I use a Deltran Battery Tender Jr on the OEM original battery. It is permanently connected using the ring lugs. When it is plugged in, I keep the plug module outside the car and it is plugged into a heavy gauge extension cord. I also do not latch the trunk when it is plugged in. So far, it's worked as advertised with no problems.
Kooks 1-7/8 LTs; Calico coated w/HFCs, Solo Mach XL catback w/J-pipes, VMax TB, C.A.I. Intake, JRE Tune, RX Catch-Can