Is inconsistent charging bad for batteries?
For most cell chemistries, deep discharge, overcharge, or very rapid (high current) charge or discharge are damaging. Simply irregular use, as long as it does not completely deplete the battery during disuse, is harmless. 

If you use a charge regulator designed for the battery, there should be no harm in inconsistent charging -- in fact, some chemistries benefit from occasional charge/discharge, rather than floating at full potential.