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Charging AGM batteries - a question.

1765 Views 1 Reply 2 Participants Last post by  DIYguy
I'm using ex-UPS AGM batteries because they're cheap (6x12v = 72).

My old charger was made for floodies and never switched to float with the AGMs, I thought there was something wrong with it. I had been keeping an eye on it and turning it off after about 4 hours, all went well until the inevitable, I forgot. The batteries virtually turned to jelly after being left on overnight. An interesting smell too!

Now I have another set of AGMs, and I got another charger too, however it's one for flooded lead acids again. I've tried it and like the old charger, after a while the voltages climb too high to be safe for AGMs (16v each when they only want 14.7v). And again it didn't switch to float. Admittedly I only left if for four hours, but by then the voltages were unsafe for the AGMs and they started to get hot.

Kelly HWC4B 72V/15A

So here's the question(s) -

1. Will my AGMs never tell the charger it's time to switch off? (just out of interest)

2. Is there a clever way I can fool the charger to avoid overcooking the batteries?

The Kelly has 3 lights to indicate state of charge, when light 2 goes on the voltage is perfect of the AGMs, it would be nice if it went to float then.

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AGM batteries require a different charge algorithm than you have discovered. You need to get the correct algorithm for your charger or one that has a profile for AGM batteries. This charge curve will not only charge your batts to the correct voltage it will increase the life expectancy.

Another thing you need to do is to have active balancing. If your batts drift out of balance (and they will) you will invariably cook one or more of them...probably one, then another etc. Floodies balance by boiling the ones that hit top first and waiting for the others to catch up. You just add water to keep up. If you overcharge AGM's and they boil...then they vent and you never get this back. Life is shortened dramatically. It doesn't take long.

Incidently, I have AGM's and am upgrading to lithium this winter. I will have some balance units available soon. If you're interested, PM me.
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