The short answer
Backup sump pumps run on a 12 V deep-cycle battery, never a car starting battery. AGM batteries are sealed and maintenance free; flooded wet-cell batteries cost less but need water topped up and venting. More amp-hours means longer runtime.
AGM vs wet cell
| AGM | Flooded wet cell | |
|---|---|---|
| Maintenance | None | Check and top up water |
| Venting | Sealed | Needs ventilation |
| Price | Higher | Lower |
Batteries we cover
- Wayne WSB1275: 12 V 75 Ah, the battery Wayne uses for its runtime figures.
- Basement Watchdog BW-27AGM: check compatibility with pre-2014 controllers.
- Mighty Max ML75-12: 75 Ah group 24 AGM, no wiring harness.
Compatibility
Check that the charger in your backup supports the battery type, and that the battery fits the box or case. Some older controllers show false alarms with AGM batteries.
Products mentioned
Questions readers ask
Can I use a car battery for a backup sump pump?
No. Starting batteries aren't built for deep discharge; use a deep-cycle battery.
How often should I replace the battery?
Check it yearly and replace it when it no longer holds a charge, often every few years.