Originally Posted by
CJD
I agree with everyone except for the battery tenders. I have several classic cars that sit for months, and I always used battery maintainers. I have recently had 3 of those batteries go open circuit after only 2 years each...so I am suspecting that long periods on tenders may not be good for batteries. The fact they all failed in the same rare way is what gives me concern. They fail showing 12.9+ volts at rest, but they have absolutely no amperage to operate anything.
I now still use a tender if I know I will be using the car/boat within 2 weeks. If it will be a whole season, like for the boat in winter, I remove the batteries and store them...untended...in a climate controlled storage. At the end of storage I charge them and reinstall.
Overcharging batteries for a long period of time (exactly what you describe) can be trouble. I believe the "tender" brand does not include a circuit to turn them off or go into a "top-up" mode. At least an older model one that I have does not include this ability. I like to use the optimate low-amp charger/maintainers on my motorcycles. I use a NOCO Genius in the boat. It's important to me that it can recharge both of my batteries overnight but I also need to be able to leave them plugged in for extended periods of time to properly "maintain" them. This unit seems to do everything very well. http://www.geniuschargers.com/GEN2
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.