Rejuvenating AGM batteries

Torleif

Hi,
I have a couple of 200 Ah (10h) Vision 12v AGM batteries on my off grid solar system. I am now experiencing that the battery capacity is extremely low (say 5A in 5 hours before the Morningstar MPPT SunSaver shed the load, given 0 charging). After checking around on the net it seems like I have to perform a heavy discharge / charging cycle a couple of times in order to 'rejuvinate' the batteries. The current scheme is; discharge current = 30A until battery is 'empty', Charge with C/10. Any comments on this approach?

Cariboocoot

Re: Rejuvenating AGM batteries

How old are these batteries?

The reason I ask is because 'rejuvinating' batteries is a process that rarely works, even with flooded cells that you can check the electrolyte level and specific gravity on.
With any type of sealed battery you haven't even got that much to work with in terms of determining what is wrong and whether or not you've corrected it.

Discharging a battery until it is 'empty' is also called "killing it to the point where recharging is highly unlikely no matter what".

Most likely; the batteries are scrap metal and all you'll be doing is wasting your time.

Torleif

Re: Rejuvenating AGM batteries

They are 3,5 years old. This is a last resort trial in order to see if I can get some extra ump out of them - before the scrap heap. The current challenge is to get hold of a decent DC load capable of handling the current,,,

Cariboocoot

Re: Rejuvenating AGM batteries

That's not very old. Maybe you should look in to the 'why' of this short lifespan before buying new ones.

Torleif

Re: Rejuvenating AGM batteries

If I knew that:-) The physical explanation is simple enough. The internal resistance in the batteries is way to high given a current over (say) 4 A. Four to five hours with I > 4A will decrease the battery voltage to 12,1 V, and my Morningstar SunSaver MPPT will give a low voltage alarm, and then disconnect (11,5V) after an additional 30 minutes. So the question is 'why the increased internal R?' Sulfat build up? The system is mounted in our cabin in the Norwegian mountains where the amount of sunshine is limited, especially during the winter. I suspect that the amount of energy that the batteries get from a 85W panel is to low in relation to our consumption pattern (typical weekend - 30 to 40 Ah) and this has gradually decreased the actual capacity. I am not sure, but it seems like there is some kind of memory effect in action.

Cariboocoot

Re: Rejuvenating AGM batteries

Lead-acid batteries do not have a "memory effect".

But an 85 Watt panel? Just one? For 200 Amp hours of 12 Volt battery? That panel will only produce somewhere between 4 and 5 Amps. On that size battery that's only a 2.5% maximum charge rate. That would be in good sun. As for the Amp hours of harvest ... you get 4 hours of good sun and maybe you get 20 Amp hours from the panel. You use 30+. Bingo! What do I always say?

(CHORUS OF FORUM MEMBERS): "You need more panels for those batteries!"

What you should have on that battery bank is about (20 * 12) 240 Watts minimum. Or the equivalent of about five of that 85 Watt panel.

Yes, your batteries are sulphated. Pretty much a certainty.

inetdog

Re: Rejuvenating AGM batteries

Cariboocoot wrote: »

Lead-acid batteries do not have a "memory effect".
...

Yes, your batteries are sulphated. Pretty much a certainty.

Two primary visible effects from sulphation:

1. Less active material is available, so the overall battery capacity is decreased.
2. The area of active material exposed is reduced, so the internal resistance of the batteries goes up.

Together this seems to be an exact match for your symptoms.

The only remaining confirmation would be to measure the SG, but that is difficult with an AGM. (I did see one post on a battery rejuvenator forum which suggested carefully removing the AGM vent while wearing full protective gear in case it is under pressure, tearing off a small piece of fiberglass and squeezing a drop of electrolyte onto a piece of glass to check with a refractometer. I am repeating this as an interesting view of how far some people are willing to go, not to suggest that it is in any way safe or even reasonable!)

With AGM batteries, you have to pay closer attention to voltages at rest and under load and using high enough (but temperature sensitive and limited) charging currents, since you do not have the luxury of checking the SG.

westbranch

Re: Rejuvenating AGM batteries

what are the loads? It sounds like you may just be using the 'surface charge' and they have been sulphated from chronic undercharging...

Torleif

Re: Rejuvenating AGM batteries

The loads are LED lights, fan (pc type), radio, TV
We use the cabin in average 6 days/ month
The energy balance (based on observed figures - all in average):
Load: 12h use/ day, off them 8 h with 2A and 4 h with 5A = 36Ah * 6 days = 216 Ah / month
Charge: 10h with 2A for 30 days = 600 Ah/month
Load/charge = 36%

In the winter season we use a generator connected to a Sterling charger to maintain the batteries at top voltage.

I have installed a Victron BMV-600S battery monitor in the system, and the monitor confirms my energy balance

Torleif

Re: Rejuvenating AGM batteries

Hi,
It is a while since I posted this post and thought I should give some feedback to the forum members. I run the deep discharge routine and recharged the batteries one time. To sum up - the procedure worked fine! I built a simple discharging rig consisting of 5x 12v/100W bulbs (car bulbs), some fuses, a amp meter and a breaker. Discharge time around 8 to 10 hours (one battery at a time). I ran the procedure until the bulbs went black. Recharge time around 5 hours (Imax = 60A). The AGU fuse holder melted.... had to change to a blade fuse type. I connected the discharged battery in parallel with the 'healthy' battery in order not to risk a non-rechargeable battery.
I did this last fall, and have now run the batteries through the winter - they have absolutely kept up, even during heavy use that would have rendered them useless after a few hours if it was done before I ran the procedure.

westbranch

Re: Rejuvenating AGM batteries

that is good to hear, congratulations. Keep up the vigilance as it may start to show signs of failure at any time.

BB.

Re: Rejuvenating AGM batteries

I am glad to hear that it worked well for you...

For what it's worth--I would not suggest taking a battery dead where one or more cells are "reverse" biased (voltage of cell(s) reverse polarity). That will usually destroy most rechargeable battery chemistries. Discharging to ~10.5 volts should be low enough for your needs.

-Bill

Torleif

Re: Rejuvenating AGM batteries

For me it was a choice between scrapping the batteries or give it a go - an easy choice. Since this was a rather rapid discharge the battery voltage after the batteries rested without load for a few minutes was higher than 10,5 V. That is before I started the recharging.