This article explains the benefits of parallel charging lithium polymer (lipo) batteries.
It does not cover lipo battery basics. It is recommended that you look at this article and become familiar with lipo selection and charging before attempting parallel charging.
When charging batteries SAFETY is critical. The usual precautions apply:
Do not charge batteries unattended.Charge in a safe place (fireproof bag or box is recommended away from flammable materials).
Parallel charging is safe; however, the following additional precautions apply:
NEVER connect batteries of different voltages together in parallel. Make sure that all the batteries are the same cell count and voltage.
Be aware that once batteries are connected together in parallel, if something does go wrong there is more energy stored in the batteries which in the worst case could damage multiple battery packs and damage property through fire.
I have been charging batteries in parallel for three years and still using some three year old batteries charged by this method. I can detect no harmful effects on battery performance and can only see benefits for the charging and discharging of lipo batteries.