Battery for backup sump pump – testing and replacement

We had our sump pump system redone after basement flooding issues in 2009.  One of the upgrades was putting in a more powerful back-up sump pump.  Our plumber used one that is comparable to the Zoeller Aquanot II pedestial pump.  It will run off of the AC wall current when we have power, but runs off DC current from a large battery when the power is out. These types of pumps can pump about 35 gallons per minute at a 10 foot head, which is significantly more volume than cheaper models that you get at Home Depot (which is what we had in previously).

When installed, the builder used a local distributor’s AGM battery (Absorbed Glass Mat) 12V with a charger to power the pump.  This battery was different from the one we had before, as it was all sealed with no place to add water.  As it was from a local distributor, there was not much information on the battery.  The charger is a 3 stage charger (or at least it has 3 different lights on it) and it’s consistently shown all three lights on which means the battery is fully charged.  In order to test the battery, about twice a year I would unplug my primary pump and the charger for the backup, let the sump pit fill, and make sure the pump ran a few cycles.  If it did that, I assumed the battery was reasonably okay.

As the battery was now over 5 years old, I figured I was pushing my luck, but I didn’t know what the typical life-span is for these AGM batteries.  Online, suppliers claim they can last 7-8 years.  These batteries are not cheap either, running about 3-4 times the cost of a similar wet lead-acid battery.  I was trying to figure out the best way to test if my battery is really okay.  I don’t like testing it for extended periods of time because with my luck, the power will go out right after I’ve run my battery down testing it.

Reading around online, it seemed the best way to test if your battery is operating properly is to do a load test.  They sell load testers that you hook up to the battery and they will tell you how your battery is doing.  I found this load tester on Amazon for under $30.  I figured it was a good investment compared to replacing a battery every 3 years as my plumber recommends if the battery has the potential to last 7-8 years.

I hooked up the load tester and ran it, and the needle hovered on the green line around the 600 rating.  This would be fine for a battery rated at 600 cold cranking amps.  My battery had no such labels, so I had to call the battery distributor, and he told me my battery was rated at 800 cold cranking amps.  Hmmm.  That puts my battery in the “weak” category.  So, time to replace it.

There are a few different options and for each one, I’ve seen a few different opinions.  Some people recommend going with the wet lead-acid batteries because they are cheap and work well.  Other people recommend the AGM batteries because they are zero maintenance.  There are also gel batteries which are similar to AGM batteries, but you’d need a special charger, which ruled it out for me.  Most of the advantages of AGM batteries seem to apply more to marine use than home sump pump use (they can be positioned in any direction, they resist shocks, and they have nothing to spill if tipped or broken).  In my home, the battery won’t be exposed to shocks and unlikely it will break just sitting there.  They do say they hold charge well, but my battery stays connected to the charger to maintain a full charge.  The biggest downside I can tell for a typical wet lead-acid battery is that you have to monitor the fluid level and replace the water from time to time.  You don’t need to do that with AGM batteries.  Also, the capacity of the AGM batteries seemed to be greater.  I could get a similar capacity using two lead acid batteries for less cost, but I opted for the AGM battery just to be consistent with what I previously had and what our plumber had recommended, assuming he knows more about this stuff than I do.

If you do go with a lead acid battery, you need to make sure it is a “deep cycle” battery.  Typical car batteries aren’t designed to provide longer term power where the battery gets discharged.  Deep cycle batteries are designed to provide longer periods of power and then can be fully recharged many times.  I found the most useful measure of storage capacity being amp-hours.  The battery I was replacing was rated at around 100 amp hours, which is pretty good from what I could tell.  Costco had a lead acid deep cycle battery that was rated at about 65 amp hours, but only cost around $100.  The AGM batteries cost around $300.  They have a high rated AGM battery on amazon rated at 125 amp hours for around $260. I figured for the peace of mind of not having to worry about maintenance, and the possibility that the battery will last longer, and the higher amp hour ratings so I don’t have to worry about multiple batteries, I decided to go with the AGM battery.  I’ll keep my old battery around as a spare, as I’m guessing it still has some life to it, even if it did test in the “weak” category.

If you decided to go another route or have better information, please share in the comments below!

2 Responses

  1. Mark says:

    In case anyone wants to know, the makers of the popular Basement Watchdog Battery Backup system recommend these specs when buying a deep cycle marine battery for your Basement Watchdog Emergency system BWE 1000:

    “12V/DC, deep cycle, 24 or higher group size

    and any one of the following

    75 or more Ah
    100 or more minutes RC @ 25 amps
    400 or more CCA”

    Maybe that helps someone since it’s a common desire to not use the Basement Watchdog brand battery with the BW pumps.

  2. Brian says:

    Pedestal pumps (motor outside pit) are pricey but very good. For in sump pumps – our primary sump is a “Ion WC33I” with digital switch and the backup we just got is a 2019 Zoeller Aquatnot 508 Fit. Sumps will last about 7 years (same as a good AGM battery). However floats are the biggest earlier failure in sumps. Electronic/Digital or dual float switches are the way to go on primary and backup pumps. Lastly look a good monitoring – every time I had a leak or water in basement – the earlier I knew about it the better I could stop and clean it. Good sump pump controllers will exercise/test your sump, test your battery, tell you if it is running or water is high, and send you texts. I also have a bunch of wireless leak sensors around my basement – near window wells, water heater, sump pit, etc. Cheap. Here is the battery my plumber suggested – great value and good specs.

    https://www.samsclub.com/sams/duracell-agm-deep-cycle-marine-and-rv-battery-group-size-31dtmagm/prod3590232.ip

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