The first Platinum battery I had started showing an error message on the smart charger after about 5 or 6 months. And the last one I had charged up ok to 13 volts but got to the point that it would only hold charge for half the day before the pump would struggle and the multimeter reading showed 10.8 volts. So I assume a faulty cell, cell fluid levels were ok and the green light on the battery was still showing the same as when new(ie. good condition).
I assumed the first one was just a duff one and as I had 5 or 6 months from it I didn't ask for a swap under warranty and just bought another one. Wasn't impressed at all when the second one started playing up.
Andy I would put your problem down to the seller it looks as if your buying batteries that have been sitting on the shelf too long.
If not you could be discharging them to a level that's causing damage.
When you next buy a battery, charge it with your smart charger and leave it to rest over night with nothing connected.
In the morning check the voltage and it should read 12.8 volts.
Anything less than this and its not working at 100% which you should expect from a new battery.
I know the guy that's supplies my batteries wouldn't stock them if he was getting complaints from his customers who
all use them for wfp cleaning.
That's the only reason I would recommend them.