We have a 12.5KW gasoline 27HP Generac for backup power for part of church. The genset was bought after Hurricane Wilma (2005) and has the original sla start battery on it. Since I had some spare time I got no-flat tires for the genset and wheeled it out of storage. I think the only time it was used was shortly after it was bought, as Wilma damaged the grid here so much that it was 3 to 4 weeks before power was fully restored. FPL contractors here are still adding cross feeders to strengthen the grid.
Not wanting to spend more money to buy a replacement start battery with no guaranty the genset would run, I removed the battery and checked it. It had -1.3v. I had read elsewhere that a dead sulfated battery like that could be restored if you trickle charged it backwards fully and then drain it slowly and then slowly charge it the correct polarity. That almost worked. It would rest at 11v. I have several different chargers and having nothing else to lose but time, tried them all. I have a Vector stupid charger with a desulfate feature. I call it stupid because it throws an F3 fault and refuses to charge most batteries. The desulfate feature also causes an immediate drop in battery voltage, and never worked, so I tried desulfating with a second trickle charger at the same time. That helped a little but the resting voltage was still below 12.
Then I tried a high voltage-low current charge with a charger I reworked after the main transformer winding failed. I pushed the voltage over 15 a few times for a few minutes (don't want to dry out the battery, it's sealed). That got the battery close to 12.5 resting.
Then I took it back to church and hooked it up to the genset and using a different charger on engine start boost, cranked for a long time until I was convinced the carberator was sludged up and the thing wasn't going to run. I left the battery on a 13.8v charge overnight and went to the local auto parts and for $4 got a can of carb cleaner that had the nastiest chemistry I could find (benzene, acetone, toluene and methanol). I took the top off the carb to expose the jets and soaked it with the spray and put the cover back on and also disconnected the fuel line and sprayed into the carb onlet port and put the fuel line back to soak for 1/2 a day. When I came back it fired and ran on the first spin.
The following day after hard charging overnight and running most of the fuel/Sta-bil mix out, the battery now rests at 12.6v
The label on the battery did not say the date of manufacture but did say that after November 2005 the battery should be charged for a couple of hours at 1-2 amps before putting it into service.
So, it appears that age alone will not ruin a sealed lead acid battery. The battery will start the unit without being boosted now.
