All summer I had 4 car batteries (in parallel) charged by solar panels, now the sun has reduced I tried to charge them with a charger but it tells me the batteries are no good now. I thought the deep cycle batteries would be ok. The system is only used to power 12v water pump to water my plants in the greenhouse. The charge control(s) are supposed to make the batteries more efficient Do I need to use special solar batteries?
The charge controller optimises the way the batteries are charged from the solar panels. It doesn't actually make the batteries more efficient.
Did you disconnect the charge controller before connecting the charger? If not, that may have confused it.
Deep cycle batteries are more forgiving of being discharged than normal car batteries, but if they have been left flat, or almost flat, for several days then they may have been permanently damaged.
You might have some luck using a dumb, old-fashioned battery charger - one that won't tell you the batteries are no good. If possible, set it to trickle charge and leave it for day or so. But if the batteries are dead, that won't magically fix them.
Is there actually such a thing as a "solar battery", or is it just a deep cycle battery with a different label stuck on it?