Originally Posted by fivekitties
May I ask what you mean by the hard and expensive way?
These batteries will be harmed permanently by letting them deep discharge....depending on the battery, and depending on who you ask, the number varies.... but basically if you let them drop below 50% (is the rule of thumb I've seen thrown out there the most) is when the damage happens. 50% is approx 12 volts.
\It may not be an absolute failure after that... meaning the battery may not be completely gone after once, but do it a few times an it will be.
Also, the batteries will self discharge over time. Even if completely disconnected from everything they will discharge maybe 2% per month.
(this is how I learned.... I'd park the thing..... flip the disconnect in the boat or pull the fuse on the pup, but it would still go dead over a several months of storage)
If you leave them connected, there are almost always some parisitic loads that will help them discharge, LP detectors, clocks, etc..... even stray currents.
If it's a golf cart battery or a true deep cycle it can handle deeper discharges... but your battery is almost certainly a marine type hybrid starting/"deep cycle".
One more comment. I had my battery replaced under warranty. I suspected it might be dead, but couldn't test it when I did my inspection since they had the coach plugged in. It soon proved to not hold a charge.
I figure this is likely a common problem.... imagine an RV sitting on a lot.
a salesman shows it to a customer
they flip on a few lights, push a few buttons
maybe even leave a light on over night accidental.
The dealer isn't likely to take care and recharge the batteries frequently..... so it's very likely to be discharged deep one or more times... maybe even to zero.
Each time that happens, if it doesn't kill the battery, it at least harms it.
1, 2, maybe 3 iterations of this.... you've got a battery that won't hold much of a charge.
Here's a great reference page
The 12volt Side of Life (Part 1)
and a nice collection of reference materials