What to do with 3 AGM batteries

[leosgarza]

I have 3 of these brand new batteries that I got my RV that I never used. Got lithium right away before my first trip. I have solar at home and I am looking to use them to mitigate time of use electricity rates. I was thinking about charging them in the middle of the night when it is 50% cheaper. Then using them to run my basement items and a mini dryer and washer. I was thinking about buying a 4th one listed and either an EG4 or MPP 48v 3k offgrid inverter. Then go lithium later. I wanted to add an external batteries but now I'm concerned about the charging rates of AGM batteries. What would you do?

Direct replacement

 

[Hedges]

AGM batteries cost about $0.50/kWh over their cycle life. Not useful to time-shift when you draw from grid.
Lifespan is typically 300 to 700 deep cycles, a year or two nightly. Float life could be 10 years.

Best application is infrequent use like weekends, whether camping or boating. Or as battery backup for grid failure.
Consider a 12V inverter/charger or all-in-one and use to back up critical loads like communications and alarm. Or no inverter, if your equipment has 12V input, just fuse each connection. Add some PV panels and charge controller for extended outages.

You have 3x 12V 100 Ah, 3600 Wh gross, good for up to about 2400 Wh cycling, so average 200W load during a night.
This could even run larger loads like TV and computer if desired. Or critical loads for a few days without power.

The three batteries in parallel could accept up to 45A ... 90A charging, but to be cheap you could get a smaller regulated charge to keep them at float. Although faster charge is better. I use a 4A charger from Harbor Freight to keep such batteries happy in standby. That could be OK for very occasional recharging. You might start with that before putting in larger PV setup.

I have large whole-house backup systems that use AGM and PV panels.

 

[sunshine_eggo]

What to do with 3 AGM batteries

I have 3 of these brand new batteries that I got my RV that I never used. Got lithium right away before my first trip. I have solar at home and I am looking to use them to mitigate time of use electricity rates. I was thinking about charging them in the middle of the night when it is 50%...

diysolarforum.com

 

[12vDC]

Since they are new, you could still get a decent amount for them if you sell them. They are good start batteries and they still have a place in various applications.

Since you’re asking, I’d say sell them and get a kickin ass deployable solar panel. Or or get a small portable power station. Or both.

??‍

I recently replaced my perfectly working batteries with mini’s to up the capacity in the same allowable space. I was going to take the old ones and make a portable power station, but by the time I would’ve gotten done with it I could’ve just bought one for same money (time/labor/frustration) and it would’ve been half the size. So I ditched the idea and just gave my old lithiums to my friend (replaced his clapped-out AGM’s).

 

[Hedges]

I recently bought 24 pieces 6V 250Ah AGM batteries (used). Cost me $0.15 on the dollar.
That keeps me biased in favor of the old standby.

The question is really whether you want 3500 cycles in a decade or only need 350.

You mentioned rates 50% cheaper at night. How much are your rates? Can you just run the laundry at the off-peak hours?

 

[leosgarza]

I go from 11cents per kw to 29 cents per kw. It is not a lot in the grand scheme of things. I am really thinking of just buying 1 extra to get my configuration to where it's needs to be and using them until they die. I will just upgrading to Lifepo4 when they die. Reason: seems like to sell them well not be that easy. People just do not want AGM.

I recently bought 24 pieces 6V 250Ah AGM batteries (used). Cost me $0.15 on the dollar.
That keeps me biased in favor of the old standby.

The question is really whether you want 3500 cycles in a decade or only need 350.

You mentioned rates 50% cheaper at night. How much are your rates? Can you just run the laundry at the off-peak hours?

 

[Hedges]

You could wear them out in 2 years, maybe 1 year, using them to store off-peak power for use on-peak. $0.29 - $0.11 = $0.18 per kWh saved. Without knowing purchase price or rated cycle life, I'll assume the batteries cost you $0.50 per kWh, so you lose $0.32 per kWh this way. Not counting cost of inverter. Will that automatically switch back to grid when it reaches low battery shutdown?

The only thing this does for you is delay when you buy a lithium battery. Then you get to address precharge (is battery willing to be connected to an inverter, or does it shut down? If necessary, use resistor to charge capacitors.) Address peak draw (when inverter starts motor in laundry machines, for an instant it draws 5x running current. Lithium battery and BMS must be sized large enough to support that.)

Lithium batteries may be cheaper still in the future. Or may be more expensive. I think the price is based on volume of reject batteries not good enough for EVs in China, and market demand for them.

AGM batteries are simple, robust, easy to use, but need to be fully and properly charged, including a couple hours of absorption. Best use for them is portable applications where their weight is not excessive (permissible as air cargo) and backup applications where they see a few hundred cycles over their 10 year float life.

I suggest using them as backup for critical loads. My internet gear has 12V plugs (I assume but don't know that they would be fine with the range of charge and discharge voltages used or AGM.) Possibly your computer would too, or you could get a small inverter for it.