I use a 1/4 A solar panel to maintain two 12V lead-acid batteries (one starting battery, the other is a deep-cycle storage battery). The solar panel is multiplexed to the two batteries as needed by the controller. Now this is a "maintainer", not a "charger", used to keep the batteries from self-discharging when the boat is not in use. These batteries are charged by the engine-mounted alternator (at a rate of ~40A) when the engine is running.
Charging a lead-acid battery can be done with much lower currents than 1A; it will just take much longer. For example, if you have a discharged 50Ah battery, it would take ~70h to charge it at 1A, 700h at 0.1A. You have to put in 70Ah to get 50Ah out, because of the Columetric Efficiency cited in the reference. At very small charging currents (~20mA), all of the charging current is going to overcome the battery's self discharge leakage, so it will never charge. This is the basis of the small unregulated dashboard-mounted solar panel automotive battery "maintainers" as sold by Harbor Freight.