See post #9 in this thread--
There is some descrepancy on the web as well. The problem is that only 1 tenth of a
volt makes a difference in charge state on a lead acid battery.
Basically a battery with no load and 12.75 volts is 100%
and a battery with 12.00 is down to 40%.
For maximum battery life it's best to keep your battey(s) charged to above 50%.
They will last longer if you charge them back up any time they drop to 1/2 charge.
Having said all that, I listen to my water pump and watch the lights.
IF a light dims significantly when the water pump cycles it's time for a charge.
If the water pump sounds tired near the end of a pump cycle it's time for
a charge. (Water pump slows down as pressure builds and get's noticeably slower
just before it shuts off.)
This is not scientific at all but seems to work for me.
Also these panel indicators are NOT very accurate but when mine gets down to
1 led when I push the button I'm getting very low on charge.
One thing I do that I really think helps coax long life out of my batteries is
I have a battery tender Jr. on my trailer at home.
These little chargers are fully automatic and will boost the battery voltage
enough to cause bubbling in the electrolyte. This keeps it mixed up and cuts way
down on sulfide build up.
I have a battery tender Jr on my truck in the garage because the electric brake controller
is a drain even with the key off. I have one on my trailer as I said.
My truck still has the original factory battery. It's a 2004.
My trailer is an 08 and just this June we camped without hookups for 5 nights and
used our MaxxAir fan every night and watched national news on TV every evening
using a little cigarette socket plug in inverter. We also watched one DVD movie.
We were careful on lighting. We have a couple LED lights that we use for moving
around inside. We have a fluorescent thin lite over the table for eating and reading.
My water pump still sounded peppy on our last day and my LED battery indicator
still had 2 lights on when we pushed the button.
I have only 2 12v regular old "crank/deep cycle" marine batteries.
I'm not an employee or anything but these guys have the best price on the
internet for a battery tender Jr.
It's $20 with free shipping. If your trailer is parked where you can reach 120v for
a battery tender Jr. I highly recommend using one.
Battery Tender Junior