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.