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Hi, Charging AGMs (longer life)

1244 Views 0 Replies 1 Participant Last post by  Asim

I'm a bit of an electronics enthusiast. That's not to say I work on stuff, just that I like it a lot. :)

I left my stuff behind when I shifted cities (don't even have a voltmeter) but I've had a solar power project prospect in mind before the summer (probably too late, maybe next year). It could even be fluid mechanics intensive; hopefully my physicist acquaintance could help me. :)

The main motivation for the project is the power ((load shedding) situation in my country Pakistan. I'd like to think I have abundant or even immense concern for people, but I'm probably looking to have some fun and do some good things along the way too. ;)

Now I've got another propect in mind. We bought 2 large AGMs for our UPS and I'm looking to provide the right charging for them.

My flooded batteries died about a week ago and I started researching true deep cycle FLAs and AGMs. I thought 2 AGMs in parallel with 2 FLAs would be ideal: old, or industrial, fast self discharging FLAs mainly handling 1-12 1hr outages (every day) and AGMs in reserve for 4hr maintenance outages (about once every month).

I wrote a long bit describing the dead batteries in the hope of reviving them, but then I read that ""Flooded cells convert 15-20% of the electrical energy into heat instead of potential power [...] but AGMs as little as 4%." (

We got 2, 200 AH AGMs. Our maximum (summer) load would only be around 25 or 30 A, but I thought it would be good to have the 4 hr outage only drain them about 50 %.

I don't know how the batteries should be treated with the low (winter) load of only around 10 A. I read that AGMs are sensitive to gassing voltages, but one website says 50% - 10% DOD gives the same longevity*, while "a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10% [...] because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film."

DOD vs. Cycles for AGMs: vs Life

* 5 batteries discharged 10% --> 5,000 cycles = 5 * 1 battery discharged 50% --> 1,000 * 5 cycles

Is it necessary to ramp up the voltage when the battery has reached 80 %? I'd like to avoid plate corrosion as much as possible (with low load shedding, there can be plenty of hours between outages for charging). And then there is the higher efficiency (before the absorption phase) which is why I bought the AGMs.

Does sulphation occur specifically because of the state of discharge? Will an 80% charged AGM be sulphation free if kept at the minimum float voltage? Regarding stratification, one article said that "As a rule of thumb, one should not extend partial state-of-charge operation beyond approx. 30 cycles, and much less in case of very deep discharges." ( electricity/2008 Energy Unlimited small vessel electrical.pdf)

This raises an important question: are AGMs kept on desulphators (I only read that equalization is only safe for FLAs)?
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