The system is great while using the car most days, but our cars have been sitting on the drive for around 10 months with only the odd run to pick up food, I was lucky I have a smart charger which I can rotate car to car and also caravan batteries, easy enough to clip on and forget, however the older chargers can wreck valve regulated lead acid and absorbed glass mat lead acid, as to using any charger while connected to a car with computer controlled charging, we cross our fingers and hope it will not upset the computer.
I know with wife's Jag at 11:40 am every day some thing happens, as the charge rate changes with the smart charger, which having an interest in lead acid batteries I have powered by an energy meter so I can see what is going on. I am told should charge from under bonnet as the car then sees it as being charged, not direct to battery.
But we know sulphate gets hard over time, so it takes longer to remove, and with a stair lift battery which had been abandoned I found it took around 10 days on the charger before it started to take charge, and was as if some one had flicked a switch, started charging, went through the cycle and then seemed not the worse from being allowed to discharge.
So where a battery has been allowed to half discharge and sit there for some time, the question is how long does it need to be on charge and fully recover, and what damage will leaving on charge do to battery is any?
The standard method for a VRLA is a regulated power supply, for 12 volt between 13.4 and 13.8 volt, but this does not equalise the cells, so the smart charger alternates between 12.8 and 14.4 volt each time the volts goes below 12.8 it switches on, or ups the charge rate, and when at 14.4 volt it either drops charge rate or switches off. For my chargers either zero and 0.1 amp or 0.1 amp and 0.8 amp to maintain the battery, and with most of the batteries they sit at 12.8 volt most of the time, then for a few minutes raise to 14.4 then drop again.
However the odd battery can be just on the edge, so one gets a charge patten like this.
I really don't know if that is a problem or not? In the main more like this
.
However since I know from the recovered stair lift battery it can take 10 days to recover with hard sulphur on the plates, I aim for a week or two each time charged to ensure all sulphur on plates is returned to being acid. The point is it takes time, so flat battery and non smart charger for long enough to start car OK, but would not want to leave that type connected to battery for more than an hour, but then you need the engine to run daily to recharge, and we can't do that, so without a regulated power supply or a smart charger it is near impossible to care for the AGM battery.
