Battery maintainers (aka trickle chargers) can only keep a nearly fully charged battery fully charged. They won't have enough output to charge a discharged battery.
A true charger will have enough output to charge a battery that's discharged.
I don't know what the output rate is to differentiate between the two, but a maintainer is probably below 5 amps, while a charger should output at least 10 amps.
A higher capacity solar charger will need a charge controller (usually built in) to keep from overcharging the battery.
