A Battery Backup Sump Pump for When the Power or the Pump Fails

The problem

A sump pump tends to fail when you need it most. Big storms bring heavy rain and power outages together, so the pump loses power right as the pit fills fastest. Even with power, a stuck float switch, a worn-out motor or more water than one pump can handle can flood a basement in a few hours.

A basement full of water means ruined flooring, drywall, furnaces, water heaters and everything stored down there. A battery backup pump is the insurance against all of those failures at once.

What you need

• A battery backup sump pump system. A second pump that sits in the pit beside your main pump, a control panel with a built-in battery charger, and its own float switch. Many systems use two floats for extra safety.
• A deep-cycle or AGM battery made for the system. Maintenance-free AGM batteries don't need water topped up, and the maker's runtime chart tells you how long each battery lasts.
• A check valve for each pump. A check valve is a one-way door in the pipe: water can flow up and out, but a flap closes behind it so the water in the pipe can't fall back into the pit, or cycle back down through the other pump. Clear check valves cost a bit more, but they let you watch the flap open and close and see that each pump is actually moving water. I was too cheap to pay extra for them, but I'd consider them next time.
• PVC pipe and fittings to tie the backup into your discharge line, or a plumber to do it.
• A few bricks for the bottom of the pit (more on that below).
• A healthy main pump. The backup is a second line of defense, not a replacement for a good primary pump.

How it works

The main pump does the everyday work on house power. The backup pump stays idle, with its float set to turn on a little higher than the main pump's. If water ever rises past the main pump's normal level, because the power is out, the main pump has failed, its float is stuck or there's simply too much water, the backup's float lifts and the battery pump starts.

At the same time, the control panel:

• Sounds an alarm and shows what's wrong, such as a power failure, a pump problem, a low or old battery, or a loose connection.
• Keeps the battery charged whenever the power is on, and recharges it after the backup runs.
• Watches itself around the clock, so a worn-out battery or a bad connection gets flagged before you need it.

How long it can protect you depends on how much water comes in. For my system and battery, the maker rates it at about 80 hours of pumping at a 10% duty cycle, meaning the pump runs about one minute in ten. A backup pump also moves less water than a big main pump: mine is rated about 2,000 gallons per hour with no lift and about 1,000 at 10 feet.

Setting it up

This involves plumbing, electricity near water and a heavy battery. Follow the system's instructions and local codes, and hire a plumber if you're not comfortable with PVC work. I did mine myself. Working with PVC isn't too difficult: you cut the pipe, glue the joints and screw on the threaded connectors.

1. Check the main pump first. If it's old or struggling, replace it at the same time.
2. Put a few bricks in the bottom of the pit and set the pumps on them. This is a trick I like: it keeps the pumps up out of the silt that settles on the bottom, so they don't clog as fast. I use bricks with holes in them so they take up less of the pit's water space.
3. Set the backup's float a bit higher than the main pump's, so the backup only runs when the water rises past the main pump's normal level. The backup pump itself doesn't have to sit higher if you can mount its float switch higher, as long as there's room in your pit. Follow the maker's height guidance.
4. Give each pump its own check valve, then join both lines into the discharge pipe as the instructions show.
5. Don't forget the weep holes. A weep hole is a small hole, often about 3/16 inch, drilled in the discharge pipe between the pump and its check valve, a few inches above the pump and below the water line. When a pump shuts off, water drains back out of that short section of pipe and leaves a pocket of air. Next time the pump starts, it can just spin in that air without pushing any water past the check valve. That's called an air lock, and the pump runs, overheats and never empties the pit. The weep hole lets the trapped air escape so the pump can prime. Some pumps have one built in and some makers say not to add one, so check each pump's instructions for whether you need it and what size.
6. Set the control panel and battery up off the floor, near the pit and an outlet, and connect them as directed. Plug the charger in.
7. Let the battery charge fully.
8. Add an extra alarm or two. I hang a battery-powered water alarm in the pit; its sensor is on a wire you can pull out to any length, so I hang it a few inches above the highest the water should ever get. If the water reaches it, both pumps are losing, and I get warned earlier than a sensor on the floor would. I also have a leak sensor on the floor outside the pit that alerts my phone.
9. Test it: unplug the main pump and fill the pit with a bucket or hose until the backup starts. Confirm it pumps out, and that the alarm sounds. Then plug the main pump back in.

Upkeep

• Test it every few months: unplug the main pump and raise the water until the backup runs. Some systems also run a self-test on their own. My Home Maintenance Checklist reminds me every 3 months.
• Pay attention to the panel. A battery warning or corroded-terminal alert is the system asking for help before a storm, not during one.
• Act on the leak sensor's offline alerts too. If the Aqara sensor beside the pit loses contact with the hub or its battery runs low, the app tells me. I want to hear about a dead sensor the day it goes quiet, not the day the pit overflows.
• Replace the battery every few years, or when the panel says so. Mine lasted about four years.
• Clean the pit once a year, removing silt and debris that can jam floats or clog pumps. I did mine this year when I swapped the battery.
• Don't let anyone drain your water heater into the sump. If someone comes to flush your water heater, they may want to empty the tank down the sump pit. It's easy for them, but it sends all the sediment from the tank through your pumps and check valves. Ask them to run a hose out to the yard or to a floor drain instead.
• Keep the outside discharge line clear, and make sure it can't freeze shut in winter.

What it doesn't fix

• Very long outages. A battery covers hours to a few days, depending on how much water comes in. Beyond that you need a generator, or someone to swap in a charged battery.
• A clogged or frozen discharge line. Both pumps push water out through it, so if it's blocked, neither can help.
• Telling you when you're away. The panel's alarm is loud, but only someone at home hears it. Pair it with an alert that reaches your phone, like a power-loss alert on the sump outlet and a leak sensor near the pit.

Next time: I'd consider paying the extra for clear check valves. During a test, being able to see each flap open is the quickest way to know both pumps are actually moving water.

Products named here are ones I bought and use myself. No sponsorships, no affiliate links.
I'm not a licensed electrician or other professional. Follow the product instructions and local codes, and call a qualified pro if you're unsure. Full disclaimer.