Caring for AGMs, Absorbion , ending amps.

zoneblue

As mentioned weve ended up with 2volt agm cells for our off grid install. This was partly naivety on my part, not realising this isnt really recommended, partly that they do happen to fit our short to mid term circumstances, and partly we were lucky enough to get a good deal on them. So if we kill them its not all bad.

But anyway i do have concerns about the way that i understand even the best charge controllers handle the absorption stage of the charge process. As i understand it you can choose ending amps or timer or both. In an ideal world where the sun shines 24 hours a day and the batterys have no load whatsoever whilst charging, ending amps will work nicely. However from what ive read, controllers can only make the best of bad situation when the sun comes and goes, when loads kick in. Not knowing precisely how the controller (Classic 150) works, do you have any tips as to how best set the classic to best handle absorb for AGM batterys?

My web search turned up this extensive piece from the marine world where AGMs rule the sea, (for now):
http://www.morganscloud.com/category/electrical/el-indepth-agm-battery-test/

His two main conclusions are to use two banks and charge one at a time to avoid loads confusing the charger, and that equalisation is as important for AGMs as for any other lead acid battery, providing that you know that the battery will tolerate a certain amount of eq. The logic there is that the battery will usually die of sulphation well prior to water loss.

I cant see any easy way to do the former, and as the documentation for the batterys is really poor, theres little hope of getting any comfort on the latter.

Obviously if the controller had access to battery load information from a built in shunt, or an input for a battery shunt, then it could use this to better inform the ending amps. I also got the sense from something Bob at Midnite said that they are working on a standalone amp counting battery monitor, as opposed to the version they already have which works on voltage only. Im guessing (hoping) that the plan there is to network the monitor with the classic? Due sometime in 2013. If so, great, problem solved.

Lastly, is the battery temp sensor also used to inform ending amps, or is that just for temp compensating the charge voltages.?

So many questions! Like i say did research this but I hope these arent FAQs, if so dont hesitate to point me in the direction of existing answers.

Cheers.

westbranch

Re: Caring for AGMs, Absorbion , ending amps.

her is some AGM info that may help, takes a few reads to digest.

http://www.cdtechno.com/pdf/ref/41_2128_1199.pdf

mtdoc

Re: Caring for AGMs, Absorbion , ending amps.

Well, I think having a shunt based battery monitor that is correctly calibrated is essential with AGMs since checking SG is not an option.

Terminating absorb based on the correct ending amps is ideal. As of now the Midnite CCs do not have the ability to end absorb based on a shunt-based measurement of end amps - but I'll bet that is coming soon.

zoneblue wrote: »

My web search turned up this extensive piece from the marine world where AGMs rule the sea, (for now):
http://www.morganscloud.com/category/electrical/el-indepth-agm-battery-test/

His two main conclusions are to use two banks and charge one at a time to avoid loads confusing the charger, and that equalisation is as important for AGMs as for any other lead acid battery, providing that you know that the battery will tolerate a certain amount of eq. The logic there is that the battery will usually die of sulphation well prior to water loss.

I read this and it seems he was chronically undercharging his batteries - admitting to not charging them above 85% SOC, never finishing absorb because he did not want to run his generator that long. It doesn't seem surprising they didn't last long.

Equalizing AGMs? well maybe if your particular AGMs manufacturer recommends it (the ones whose manuals I've looked at don't). Quality battery manufacturer's should have some specific recommendations about charging. If they don't recommend equalizing, I wouldn't do it except as a last ditch effort to revive dying batteries.

I guess in his case since he was chronically undercharging and sulphating his batteries then attempting to equalize may be worth the risk of popping the valves (or even worse, thermal runaway...) But really I think he is just trying to find a way around proper charging.

In a solar or other RE system if your system is balanced correctly then you should be able to regularly get through absorb and what I've been told is that sulphation with AGMs in that case is not a concern.

westbranch - Nice link - thanks!