WD-40
Mine ER-GN30 has always seemed to perform as though it has a low battery. I first noticed corrosion on the negative battery contact, the one up in the body of the unit. I treated it with Caig Deoxit D5 (an amazing product) which helped, yet the issue continued to return.
Recently, the trimmer stopped working all together. I used a power supply to "bump it" with 3v which got is spinning. So I know the switch is good and the motor "can" turn.
I removed the battery and pointed the trimmer blades toward the ground and then liberally sprayed WD-40 up into the body of the trimmer. I sprayed a lot, enough to completely saturate the internals. Then I tipped it over and used some compressed air to blow out any remaining fluid.
The trimmer once again works great, the blades spin at full speed again.