Is slow charging bad for the battery?
Slow charging is great for a lithium battery. Two things kills lithium batteries


Temperature
Charge level


Charging too fast causes excess heat, that damages lithium batteries
Keeping your battery charge too high also damages lithium batteries



Ideally you want to slowly recharge and keep it between 20% - 80%. Also, many short small charge cycles is better than a lengthy large charge cycle.

Sources: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries