Intermittent AC Charging AGM Battery Bank - Worth it?

[PaleBlueDotFarm]

Hello! I have 12x 2V 1000ah AGM Batteries. Monitoring SOC using a VictronSmart Shunt and resting voltage.

Due to these batteries low (2.3V per cell) charging voltage, they never realistically get to 100% in a day. My current threshold to run the AC generator and charge them is a morning deficit of >200 ah (80%). After a month, we have only reached this point once, after several days of no sun.

Is there value/need to run the generator to charge them regularly (weekly, biweekly?), to hit the magical 100%? I understand these bats are not suited well to solar, but they're what we've got!

 

[sunshine_eggo]

Probably not unless you see signs of degradation. If your goal is to get 1-2 years out of them, and you're only using 20% of the rated capacity, it's probably not worth it unless you see signs of degradation not associated with temperature, i.e., when the battery is at its lowest state of charge and lowest voltage, if you see the voltage going lower, and that's not explained by an increase in temperature, it might be worth running the generator for 12 hours (obviously targeting warmer temps to get the 12 hour run if you can. 24 hr seems nutz).

 

[PaleBlueDotFarm]

Thanks @sunshine_eggo! You're getting part of this homestead named after you at some point.

 

[sunshine_eggo]

Outhouse?

 

[Zwy]

Hello! I have 12x 2V 1000ah AGM Batteries. Monitoring SOC using a VictronSmart Shunt and resting voltage.

Due to these batteries low (2.3V per cell) charging voltage, they never realistically get to 100% in a day. My current threshold to run the AC generator and charge them is a morning deficit of >200 ah (80%). After a month, we have only reached this point once, after several days of no sun.

Exactly how many Ah per day do you use? Running in a range of 100% to 80% is different than running in a range of 80% to 60%. The latter will sulfate cells excessively.

Is there value/need to run the generator to charge them regularly (weekly, biweekly?), to hit the magical 100%? I understand these bats are not suited well to solar, but they're what we've got!

With any lead acid battery, the sulfation created by not reaching full charge will eventually reduce capacity. Read here from the manual, you need to monitor cell voltage and perform an EQ charge when cells stray according to the manual. In order to check if EQ is needed, you will need to hit float voltage by measuring string voltage and dividing by number of cells.

If you don't have enough recharge capacity, then you should add some. PV panels are cheap, ground mounts can be inexpensively built with just lumber.

 

[time2roll]

I would press them at least once a week to the magical 100% charged. Top charging with a generator might be wasteful as the current tapers into the last 5%. Consider a few hours max charge in the morning and let solar take it to 100% during the day. Choose a day when you might be out and using less power.

Real solution is more solar. Eventually replacing with LFP batteries, when needed, will eliminate the AGM need to get 100%.

 

[PaleBlueDotFarm]

Exactly how many Ah per day do you use? Running in a range of 100% to 80% is different than running in a range of 80% to 60%. The latter will sulfate cells excessively.

Our average is 100Ah deficit by sunrise. The monitor has never hit below 82%, and I have alarms set at 80.

A 'normal' day for us is waking up at 92%, sun rises, we hit absorption within an hour or two depending on clouds, and sit at the temp compensated Absorb=Float voltage for the remainder of the day. That will eliminate all but ~35 Ah of the deficit before sunset, as the current is only 4-5 Amps at that point to maintain voltage.

I would press them at least once a week to the magical 100% charged.

Tried it in the early morning hours this weekend and the charging performance was almost identical to just a sunny day. Hit the absorb limit maybe 30 minutes faster than the MPPT, and then it's just watching decreasing tail current. Eventually I just switched back to the MPPT since it's a better charger (vs. the Trace Xantrex SW4024), and the giant loud LP generator was only sending <200W to the batteries on a bright sunny day.

If you don't have enough recharge capacity, then you should add some. PV panels are cheap, ground mounts can be inexpensively built with just lumber.

I have 1500W on right now which seems to be doing the trick - or at least as well an LP generator. I had ordered 6x 455W panels that arrived smashed before I found these batteries - seems like that would have been a waste on this bank!

Eventually replacing with LFP batteries, when needed, will eliminate the AGM need to get 100%

I'm excited to see how the current setup performs over time. I paid ~$50/kW for these, cabling included. These are meant to bridge us until the completion of an addition to the cabin. Our priority was initial cash outlay since the future is murky. We might watch them fall off the cliff, but at least I think we know what we're looking for now!

We live on a former ranch inside of BLM land (Central CO, 9000'). All of our neighbors are off grid as well. It's absolutely bat**** crazy to see our neighbors hitting low voltage and using their generators DAILY with much higher end equipment, plunging thousands of $ each week in having solar installers coming and 'tweak' their setup to try and get by. You all and this forum rock!

 

[Zwy]

More charging capability won't help if you are hitting Absorb early in the day. The problem will be you never hit full charge and the resulting sulfation over time. This will reduce capacity.

Running the generator to get to full charge is a waste. I'd look at acquiring a small LFP battery and MPPT to charge it. Use a small inverter to power an AC battery charger to get a full charge into the battery every few weeks and EQ if needed. Or acquire another cheap lead acid battery and that might allow switching from one battery to another in order facilitate full charge and EQ when needed. The advantage of a small LFP is you can throw amps at it and it will take it until bulk setting is hit.

I assume you are using this as a 24V bank?

 

[time2roll]

Tried it in the early morning hours this weekend and the charging performance was almost identical to just a sunny day. Hit the absorb limit maybe 30 minutes faster than the MPPT, and then it's just watching decreasing tail current. Eventually I just switched back to the MPPT since it's a better charger (vs. the Trace Xantrex SW4024), and the giant loud LP generator was only sending <200W to the batteries on a bright sunny day.

Yes these short days do not leave much time for absorption. Need to start the generator before the sun comes up to have more solar hours of absorption. Possibly turn the charging voltage on the generator powered charger up a bit. As much as 14.6 or .1 over spec what ever is more.

 

[sunshine_eggo]

I would press them at least once a week to the magical 100% charged. Top charging with a generator might be wasteful as the current tapers into the last 5%. Consider a few hours max charge in the morning and let solar take it to 100% during the day. Choose a day when you might be out and using less power.

Real solution is more solar. Eventually replacing with LFP batteries, when needed, will eliminate the AGM need to get 100%.

I still wouldn't. To charge at this level, he's looking at at least 12 hr at absorption per the data sheet, possibly 24 hours depending on temperature. With DoD a mere 20% and with the stated 1-2 year goal, I just don't see the need unless the voltage trend diverges. 70% is where damage really starts to happen, and he's nowhere near there.

Given the 6+ hours of "absorption" per day, I gotta believe

Yes these short days do not leave much time for absorption. Need to start the generator before the sun comes up to have more solar hours of absorption. Possibly turn the charging voltage on the generator powered charger up a bit. As much as 14.6 or .1 over spec what ever is more.

I think you're missing the specs. These batteries explicitly state that cells should NEVER be over 2.35V/cell in any conditions EVER, and that's not for routine operation - only for "Freshening charge." 14.1/28.2V

He's limited to 2.32V/cell for absorption/float, 13.92V/27.84V, and that's WITH temp comp active.

These are essentially UPS batteries he's using for a cyclic application.

 

[time2roll]

Ok maybe the pack should be split. Charge half the stack a few days then swap to the second stack.
Just not going to last or perform like ordinary AGM. Not made for daily cycling.

I would still press the voltage a bit.... they are going down anyway.

 

[PaleBlueDotFarm]

A few updates since I missed notifications!

We had multiple gross days and some excess demand, so I finally did a fully 12 hr generator charge. We got to 98% (-45Ah) before calling it quits. Infuriatingly, it seems the Ah deficit is almost always 10x the current required to stay at float. -120 Ah deficit, bats take 12A @ 27.6, -80 Ah and we're at 8A @ 27.6. From 96% to 98% seemed like 6 hours.

All this to say, I would not be doing any of this generator garbage if it wasn't for the combs meter. I track the lowest daily voltage (6 am well pump turns on), and have NO drop over the past 2 months as measured on a process control chart. I've checked resting voltages after 3 hours a few times, and measure 26.0. Nothing other than the total 2 month Ah input vs. output leads me to think anything is a problem.

I'm planning to wait for sunny days to do AC charging in the morning, then top off at night. I like the idea of using a seperate battery to top-off and will definitely explore that. Although a lithium battery, inverter, and AC charge controller are just about equal the replacement cost of the batteries I'm abusing!

 

[Eichfeld]

Another thing to try, is more battery. That way, you wont get below 80% SOC. If you are getting absorbtion early in the day, you should have solar capacity to charge more batteries to say 95~98% SOC.

 

[RCinFLA]

Is there value/need to run the generator to charge them regularly (weekly, biweekly?), to hit the magical 100%? I understand these bats are not suited well to solar, but they're what we've got!

You should do a full absorb charge with current taper down at least once a week.

Each charging cycle you cut absorb time short you get a little less available capacity for lead-acid batteries.

If you have to do the full absorb period with a generator, plan on doing something that draws a lot of power like washing clothes or heating hot water during absorb charging period to get the most efficiency from fuel used by generator. You want at least 25% rated load on synchronous generator, or its fuel efficiency is poor. You get a little better efficiency if using ECO mode on inverter-generator when it reduces rpm at lighter AC load.

 

[PaleBlueDotFarm]

Providing an update in case others come across these batteries in their journey...

Running the generator to get to full charge is a waste. I'd look at acquiring a small LFP battery and MPPT to charge it. Use a small inverter to power an AC battery

This ended up being the key! I repurposed a Bluetti AC200Max to power a Victron 24V 12A charger. On a sunny day, I charge the Bluetti off the house to maximize what comes in through the MPPT. I disconnect the charging cable at sunset, and switch on the AC charger powered by the Bluetti. I can now fully recover SOC after a 3 day snowstorm with 2 day of reasonable sunlight using the AC charger overnight.

It. Works. AWESOME. The propane generator hasn't be turned on in over a month. It's more complicated/manual of a process than most would like, but we seem to be pulling through just fine!

Thanks for all the great feedback and suggestions!

 

[Zwy]

Glad to hear you were able to get the battery to full charge. Absorption can become a huge problem with any of the lead acid chemistry due to the time required to get to full charge. Not enough hours of daylight to get there, then loads at night can use up all the watts you charged during daylight. This leads to sulfation and reduced battery life.

Great followup, thank you.

 

[time2roll]

Perfect. Next step will be planning for a full conversion to LFP when the AGM are getting tired.