I have no experience with deep-cycle batteries. But with conventional batteries that permit the user to add water, it is good to slow-charge immediately after adding water to be sure the battery is fully charged. A partially discharged battery will sulphate and the ampere-hour capacity is reduced.

Deep cycle batts. can be discharged quite
a bit and not be harmed, unlike typical
motor vehicle batts. However, the loss
of that much water may have permanently
damaged the battery. I would guess it
was overcharged, or charged at too high
a rate. Now that you have the electrolite
level back up, hook up a charger and
check the charging rate. A battery for
this application should be on a trickle
charger, which should keep it fully charged
without over charging. It is best to charge
it at 10% of the amphour capacity of the batt.
Good luck.

People like that are better off with an AGM battery. She will have this one ruined in no time if not ruined already.

Was the pump used in the backup mode that required discharge and charging of the battery? If so, how much? This seems like excessive water loss for a stand-by or lightly used battery. It indicates that the battery charger is not operating properly. I advise you to check into it and get help while it is still covered by warranty.

Since that battery holds less than ~12 quarts of fluid, it sounds like it received a sever overcharge.

It may have been damaged, and the most common damage reduces its reserve capacity (amp-hour rating).

Even a simple load test for a few minutes may not detect changes in reserve capacity.

It might be time to check whatever charger it was connected to.

A little more background. It's a Blue Angel back-up system by Wayne. When the battery was replaced, I also replaced the charger (a wall wart) and the circuit board.

To the best of my knowledge the battery back-up has not kicked since the battery was replaced.

A size 27 battery is pretty big one, but I agree with Brick: either long long neglect or overcharging. If you added that much water I'd check the electrolyte SG (you can get the tester at the auto parts store for a couple of bucks) as well.

Here is a link that might be useful: Deep cycle info

A battery in stand-by mode, that has lost 25% of its fluid in 4 months has been both overcharged and neglected.

If the battery is being overcharged, is there any way to test that? As I said, the both the power supply and circuit board are new.

You need to use an 'amp meter', also
called and 'ammeter'. Most DMMs
have an amp function. Take a reading
and let us know what you find.

Well, while measuring current using a DVM in series will give an idea of the charge current - at that particular moment - it does not tell you if the controller circuit board is is regulating the charge being applied to the battery.

A one amp float charge (which is what the wall wart looks like to me) ran continuously on a deep cycle battery will cause loss of electrolyte, but three quarts in four months does sound like quite a bit. Maybe the battery was a little low to start.

If the electrolyte level in the new battery was correct when installed, then I'd suspect the controller board is not correctly regulating the charge - either a bad board or possibly a connection error.

However, why did you change the board in the first place? PM (preventative maintenance) or was there some other problem?

I have a digital multimeter but not one that measures DC current.

The original circuit board was replaced because it failed. The manufacturer recommended replacing the power supply at the same time.

I think your best option would be to contact the manufacturer about this for their opinion and advice. I'd be interested to hear what they say.

As I mentioned, the battery is at a friend's house. When I get a chance I'm going to check the electrolyte level and see if it has changed significantly before calling the manufacturer.

"When I get a chance I'm going to check the electrolyte level "

Not just the level but the acid strength.

Adding water can result in a low acid concentration.

The battery will not deliver its rated current load current (or capacity in amp-hours) if the acid concentration is low.