Sump pumps · battery backup
Backup pump batteries: AGM vs wet cell, and upkeep
A battery-backup sump pump is only as dependable as its battery. The pump and controller can sit ready for years, but a battery that has quietly aged out will not carry the backup through a long outage. Here is how sealed AGM and wet-cell (flooded) batteries compare, how to check one, and when to replace it.
Bottom line
Use the battery type and size the backup pump maker specifies — usually a 12-volt deep-cycle battery, not a car starting battery. Sealed AGM batteries cost more but need no water top-ups and suit a basement with little ventilation; wet-cell batteries need fluid checks with distilled water and good ventilation. Date the battery, test the backup on a schedule, and replace the battery on the maker’s timeline or when the controller flags it.
AGM vs wet cell at a glance
| Sealed AGM | Wet cell (flooded) | |
|---|---|---|
| Maintenance | No water top-ups | Check electrolyte level and add distilled water as needed |
| Venting | Sealed, very low gassing in normal use | Vents hydrogen while charging; needs ventilation and a vented battery box |
| Spills | Spill-resistant | Acid can spill if tipped or overfilled |
| Cost | Typically higher up front | Typically lower up front |
| Charger match | Needs a controller that supports AGM charging | Needs a controller that supports flooded charging |
| Best fit | Finished basements, owners who will not check fluid | Owners who will do regular checks in a ventilated space |
Pick the right battery
- Follow the pump maker. The controller’s charger is designed for certain battery types and sizes. Basement Watchdog, for example, recommends its BW-27AGM and advises against automotive batteries.
- Deep-cycle, not starting. Car batteries are built for short bursts and wear out quickly under repeated deep discharge.
- Size affects runtime. Larger capacity (amp-hours) runs longer; some systems allow two batteries in parallel. Runtime claims are estimates at a stated duty cycle — heavy inflow drains a battery faster.
- Group size and terminals. Confirm the battery fits the box and the terminals match the controller leads.
Upkeep checklist
- Write the install date on the battery.
- Check the controller monthly for alarms or battery warnings; many have a test button.
- Run the backup seasonally — unplug the primary and fill the pit, or use the controller test — see how to test a sump pump.
- Inspect terminals for corrosion and loose connections; clean per the battery maker’s instructions.
- Wet cell only: check fluid level and top up with distilled water to the indicated level — never tap water, never overfill.
- Keep it off the wet floor and in its box, above the pit and away from splashes.
When to replace the battery
- The controller reports a weak or failed battery.
- The backup runs noticeably shorter during a test than it did when new.
- The case is swollen, cracked, or leaking.
- It has reached the replacement interval in the pump or battery manual.
Recycle old lead-acid batteries where you buy new ones or at a local collection site — do not put them in household trash.
Safety
- Lead-acid batteries hold enough energy to cause burns or sparks; remove jewelry and use insulated tools.
- Connect and disconnect in the order the manual gives, and keep sparks and flames away from wet-cell vents.
- Wear eye protection and gloves when handling wet-cell batteries.
When to call a pro
- You are unsure about wiring, need a dedicated outlet, or the controller trips a breaker — call a licensed electrician.
- The backup pump runs but cannot keep up with inflow — a licensed plumber or waterproofing pro can size a better system.
- You want a backup that does not depend on a battery — ask about water-powered options and local backflow rules.
Getting help
For wiring questions start with a licensed electrician; for pump sizing and installs, a licensed plumber or waterproofing pro. R.E. Merrill & Associates has handled industrial and municipal pumps and fluid systems since 1987 — contact R.E. Merrill & Associates with general pump questions.
Related across the site
- Freeze protection for well houses and pressure tanks — another outage-season checklist
- Is my softener working? — a quick check for the other basement water system
Sources
- Battery-backup sump pump manufacturer manuals — approved battery types, charging, runtime ratings, and test procedures.
- Lead-acid battery maker guidance — AGM vs flooded maintenance, ventilation, safety, and recycling.