Help: 300Ah AGM battery, substantially decreased performanc

Your AGMS are basically sulfated from never reaching a true full state of charge. It does not matter is the battery monitor says they get full daily, battery monitors are often wrong as they need to be programmed correctly and reset regularly when the bank is known to be chock full.

HOw much of the capacity is recoverable with an extended time held at absorption voltage, and then perhaps some voltage over that?

Totally Unknown. You will have to try it.

But, you need a charging source which can indeed hold them at 14.4 to 14.8v for as long as it takes to get amps to taper to 0.5% of capacity.

Can your converter do this? Which converter is it? If it is a Wfco it certainly cannot. Few converters actually can and asking a rarely above 13.6v to recover sulfated batteries is one definition of
ignorant insanity.

Amps might not even taper to 0.5% of capacity at absorption voltage, but stop somewhere above that and start rising again.

It's my opinion AGMS make for poor batteries when Solar is to be the main recharging source. I feel AGMS, when they are at their most depleted, need to be hit with no less than 20amps per 100Ah of capacity and preferably 33amps, at least every few cycles and high$$ batteries like Lifeline Northstar and Odyssey, more initial amperage is always better.

My Northstar AGM-27, when its voltage retainment was showing signs of not being as high as I was used to seeing during discharge, and this was usually after many days of low and slow solar only charges, the trick to restore it was to drag it to well below 12.0v under load and then instantly hit it with 40 or 65 amps( a 90Ah battery). It was like a smack across its face, and it always responded.

Ultimately I got over 1300 deep cycles from this battery in my rig used for house loads and engine starting. Only recently did I replace it with another Northstar, but a group 31 this time, as the 27 got too weak to start my engine reliably. However its voltage retainment under lighter loads is still impressive. And it is now a workshop battery.

Odyssey AGM, (which is very similar to the thin plate pure lead Northstar I've gotten such great service from) has a reconditioning procedure different than that of Lifeline. They basically want the battery to be discharged to 10v under a significant load, then no less than 40 amps per 100Ah of capacity applied until voltage hits 14.7v, never above 15.0v, and constant voltage limited @14.7v(77f) until amps taper to very low levels, lather rinse and repeat.

Lifeline's procedure is after a regular 'full' charge, to bring them to 15.5v for 8 hours.

Will either of these procedures desulfate your young chronically undercharged AGMS back to usefulness? Perhaps, perhaps not.

The steps you take and the equipment needed to do a proper reconditioning by either the lifeline or Odyssey method, will help you to keep future batteries from degrading so quickly.
Ultimately AGMS are not going to be happy deep cycling and never getting a larger charge rate. Solar alone is not enough unless the cycles are shallow and the charge times at absorption extended.

A say take them down to a resting mid 11's with lots of high loads and apply no less than 60 amps, and preferably 90 or more, and be able to achieve this amperage blast at the battery's most depleted state, at least once every 14 deep cycles.

https://www.odysseybattery.com/documents/ODYSSEY_Battery_Reconditioning_Charge_Procedure.pdf

https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101F-Lifeline-Technical-Manual-Final-5-06-19.pdf

AGM'S are not the super duper magically resistant to abuse batteries that their price seems to indicate to most of the general public. If anything they are more finicky in regards to chronic undercharging and to too little initial charge current when they are cycled deeply.

HItting your AGMS with 90+ amps when they are well depleted will be a challenge, but BFL-13 will link you to the powermax adjustable 100 amp unit which can work off of a 15 amp household outlet, and this charger/converter, can in the future keep your batteries healthy by not only feeding them high amps when well depelted, but by holding them at absorption voltage as long as is required for amps to taper to 0.5% of capacity.

If you are not able to muster the high amps, or the ability to hold absorption voltages for longer on a regular basis then you will have to accept poor AGM battery longevity.

Don't be one of the many who think a long slow 'trickle' charge is what always works to set batteries right, AGMS are tickled to death with too low of charging currents, and can easily accept huge charging currents, just limit their voltage to manufacturer specs and DO compensate for battery temperature, especially when high rate recharging.

I hope you can recover their capacity, but you will likely need new equipment in order to do so, otherwise you will just waste time and effort.

What did you leave on while gone,
Residential fridge?
Heater/furnace set to high, something sucked all those ampHrs from the batteries,
Yes properly charged batteries would have faired better, but something used that power,
Solar was not enough to resupply the batteries

What make of battery?

Does the maker have a charging profile?

AGM do need a "wake up" charge with higher voltage. Unless you can disconnect the fridge (disconnect, not just turn off), don't go above 15.4 volts. Best to totally isolate the battery bank when doing the reconditioning, then higher voltage can be used--but follow the battery maker's recommendations.

Nomadlyweds wrote:
Thanks to all for the suggestions -- seems like conditioning is the right answer, I'll give that a go.

I do have a Victron charge controller with temperature compensation.

MrWizard wrote:
What did you leave on while gone,
Residential fridge?
Heater/furnace set to high, something sucked all those ampHrs from the batteries,
Yes properly charged batteries would have faired better, but something used that power,
Solar was not enough to resupply the batteries

Leaving the inverter turned on with no load will drain the battery as will the refrigerator on propane. The batteries will also self discharge if left for a while.

I disconnect my batteries if they are going to be idle for long. I have a "battery tender" coming to keep my brand new batteries healthy through the Winter.