 

You can see from the chart below, that for an AGM battery (SEALED/VRLA column), 50% DOD equates to 12.5v.

A discharge down to 12.2v equates to 75% DOD.

 

The test therefore suggests the battery is well down on capacity, by about 25%.

The industry regard a battery as exhausted once it goes below 80% capacity, so this battery isn't in it's prime.

 

A Loss of 25% capacity may not sound like it is past it, but when it is recommended a battery is only discharged down to 50%, a 25% loss of capacity leaves you with just 25% 'working capacity', or about 22Ah capability on a 90Ah battery.

 

 

If anyone carries out a battery test, it is important to know the batteries technology so you apply the correct working voltage range.