AGM batteries don't seem to be holding charge

[Rutherama]

I'm a total newbie and do not understand more than just the basics of how batteries, solar and inverters work. All the info I can find is technical or
too mathematical etc. I am hoping someone can help me understand my issue in simple terms!
We bought a professionally converted skoolie. We have 2 X 12v/200ah/10h AGM batteries, 4x 100w solar panels wired to a rover mppt controller, a renogy pure sine wave inverter and a remote for the invertor.
We picked up our bus in July and drove around for about 5 weeks rarely plugging in (30amp) as we prefer boondocking. We don't use much power - just to charge laptops and phones. Then we hit forests and our battery ran down and the invertor alarm sounded We plugged in but the renogy settings were wrong and it wouldn't accept the 30v. Eventually we got it sorted but it went down to 8.5v on the renogy remote display. I didn't check what it said on the MPPT.
We charged up and all seemed fine. The RV repairer told us that it would have done no damage to the batteries. We noticed that it started dropping in volts much more quickly though and we feel as if our batteries are not holding power. They go up to 14.8 when being charged with a cable and are at 13.3v after float They drop down to 12.8v quickly after we unplug. After a day in the sun and 90 mins driving they were down to 12.6. By morning 12.3. The MPPT says that is 60%. The only draw is the invertor itself and the compost toilet fan overnight. Is this normal?
We wanted a system that allowed us to to off grid for as long as possible but this only now lets us stay away from power for about three days. Help

[PNW_Steve]

What loads do you have the inverter running?

[Rutherama]

How do I tell? Do I take the reading from the MPPT controller?

[john61ct]

You need to get better acquainted with your system and basics of DC electrics.

Laptops can use a lot of Ah per 24hrs depending on specs and what you're doing.

I bet you have other loads too, the compost toilet fan as you stated, also in the galley, lighting, ventilation, pumps?

Your MPPT SC has no clue about bank SoC%

Get an Ah-counting battery monitor, and get to know how many Ah you use in total each 24hrs.

This must be kept lower than your daily energy inputs, whether alternator, solar or a portable genny.

Called an energy budget, worksheets available online.

Sitting below 100% SoC for long does damage batteries, but it may not be obvious, just drastically reduced lifespan.

A bank that could last 6-8years coddled can be murdered in a few months if abused.

The charge specs, definition of 100% Full from your battery maker is critical, please get and post that first.

You need a decent ammeter and DMM, ideally known accurate voltage capable of being calibrated, and to learn how to use them.

Try to avoid using the inverter, phones and other devices with batteries should be run directly off DC. Inverter should only be turned on when actually needed, ideally with charge sources active.

And obviously getting as much solar as will fit on the roof, maybe a portable gennie.

Don't replace your bank yet until you know how to care for them, unless they're completely shot, or you'll just do the same again to the new one.

Very few RV industry professionals know much about this stuff for an off-grid living use case,

put in a bit of reading, googling, asking smart questions and you will soon know more than 99% of them.

DC Electrics 101 learning thread http://www.cheaprvliving.com/forums/....php?tid=28197

[Rutherama]

Thanks! We are going to buy a battery monitor. I assumed the inverter display or mppt would have the info we needed, but renogy confirmed they do not.
We have also bought a multimeter to test them full and after a night with no load. Renogy said this was the best way to see it they are faulty. They show full on the inverter at 14.7 so we'll also find out if that is correct.
I'm also going to turn the inverter off when we are not plugged into ac. All it does is power the toilet fan and the ignition on the propane gas oven. I still don't know see how these are draining the batteries so quickly but I will find out and will make myself an expert on the system for sure!

[john61ct]

Toilet fan should be converted to 12V, very easy.

And of course sparks too, no need for AC for that

[Dem0072]

Everything John said. Plus you might want to look into a battery balancer, especially if you're running lead acid and not a sophisticated bank of cells that were beautifully balanced before install. They'll fight each other for voltage to equalize, which is terrible under load. A good balancer and charge controller with a monitor goes a long ways to protecting them.

Other thoughts: If you have it to invest, LiFePo4 batteries save a lot of headache.

If you hit the sticks you're probably getting a fraction of the charge from impeded sun exposure, and its causing the system to struggle to top off the batteries, so they're running undercharged.

Bear in mind too, with lead acid you only get to use about 20%-30% of the amp hours you're rated at before you start abusing and damaging them.

[Ronnie]

The OP has AGM batteries, I thought one of the reasons to use them is the ability to discharge more without harm? 50% is the value I have heard many times on this forum for a regular lead acid battery for maximum discharge. Can an AGM go more safely? By safely I mean without harm to the battery.

[john61ct]

Avg DoD vs lifetime cycles is a continuous slope, there is no magic number. The reasonable guideline is try to stay most of the time above 50%, only go a bit lower when you need to.

Shallower than that will give longer life, but at greater weight/space/cost.

AGM does not do better, in general poor value per Ah per year than FLA and fussier to care for.

Getting back to 100% Full as per endAmps as quickly and frequently as possible is also critical to longevity.

Ideally a bank has just one string of 2V cells each sized for the total Ah needed.

Paralleling strings does indeed cause balance issues, but 3 or fewer should be manageable. Regular equalizing should help another FLA advantage, Lifeline as well. Balancing gear is rare for lead.

[TheHubbardBus]

Originally Posted by Rutherama

They go up to 14.8 when being charged with a cable and are at 13.3v after float They drop down to 12.8v quickly after we unplug. After a day in the sun and 90 mins driving they were down to 12.6.

Depending on temp, anywhere from 12.6 to 12.8 is a fully charged battery. 14.8 is a healthy charging voltage, and 13.3 is exactly what you'd expect to see right after pulling them off a charger (they'll drop down to the ~12.7 shortly after). So there's no problem with any of that.

By morning 12.3. The MPPT says that is 60%. The only draw is the invertor itself and the compost toilet fan overnight. Is this normal?

To help you understand why 12.3v is 60%, 1) a 12v battery is considered discharged at ~11.5V, and 2) The discharge curve it not linear. Whether or not this is normal in terms of draw vs capacity I can't say w/o digging into the specs of the inverter & loads. But inverters can draw A LOT in standby, particularly if they're oversized for the system, and small, constant loads (like a toilet fan that runs 24/7 if it's AC) can keep them from going into sleep mode - if they have one - which means they're constantly burning energy just to exist. In small systems, especially if they're poorly selected, the inverter can commonly be THE most energy-consumptive appliance. What is the exact model of inverter that you have? What is its rated wattage?

The toilet fan should be DC, which would (ideally) bypass the inverter & be run directly off the batteries. Is this the case, or is it being run off AC via a wall-wort? If the latter, you should change that. Once again to keep allow the inverter to sleep at times when that or other DC-capable loads are the only thing being drawn.

If you don't already have them, a USB port or two, once again DC-powered, would be a good idea for charging phones. It's highly inefficient to convert from DC (batteries) to AC via the inverter and then back to DC just to charge a phone ;) Maybe you already have this?

[john61ct]

Inverters should be sized no larger than needed for the load powered, and turned off when not actually being used.

A bunch of small ones often is both cheaper and more efficient.

Best of course to avoid using them as much as possible, I consider them in most cases an unnecessary luxury.

For living off-grid / mostly solar, I should say.

If you have regular access to shore power overnight, then all you need is a big bank and bigger charger