Deep Cycle batteries

[pmrid (Peter)]

I have a couple of 100AH GEL batteries in my caravan and I'm concerned about their health.

I am wondering if anyone knows how to decide when a battery is cactus. I know, of course, that just putting a multimeter across the terminals will give me a voltage and if, after a good charge, it drops below about 12.3 or 12.4 then there is probably a cell on the way out. Are there any other indicators or tests, that a layperson can do?

peter

[GrampianStars (Rob)]

G;day Peter
12.3 / 12.4 volt is 50% flat
charge & rest for 4 hours with no load check voltage again and you will get an indication of capacity by the voltage.
FYI
State of Charge
Gel battery
100% 12.85+
75% 12.65
50% 12.35
25% 12.00
0% 11.80

[pmrid (Peter)]

Thanks Rob.
Peter

[deanm (Dean)]

Rob - you're clearly a 'batteries man' - could you give us an informed idea of how many charge/discharge cycles various types of batteries can sustain?

Cheers!
Dean

[GrampianStars (Rob)]

Hello Dean
Wow how long is a piece of string ?
or how many kilometers per litre ?

Good reading here http://bdbatteries.com/marinegraphs.php

I live rural and the folks just put batteries in with mixed capacities and chemistry with unpredictable results

Rule of thumb Deep Cycle AGM batteries lifespan are generally rated to 50% discharge for 800 charges
i.e 12V 100Ah AGM discharged 50% to 50Ah = 800 charges
i.e 12V 100Ah AGM discharged 25% to 75Ah = 1200 charges
i.e 12V 100Ah AGM discharged 75% to 25Ah = 400 charges

Hope that helps

[deanm (Dean)]

Thanks Rob!

"..how long is a piece of string?"

X cm!

I plotted your useful numbers on a graph & was surprised at complete linearity (no hysteresis or the like).

And I was pleased that the 400-odd charge cycles I get out of my batts (treated & depleted cruelly, especially for the dew heaters) is entirely within expectations.

Thanks!

Dean

[uwahl (Ulrich)]

That is extremely useful information - many thanks!