How long does a sump pump battery last?
Think in cycles pumped, not hours elapsed. And replace the battery on age, because it will test fine right up until it does not.
Two different questions hide behind this one, and conflating them is why the published answers vary so wildly.
Question one: how long will it run during an outage?
The honest answer is that it depends almost entirely on how fast your pit refills, because the pump only draws current while it is pumping. A battery that lasts a day and a half in light inflow can be flat in under six hours of continuous cycling.
A useful way to think about it: a group 27 AGM holds roughly 90 to 100 amp-hours, of which about half is usable without damaging it. A backup pump drawing somewhere between 6 and 20 amps while running therefore gives a few hours of actual pumping time — spread across however long it takes your pit to demand that much work.
| Factor | Effect on runtime |
|---|---|
| How fast the pit refills | The dominant variable by a wide margin |
| Battery capacity | Roughly linear — twice the usable amp-hours, twice the pumping |
| Battery age | A three-year-old battery may hold half its rated capacity |
| Pump current draw | A 6.5 A pump runs roughly three times as long as a 20 A pump |
| Temperature | A cold basement reduces usable capacity noticeably |
| Lift | Higher lift means longer run time per cycle for the same water |
Question two: how many years before replacement?
Three to five years for a sealed AGM in typical basement conditions. The important thing about this number is the failure mode: a backup battery sits on float charge for years, tests fine on the controller's indicator, and then collapses the first time it is asked for real current.
That is why the advice is to replace on age rather than on symptoms. There frequently are no symptoms.
Universal Power Group UB121000 (12 V 100 Ah AGM)
100 Ah of sealed AGM is as much as a single-battery controller can use.
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| Type | Sealed AGM deep-cycle, maintenance free |
|---|---|
| Nominal | 12 V, 100 Ah |
| Use | Group 27/31 class capacity for a 12 V backup pump controller |
Runtime scales almost linearly with usable capacity, and capacity is the one thing you can simply buy.
What it gets right
- About 50 Ah usable without harming the battery
- Sealed, no topping up
- Standard group size
Where it falls short
- Heavy
- Check your battery box fits it
Specifications from Amazon listing (search).
Basement Watchdog AGM standby battery
Matched to the controller, which keeps the charge profile and monitoring meaningful.
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| Type | Maintenance-free AGM standby battery |
|---|---|
| Designed for | Basement Watchdog backup controllers |
Worth it where the controller monitors battery condition and you want those readings to mean something.
What it gets right
- Designed around the controller
- Maintenance-free
- Fits the supplied box
Where it falls short
- Less capacity per dollar
Specifications from Basement Watchdog battery range (retail listing).
Getting more runtime
- Buy the largest battery your box will take. The cheapest runtime available.
- Add a second battery in parallel if your controller supports it — same type, same capacity, same age.
- Reduce the inflow. Gutters and grading outside do more for backup runtime than any battery upgrade.
- Choose a lower-draw backup pump. A 6.5 A pump stretches a given battery dramatically further than a 20 A one.
- Add a generator for the long tail, so the battery only has to cover the first few hours. See battery backup vs generator.
Questions people ask
How many hours will a sump pump battery last?
A few hours of actual pumping, which can be anywhere from six hours to over a day of real time depending on how often your pit demands a cycle.
How often should I replace a backup battery?
Every three to five years for a sealed AGM, regardless of how healthy it appears. The characteristic failure is a battery that tests fine and then collapses under load.
Can I add a second battery?
If your controller supports it, yes, and it roughly doubles runtime. Both batteries must be the same type, capacity and age.
Does cold weather affect the battery?
Yes. Usable capacity falls as temperature drops, so a cold basement in winter gives less runtime than the same battery would in summer.
Sources
Read next
The best batteries for a backup sump pump
Your backup system is only as good as the battery behind it, and the battery is the part everyone forgets until the night it matters.
The best battery backup sump pumps
The storm that fills your pit is the storm that takes the power out. This is the half of the system that works when the other half cannot.
Do I need a battery backup sump pump?
The storms that fill your pit are the storms that take the power down. The question is what a few hours of no pumping would actually cost you.
Water-powered vs battery backup sump pump
One has no runtime limit and four conditions attached. The other works anywhere and runs out. That is the whole decision.
Battery backup vs generator for a sump pump
One works while you sleep at reduced flow. The other gives full flow but needs you awake and outside. They are not really competing.