Help: 300Ah AGM battery, substantially decreased performanc

Hi, I am a full-time RVer with a 400W solar setup that charges my 300Ah AGM battery bank (2 x 6V 300Ah AGM batteries). I also have a 2000W inverter.

One month ago I left my RV for 4 days at an RV park while I traveled elsewhere. My batteries were near 100% charged, and I turned off all systems in my RV so the draw was less than 2A per hour with solar panels still charging about 800-1,000 Wh per day at about 22-24V (around 40 Ah).

When I returned, the batteries had fallen under 12V and were so weak that I couldn't even turn on the TV powered by my inverter. I immediately plugged them into 30A shore power to charge them up to 100%, but ever since then they have been seemingly unable to hold a full charge.

Where I probably made my biggest mistake: It got down to 25-30 degrees F each night while I was away without the batteries plugged into any shore power or trickle charger.

For example, this weekend I had them plugged into 30A shore power for 48 hours and the batteries read 12.75V after detaching according to my BMV-712 battery monitor. Two hours later they had fallen to 12.5V with very minor usage (about 7Ah draw, 14A total or 5% capacity) and they seem to fall to 12.2-12.3V with just one day of normal use (about 75 Ah). With a 300Ah battery bank that's only 25% draw, so the batteries should be about 12.4-12.5V even if I didn't have solar providing charge throughout the day.

I used to be able to go weeks with the batteries staying above 12.2V thanks to my 400W solar setup and minimal usage, only occasionally plugging into shore power or running our 3500W generator. Now I need to run the generator for 1-2 hours each day to keep the batteries from falling below 12.2V. The decline in performance after my 4 days away is stark.

My question: Based on the above, is there anything I can do to restore these batteries to proper working order? Or am I resigned to frequent generator usage or purchasing new batteries?

Thanks in advance for your advice!

2oldman
Explorer II
Dec 09, 2019
AGMs can be equalized. Perhaps that would help. How old are these guys? AGMs are usually pretty forgiving of what you describe, but I'm not all that familiar with undercharged AGMs in freezing weather. What you describe isn't that cold.

Nomadlyweds wrote:
the draw was less than 2A per hour ...!
It's just 2 amps.
Nomadlyweds
Explorer
Dec 09, 2019
The batteries are just 6 months old so brand spankin new.
jkwilson
Explorer III
Dec 09, 2019
Measure each battery individually. 12.75V is on the low side for a fully charged AGM if that was OCV. For example, the Trojan T105 is 6.42V OCV at full charge so you should measure 12.84V. Most brands are that voltage or higher. My guess is you’ll find one lower than the other.
time2roll
Nomad
Dec 09, 2019
What brand battery? Lifeline has an equalization/recondition charging spec for batteries with reduced performance.

Does the solar get them up to 14.6-14.8 volts every day for a few hours?
BFL13
Explorer II
Dec 09, 2019
You first need to charge them to true full. Then do a "recondition" charge.

To get them to true full, you need a couple days on shore power so your rig's 12v will be from the converter and nothing from the batteries.

Now you disconnect the pair of 6s and charge them with something that brings them to 14.4-14.6 ish volts and holds that voltage for an indefinite time.

Use your monitor or another ammeter to observe the charging amps taper, once they start doing that, until they get down to 0.5a/100AH, or 1.5 amps in your case--if amps go down more than that then let them keep going until they bottom out.

Now they are full. So the overcharge will be to crank the voltage up from 14.4-6ish to 15.x and leave them doing that for a while, observing casing temperature with your IR gun. Temp will rise slowly. If it starts going up faster stop. Otherwise give it a few hours, then stop.

So you are asking, howinheck can I keep the voltage constant to do that when my converter or charger drops to 13.6 all by itself before the battery is fully charged?

A. if you have a PD converter with charge Wizard, just keep hitting the boost button as required to keep it at 14.4.

B. own a charger with manually set adjustable voltage that you control. Example:

https://www.boatandrvaccessories.com/products/powermax-pm3-60lk-12-volts-60-amp-power-converter-battery-charger-w-led-light

Assuming your AGMs are recovered ok, now you have to keep them that way, by getting them to true full as described above, as often as possible. Follow the AGM guide in here even if you have some other brand.

https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
pianotuna
Nomad III
Dec 09, 2019
Hi,

Sorry for your issues. To prevent this happening in the future change to a charge controller that has temperature compensated charging. What charge controller do you have? Does it have adjustable voltage set points?

Basically the jars may have been under charged for the whole time you have owned them--because the charging voltage is too low.

A trickle charger would have made zero difference to this problem--unless it was temperature compensated.

Do follow the advice on the Lifeline page to recondition the existing batteries. You may need to "dance on the needles" to recover the jars.
Nomadlyweds
Explorer
Dec 09, 2019
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.
BFL13
Explorer II
Dec 09, 2019
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.

You normally can't do it with solar, because it gets dark too soon and the voltage falls off.

That is why you normally need a charger that can hold 14.4ish volts indefinitely until the amps get down to 0.5/100AH.
PNW_Steve
Explorer
Dec 09, 2019
I would be thinking sufating.

I am far more familiar with FLA but I do understand that AGM are subject to sulfating just like FLA.

With FLA you want to charge at around 10% of C20. AGMs will handle a higher rate of charge than FLA.

If you don't charge aggressively the plates can sulfate reducing battery capacity. I would look at having at least enough solar to be able to charge at 40 amps. Taking into account the fact that you are unlikely to ever see much over 70% of nameplate ratings from your panels. 800 watts of panels will get you in the ballpark.

Anything less can reduce the lifespan of your batteries significantly.

There is a solar forum with some really sharp mods and members when it comes to solar. Http://www.solarpaneltalk.com

I have found that RV specific knowledge is a bit weak but I have not found a better source of quality solar and battery information. Take a look at the stickies in the battery section. They are quite an education.

Good luck.
PNW_Steve
Explorer
Dec 09, 2019
I would be thinking sufating.

I am far more familiar with FLA but I do understand that AGM are subject to sulfating just like FLA.

With FLA you want to charge at around 10% of C20. AGMs will handle a higher rate of charge than FLA.

If you don't charge aggressively the plates can sulfate reducing battery capacity. I would look at having at least enough solar to be able to charge at 40 amps. Taking into account the fact that you are unlikely to ever see much over 70% of nameplate ratings from your panels. 800 watts of panels will get you in the ballpark.

Anything less can reduce the lifespan of your batteries significantly.

There is a solar forum with some really sharp mods and members when it comes to solar. Http://www.solarpaneltalk.com

I have found that RV specific knowledge is a bit weak but I have not found a better source of quality solar and battery information. Take a look at the stickies in the battery section. They are quite an education.

Good luck.