What Kind of Battery for a Sump Pump: Sizing and Type Guide?

A deep-cycle AGM battery in the 75Ah to 120Ah range is the standard choice for most 12V residential sump pump backup systems, sized to the pump’s running amp draw.

When the power drops, a backup sump pump is only as reliable as its battery. Most homes run a 12V backup pump, so the choice comes down to chemistry and capacity — get those right and the system runs for hours or days on intermittent pumping. Below: which battery belongs in a basement, how to size amp-hours, and the mistakes that kill runtime.

What Type of Battery Does a Sump Pump Backup Need?

Deep-cycle lead-acid is standard, and AGM (Absorbent Glass Mat) is the most commonly recommended sealed subtype. Deep-cycle handles repeated discharge and recharge — unlike a starting battery built for one big jolt. AGM earns its place because it is:

• Sealed and spill-resistant, safe in a basement without venting concerns.
• Low-maintenance, with no water levels to top off.
• Float/standby friendly, staying charged between outages.

Consumer guidance discourages car batteries: a starting battery degrades fast under deep, repeated discharge. One hard compatibility rule: 12V pumps run on a single AGM battery (or several in parallel for longer runtime), while 24V pumps require two AGM batteries in series. Wrong voltage is a common, avoidable failure.

How Many Amp-Hours Do You Actually Need?

Practical range is 75Ah to 120Ah — typically several hours of intermittent pumping, stretching to several days in an extended outage depending on duty cycle and pump load. A major retailer guide sets the floor at a minimum of one deep-cycle battery, with multiple batteries allowed for longer runtime.

Start with your pump’s running amp draw. Match the Ah rating to that load and your desired coverage time; a higher Ah rating simply buys more runtime. Watch vertical lift, too: manufacturers list capacity at different lift heights, and higher lift reduces output while increasing power needs. Check the pump manufacturer’s own recommendations first — that overrides generic sizing charts.

Most sump pump battery boxes accept BCI Group 27 or Group 31, so confirm the box before choosing capacity. To see which deep-cycle batteries hold up best, our tested roundup of sump pump batteries compares the models worth considering.

How to Size and Maintain the Backup System

Sizing comes down to amp draw, duty cycle, and desired outage coverage. Add up the pump’s running amps and pick the Ah rating for the runtime you need; a pump that cycles on and off drains slower than one running continuously. The most common mistakes share a root cause — ignoring the numbers. Underestimating amp draw, duty cycle, or vertical lift leads to a battery that quits early; mismatching voltage or box size fails just as hard. The U.S. Consumer Product Safety Commission’s sump pump buying guide covers backup-system ratings worth checking.

Owner’s manuals recommend monthly testing of the backup system and monthly battery condition checks — a quick visual and a test cycle proving the pump runs on battery power.

FAQs

Can I use a regular car battery for my backup pump?

No. Consumer guidance discourages car batteries because they are starting batteries built for short bursts, not the deep, repeated discharge a backup pump demands. A deep-cycle AGM is designed for many drain/recharge cycles, so it lasts far longer in standby service.

How long will a 100Ah battery run my sump pump?

Runtime depends on your pump’s running amp draw and duty cycle, not amp-hours alone. A 100Ah deep-cycle battery within the recommended 75Ah to 120Ah range typically supports several hours of intermittent pumping, and can cover several days in an extended outage if the pump cycles rather than runs continuously.

Do I need one battery or two?

A 12V pump runs on a single AGM battery, or multiple in parallel for longer runtime. A 24V pump requires two AGM batteries in series. A major retailer guide sets the minimum at one deep-cycle battery — add capacity only if you need more hours.