My point was that there can be a situation when the batteries are fully charged, but the solar still pumps more current into them. What's going to happen then?
As perviously stated, it depends on the battery type and capacity!
Some, like NiMH and Lead Acid batteries (automotive and gel SLA) it won't be a problem,
provided the current is kept small enough to function as a trickle charge otherwise it can damage the battery and at high enough currents lead to the production of hydrogen gas (always very flammable in normal air and potentially explosive if sufficiently concentrated). Other battery types you'd have to have a way to automatically electrically disconnect a fully charged battery from the solar panel to prevent it from being overcharged. LiPos, for example, are easier to damage and have a lower current threshold for self-ignition or explosions. With only 100 mA of overcharging the more spectacular failure modes are still possible (especially with small capacity LiPos and Li-Ion), but the far more probable issues will be faster than normal degradation of the batteries and/or total failure where physical damage is limited to the battery itself.
In any case, even if you are using the cheapest possible recharagable batteries, it's usually more cost effective, and always a bit safer, to prevent minor overcharging than to live with its consequences.
