Best Sump Pump Backup Battery

Battery-powered sump pumps are a type of emergency equipment that must operate reliably when required, and saving money on these pumps or their batteries may be a very expensive thing to do.

The sump pump backup battery must be strong enough to power the sump pump motor, and it must store enough energy to power the whole unit long enough to prevent as much damage as possible until mains power is restored.

Updated: November 10, 2022.

On This Page:

Battery Powered Sump Pumps Features and Specifications Sump Pump Battery Types Sump Pump Battery Boxes and Chargers Most Popular Group 27 Batteries Most Popular Group 31 Batteries Sump Pump Motors Operating Time Charts Best Sump Pump Backup Systems Few Final Words

 

Battery Powered Sump Pumps Features and Specifications

The most important feature of the battery-powered sump pump is the nominal strength of the pump motor, and it is usually given in the HPs.

The most common battery-operated sump pumps come with 1/6 HP, 1/4 HP, 1/3 HP, 1/2 HP, 3/4 HP, or even 1 HP motors.

Note: Mains-powered sump pumps, especially those powered by three-phase electric power, may feature several HPs motors, but these are mostly commercial units, rarely used at homes.

The most common type of battery-operated sump pumps are 12V sump pumps, with 24V sump pumps being used where a larger flow (gallons per hour, gallons per minute) or larger lift (given in feet) is required - or both!

• 12V sump pumps, normal conditions: go for a single, high-quality AGM battery,
• 12V sump pumps, a long operating time required: go for 2 or more high-quality AGM batteries and connect them in parallel - use the same battery model of the same brand, preferably from the same batch,
• 24V sump pumps: choose 2 high-quality AGM batteries and connect them in series.

Most popular sump pumps come with their own battery boxes and their own battery chargers, somewhat limiting the users regarding battery selection.

But, in the case of emergencies, one has to know that sump pumps don't operate continuously. For example, some motor controllers, when the alarm is triggered, will run the pump motor for 10-20 seconds continuously (to prevent short cycling) or until the float switch resets itself.

How often will sump pump cycle, depends on the sump pump volume (gallons per minute, for example) and the amount of incoming water.

In the case of a "worst case scenario" emergency, one may assume that the basement will be fully flooded and that the sump pump will work continuously until the battery is fully discharged - and that is really the worst-case scenario!

In that case, it is important to know how many gallons the sump pump may pump out on a single battery charge, depending on the required height.

All these values are good to know BEFORE choosing and installing a battery-powered sump pump.

Sump Pump Battery Types

The most popular sump pump battery types are lead-acid batteries:

• A Wet/Flooded Lead-Acid Battery is not a maintenance-free battery, nor is it a spill-proof battery. It is up to the user to periodically check the battery electrolyte and, if required, add distilled water. Wet lead-acid batteries may operate only in the upright position.
• A Gel-Cell Lead-Acid Battery features electrolytes in the form of silica gel, hence the name. Gel-cell batteries are maintenance-free and spill-proof batteries that can operate in almost any position except upside-down. They also cycle well and feature a long standby operating life.
• An AGM Lead-Acid Battery uses Absorbent Glass Matt and is also a maintenance-free and spill-proof battery that can operate in any position except upside-down. AGM batteries also cycle well and feature a long standby operating life.

Deep-cycle lithium batteries based on the Lithium Iron Phosphate (LiFePO₄) chemistry offer many benefits over AGM and Gel-Cell lead-acid batteries, including weight savings, longer standby life, a larger number of supported charging/discharging cycles, can operate in any position (even upside down), etc.

Also, with the advances in electronics and chemistry, lithium batteries don't cost (much) more than lead-acid batteries.

But, to operate safely, lithium batteries come with a Battery Management System (BMS) that monitors the battery charge and discharge parameters and, if required, protects the battery by shutting off the battery when some unwanted event happens.

That also means that if there is a glitch of some kind in the electric system, there is a risk of BMS shutting off the battery - the last thing one needs when the basement starts to flood.

Personally - when looking for a new sump pump backup battery, go for a good AGM or Gel-Cell battery and don't look any further.

Sump Pump Battery Boxes and Chargers

Sump Pump battery chargers operate as both chargers and maintainers, keeping the battery fully charged and in optimal condition over a long period of time.

If the sump pump doesn't have a reliable battery charger/maintainer, consider buying a dedicated one and connecting it to the battery.

Note: battery-powered sump pumps and similar emergency equipment MUST work reliably in the case of emergencies - if they don't operate reliably, what's the point of having them? Thus, any work on the sump pumps and their electric and electronic systems should be done by certified personnel or at least by someone who actually knows what to do. Seriously!

The preferred battery types of sump pump batteries are dual-purpose or deep-cycle AGM (Absorbent Glass Mat) SLA (Sealed Lead Acid) batteries or Gel-Cell SLA batteries.

Note: Car batteries are designed as starting/cranking batteries, being able to provide short bursts of power for shorter periods of time and thus are not the best choice for, what is easily possible, deep cycle applications. However, if your sump pump battery is dead for who knows what reason, hook your charged car battery and let it work - one(!) deep discharge cycle will not harm starting battery, but the cycle use may shorten its operating life significantly.

Thus, when choosing a sump pump battery, go for a reliable deep cycle (or maybe dual purpose) AGM battery from a reputable brand.

Most sump pump battery boxes accept BCI Group 27 batteries or BCI Group 31 batteries.

Their sizes are:

- Group 27 batteries (L x W x H): 12.0625 x 6.8125 x 8.875 (12 1/16 x 6 13/16 x 8 7/8) inches, 30.6 x 17.3 x 22.5 cm,

- Group 31 batteries (L x W x H): 13 x 6.8125 x 9.4375 (13 x 6 13/16 x 9 7/16) inches, 33 x 17.3 x 24 cm.

As one can see, the battery box designed to accept group 31 battery may also accept group 27 battery or any other battery group that is smaller than group 31 battery.

However, the smaller the battery, the smaller the capacity in Ah and, thus, the shorter the operating time of the sump pump when there is no mains power available.

The following two charts list the most popular group 27 and group 31 dual-purpose and deep cycle batteries.

Most Popular Group 27 Batteries

Note: Amazon links in the table open in the new windows ('Model' column), so feel free to check them for the most up-to-date prices and offers.

On average, Group 27 AGM batteries feature 20h capacity in the 90-100 Ah range, with Reserve Capacity (RC) values of 190-220 minutes.

Note: Optima batteries use spiral wound cells and are somewhat lighter than other batteries of similar size and feature smaller capacity and shorter RC. Other than that, they are popular and rather reliable batteries.

Most Popular Group 31 Batteries

Note: Amazon links in the table open in the new windows ('Model' column), so feel free to check them for the most up-to-date prices and offers.

On average, Group 31 AGM batteries feature 20h capacity in the 100-125 Ah range, with Reserve Capacity (RC) values of 200-260 minutes.

For smaller sump pump motors (1/2 HP or less), it is recommended to go for deep cycle/solar batteries, while for stronger sump pump motors (3/4 HP or more), it is recommended to choose dual-purpose batteries.

The reason is simple - at 1/2 HP (US HP -> 746W), a 12V motor draws at least 31.1 Amps, and that is not a problem for 100Ah deep cycle AGM batteries. However, at 3/4 HP, a 12V motor draws at least 46.7 Amps, and dual-purpose batteries may perform better.

At 1 HP, 12 motor draws at least 62 Amps, and the actual capacity of deep cycle batteries optimized for low discharge applications may deteriorate too much.

Rule of thumb: don't overthink - go for a reputable brand, just check the size of the battery and its Reserve Capacity value.

Sump Pump Motors Operating Time Charts

When trying to determine if the battery is good for certain applications, it is recommended to check the battery's constant current and constant power discharge charts.

One of the most popular and affordable 100 Ah batteries is the Mighty Max ML100-12 100 Ah AGM deep cycle battery.

The following charts list Mighty Max ML100-12 discharging features - constant current discharge characteristics (in Amps, at 77°F/25°C) and constant power discharge characteristics (in watts, also at 77°F/25°C):

Constant Current:

End Voltage Per Cell	Battery End Voltage	10 min	15 min	30 min	45 min	1 hour	3 hours	5 hours	10 hours
1.60 V	9.6 V	207	162	92.0	66.5	53.5	24.0	16.2	9.20
1.65 V	9.9 V	195	154	89.3	65.1	52.5	23.5	16.0	9.15
1.70 V	10.2 V	183	145	86.4	63.6	51.3	23.0	15.7	9.10
1.75 V	10.5 V	170	136	83.2	62.0	50.0	22.5	15.4	9.05
1.80 V	10.8 V	155	126	80.0	60.0	48.5	21.8	15.0	9.00

Constant Power:

End Voltage Per Cell	Battery End Voltage	10 min	15 min	30 min	45 min	1 hour	2 hours	3 hours	5 hours
1.60 V	9.6 V	2250	1830	1032	750	618	372.0	280.8	189.0
1.65 V	9.9 V	2178	1734	1008	738	606	366.0	276.0	187.2
1.70 V	10.2 V	2100	1686	978	726	594	358.2	270.6	185.4
1.75 V	10.5 V	2010	1626	948	708	582	349.2	265.2	183.0
1.80 V	10.8 V	1890	1548	912	690	570	339.6	259.6	180.0

Note: listed values are average values, and they vary slightly from the battery to the battery.

Sump pump motors come in different sizes:

- 1/6 HP: ~125 W, ~10.5 Amps, 5+ hours.

- 1/4 HP: ~187 W, ~15.6 Amps, ~5 hours, but end-voltage is rather low.

- 1/3 HP: ~250 W, ~21 Amps, 3+ hours,

- 1/2 HP: ~375 W, ~31.5 Amps, almost 2 hours,

- 3/4 HP: ~560 W, ~46.7 Amps, 1+ hour,

- 1 HP: ~746 W, 62.2 Amps. around 45 minutes, but the end-voltage is rather low.

These values are given for the brand-new, fully charged battery. Also note that sump pump motors are not working constantly, and these values are for constant operation.

For example, the sump pump features a 12V 1/2 HP motor, and it operates 15 seconds every 30 minutes. In that case, one may expect this battery to power such a sump pump for ~10 days!

If there are issues with the underground water levels and such a motor operates 15 seconds every 1 minute, one may expect this battery to power such a sump pump for ~8 hours.

Mighty Max ML100-12 offers good performances at an acceptable price, however, if the money is not an issue, there are better performers, for example, Renogy AGM and Gel-Cell batteries, Odyssey 31M-PC2150, NorthStar NSB-AGM31, and similar.

For example, Odyssey 31M-PC2150 is an expensive but otherwise excellent dual-purpose group 31 battery suitable for almost any situation, especially during cold weather.

The following chart lists the discharge features of the new, fully charged Odyssey 31M-PC2150 battery.

Time	Watts (W)	Amps (A)	Capacity (Ah)	Energy (Wh)
2 min	7025	678.5	22.4	231.8
5 min	4740	438.5	36.4	393.4
10 min	3176	285.9	47.7	530.4
15 min	2428	215.5	53.9	607.0
20 min	1980	174.1	58.0	659.2
30 min	1460	127.0	63.5	730.0
45 min	1059	91.2	68.4	793.9
1 hr	835	71.5	71.5	835.2
2 hr	461	39.0	78.0	922.2
3 hr	322	27.1	81.4	966.8
4 hr	249	20.9	83.8	996.8
5 hr	204	17.1	85.6	1020.0
8 hr	134	11.2	89.7	1070.4
10 hr	110	9.2	91.9	1095.9
20 hr	60	5.0	100.3	1191.9

As one can see, it is a much better performer than the  Mighty Max ML100-12 battery, but again, it costs more.

Best Sump Pump Backup Systems

There are many models of sump pump backup systems on the market, with each model offering somewhat different performances in terms of volume (gallons per minute, for example), lift height, and similar.

Here are some of the most popular models that have already been tried and tested by numerous real-life users.

WAYNE BGSP50 Guardian Premium Wi-Fi Connected Total Basement Protection System

AS its name suggests, the WAYNE BGSP50 Guardian Premium Wi-Fi Connected Total Basement Protection System is an all-in-1 basement protection system.

WAYNE BGSP50 features two sump pumps:

• AC sump pump, which can pump up to ~3800 GPH up to 10 feet,
• 12V 1/2HP DC sump pump which can pump up to ~2000 GPH up to 10 feet.

The unit doesn't come with a battery by default, but the manufacturer recommends their WAYNE 12V 75Ah battery.

One of the very interesting features is the support for the smartphone App, allowing the user to receive alerts.

WAYNE BGSP50 is a very sturdy sump pump system built from cast iron and aluminum.

WAYNE BGSP50 comes with a 5-year limited warranty.

Note: this is not a cheap unit, but what You pay is what You get...

For the most up-to-date offers and prices, feel free to check the Basement Watchdog BWE Basement Emergency Back-Up Sump Pump Amazon link (the link opens in the new window).

Basement Watchdog BWE Basement Emergency Back-Up Sump Pump

Basement Watchdog BWE Basement Emergency Back-Up Sump Pump is able to pump 2000 GPH at 0 feet height and 1000 GPH at 10 feet height.

Basement Watchdog BWE sump pump features a control unit with a dual float switch, a battery fluid level sensor (very helpful when having wet/flooded lead-acid batteries), a pump, a battery box, and a charger.

Note: Basement Watchdog BWE supports both wet and AGM lead-acid batteries. By default, the unit doesn't come with a battery.

Basement Watchdog BWE sump pump battery comes with a 2-year limited warranty.

For the most up-to-date offers and prices, feel free to check the Basement Watchdog BWE Basement Emergency Back-Up Sump Pump Amazon link (the link opens in the new window).

Zoeller 508-0006 Aquanot 508 Sump Pump Backup System

Zoeller 508-0006 Aquanot 508 Sump Pump Backup System features a 12V 1/3HP sump pump rated at ~1800 GHP with a 10 feet lift (~2000 GPH at 10 feet lift, AC powered).

The unit comes almost fully assembled, helping the user with installation - as always, it is highly recommended to have a professional install it and test it.

Note: the package does NOT feature a backup battery, but the battery box accepts both group 27 and group 31 batteries.

Zoeller 508-0006 Aquanot is a durable unit built from cast iron and thermoplastics.

The unit comes with a 3-year limited warranty.

For the most up-to-date offers and prices, feel free to check the Zoeller 508-0006 Aquanot 508 Sump Pump Backup System Amazon link (the link opens in the new window). 

---

Few Final Words

Best sump pump battery must be reliable and must tolerate float/standby use well - for at least 5-7 years, preferably 8+ years.

AGM SLA batteries are practically maintenance-free batteries, offering great reliability at affordable prices.

When looking for a new sump pump battery, check the dimensions of the available battery box/battery compartment and go for a reliable deep cycle/dual-purpose battery from reputable brands - don't go cheap since your home may depend one day on this battery.

The most common sump pump batteries are group 27 and group 31 batteries, so choose according to your own needs and preferences.

Note: when setting up a sump pump battery backup system, it is highly recommended to check the features of the battery charger and to replace it with a new one optimized for the battery that You have - it is an additional cost, but can prolong the battery lifetime...