I've got a sump pump backup power supply that calls for two 12V lead
acid "Battery Council International (BCI) Group 27 Deep Cycle Marine
Batteries, such as DieHard Model 27524. The output is 120VAC at 15
amps (I have my doubts about this, because it won't power a 700 W
halogen lamp at nearly the same brightness as the regular line outlet
will. Unfortunately, the life of these batteries seems to be about 2-3
years, and the warrantee is is only a year or less. My question is
whether a DieHard Gold battery with a 72 month  warrantee would
improve my battery life and/or expenses. I realize that any decisions
to change battery types would be fully my responsibility. I just want
the rationale for the deep cycle specification. Are there
performance/safety issues involved?

A deep cycle battery is required.  A DieHard Gold battery (like all
car batteries) is designed for shallow discharge at high current
(starting a car).  A deep cycle is designed for the deep discharge
that will occur when running a pump in a blackout.  Deep cycle
batteries are also used in boats, RVs, fork lifts, and computer UPSs. 
The difference being a 50-80% discharge and a 10% discharge and due to
cell geometry.

I am wondering why are your batteries only lasting 2-3 years?  Do you
have many black outs where they are being used?  An unused backup
should last longer (2 times as long?).  Battery life can be extended
by limiting "depth of discharge" (DOD) on the batteries.  In this
case, that would mean adding additional batteries.  A rule-of-thumb
that I've seen used is 50% DOD.  There may also be maintenance that
could/should be performed on the battery.

The warrantee on the battery is going to be calculated by the
manufacture to limit their exposure, which is why the 12 month versus
72 month difference.

Also, your inverter probably only supplies a 15A surge to start the
pump after which the continuous output current falls to some lesser
value.  This is why your halogen is dim.  I would expect this rating
to be somewhere on the casing.

Search terms: (Goggle and groups.google.com)
deep cycle maintenance
sump pump back up
deep cycle faq

Bradley

Hey Bradley, thanks for the input. I'm wondering, though, why the auto
battery wouldn't work, and work well. The pump only runs long enough
to drain the sump, which is usually less than a minute. I used to have
cars that would have to crank almost that long to start, and turning
over a big V8's gotta take a lot mor amps than running the pump. As to
the 2-3 year life, the guy at the sales company told me that was
typical. I suppose that it could be overcharging. We have rarely had a
power failure, and at that, I can't remember one where the pump
engaged. So no, it's not overuse. Hmmmmmmm

Boomering

I would expect the front end battery charging circuit to be able to
supply most, if not all, of the pump load when powered from the AC
line.  This would imply that the battery is never being discharged
just like operation in a UPS.  I've heard figures of six to ten years
for UPS battery replacement.  I bet someone else can testify to the
actual replacement schedule for little used UPS batteries.  I have six
year old UPSs that have not been taken care of, which still provide
some off-line usage.

If you don't experience power outages where the sump must run, then I
would go ahead and give the Diehard a try.  It may only last one or
two rundowns, but if they never occur then there should be no problem.

To test the charger, I would measure the output voltage when the
charger should be in "float" mode.  Compare it to the chart at:
http://uuhome.de/william.darden/carfaq13.htm
If the batteries are undercharged, "sulfation" can occur which is a
build up of lead sulfate, which completely insulates the cells.
Also, what is indicating a bad battery?  Does the sump have some sort
of test mode?

You might also contact the manufacturer of the pump/charging circuit
to see if there were any recalls on defective chargers.

Bradley

Hey Bradley,
Yeah, the pump is really never used during a power outage, but the
possibility for the need of it during an outage is why I bought it.
The status is indicated by a pair of LEDs, and whether they're steady
state or flashing. When the system failed, it was such that it would
always be in the "hard charging" mode, which runs with a cooling fan.
When I called the sales office (I don't have access to the
manufacturer), I was told that this constant charging was a symptom of
battery failure, and that replacing the batteries should restore
normal operation, which is what happened. I was also told at this time
that 2-3 years was normal life, and of course, they couldn't tell me
why. Replacement of both batteries costs approximately $160.
I never really did basic voltage and current checks, so I don't know
for sure whether it's over- or undercharging. I've only replaced the
batteries once (this Spring), so it may be that the original pair were
defective. It's just that if they need replacing again in a year or
two, I'd like to use a model with a longer warrantee so I don't keep
gettin clobbered with that replacement cost.

At any rate, your input is greatly appreciated. Thanks,

Boomering