A New Off-Grid Battery

[ChrisOlson]

We have two strings of 25P 6V Surrettes that are now 7 years old. The bank was 27kWh capacity when it was new, discharged to 50%. But the last two years our Conext system says the charging efficiency has dropped to 80.4% (kWh in vs kWh out) and it was 92% when they were new, after they were cycled the first few times.

They probably have 50% of the their useful life yet but we are going advertise them and sell them to people that have off-grid hunting cabins this fall and put in a fresh battery. At $17,000 for new Surrette's to replace them we are not going with Surrette's again. They are not economically feasible.

The Surrette's have no real trade-in value if we keep using them until they are totally junk. So now is the time to do something about it. And I suspect they will last another 5-7 years being lightly cycled and charged with an inverter/charger and generator like most seasonal off-grid hunting cabins do.

We can get a rebuilt 48V forklift battery from GB Industrial for $5,500.

We are still a little confused about what size of forklift battery to get. GB industrial says they should be cycled at 80% DoD, lighter cycling will shorten the battery's life. Their amp-hour rating is 6hr instead of 20hr so I THINK we need a 750Ah battery for our daily 12kWh AC load. But not sure yet.

[phil b]

I have a pair of GNB GJP2206 with an C6 amp hr of 1120. I had them set for 24 volts at the factory. They're tubular 'demo' batteries. 1856 lbs each. I charge them with 7.1 kw of solar.
 
For the first few months, it took all day to get an 80% charge. Now 6 years later, they're done by 2pm at 100%. Charge controllers are set at 30 volts and it's a struggle to get to 29 volts. That's all they will take.
Between both batteries, they use a gallon of distilled water every month.

I also have a different system with 24 Energizer GC2 6 volt golf cart batteries that have never came close to equaling the output of the traction batteries. Energizers are limping along and nearly dead after 7 years.

My battery man tells me his rebuilt batteries have been subjected to 100 amps and the fluid levels checked occasionally during the process. That's it.

[ChrisOlson]

I'm not sure what GB Industrial does to rebuild them. But the guy we talked to there tells us that if we have one suitably sized to our load and capacity to cycle it at 80% it will put our Surrette's to shame, even when they were new. The one we are looking at is this one, it weighs about 3,000 lbs

https://gb-battery.com/products?olsPage=products%2Fnew-24-125-13-battery-48-volt-750ah

[Mary B]

Jealous! I would need to build a battery room for that LOL

[phil b]

It may also be worthwhile to buy one with removable cells, just in case.
The GNB 10 year warranty looks good.

The local scrap dealer and the battery companies will not take the bad traction batteries even if they are out of the metal case. Maybe GNB will.

[ChrisOlson]

Yes, I believe they do take them in trade on a new or rebuilt one. They have free shipping using their carrier in the US and Canada too. The customer must have means to unload the battery from the truck when it comes.

[kitestrings]

We looked at quite a few different options, and eventually settled on these.  They are 4-volt cells, so it ends up being a single series-string of 12, 60 kWh (30 usable at 50% DOD), with only 24 watering points.  They are about 370#, and I think Trojan also makes a 29 cell version.  Too soon to offer anything on longevity, but maybe an option:

https://www.fieldlines.com/index.php/topic,149040.0.html

[ruddycrazy]

Hi Chris,
             About 6 years ago I scored some forklift batteries, a 24 volt 600AH for the house and a 24 volt 735AH for my shed. Both batteries are still doing well and with my shed battery I can be machining and welding all day and the sg may drop a point from .24 to .23 after days work. Those battery will run the house no worries and I do have a 80 amp Stanbury Scarf solid state battery charger that is bullet proof.

         Now as forklift batteries are designed for a high current for 8 hours then just sit on a charger overnight using them for off grid where a constant charge can be coupled with the discharge those forklift batteries come into their own. With the house battery the aim is to keep it charged so yes I do check the sg daily and run the battery charger if needed. A auto watering system is essential as they do use a heap of water.

  But bang for buck the forklift battery is in my opnion the best for an offgrid system.

Cheers Bryan