Deep Cycle Marine Battery

Mighty Max ML55-12 12V 55Ah Deep Cycle AGM Battery

Mighty Max ML55-12 is a very popular general-purpose Absorbent Glass Mat (AGM) Valve Regulated Lead-Acid (VRLA) battery, commonly used as wheelchair battery, off-the-grid battery, lightweight trolling motor battery, in medical and security systems, and similar.

Mighty Max ML55-12 is a sealed, maintenance-free battery, that features a low self-discharge rate and can be stored for a longer period of time, especially at lower temperatures.

Published: May 26, 2021.

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Mighty Max ML55-12 Features and Specifications

Mighty Max ML55-12 is a spill-proof, maintenance-free battery that features physical dimensions of (L x W x H) 9.0 x 5.5 x 8.19/8.93 inches (228 x 140 x 208/227 mm) and weighs ~37 pounds (~16.8 kg).

Mighty Max ML55-12 AGM deep cycle battery is a general-purpose battery that belongs to the BCI Group 22NF, a medium size and weight group of batteries used in a broad range of applications, including automotive, marine, and even light industrial applications.

The battery can be used for both deep cycle and float/standby applications - it should not be used as a cranking/starting battery, except for very small gas engines, commonly found at small power generators and similar applications.

The grid is built using lead, calcium, tin alloy, ensuring high power density, low self-discharge rate, and tolerance to deep discharges.

Nonetheless, in order to increase the operating lifetime of the battery, the battery should not be discharged below 80% DoD or preferably not below 50% DoD.

The actual capacity of the battery over time drops and it depends, among other things, on the number of charging/discharging cycles:

- 100% DoD: ~180 cycles down to 80% of nominal capacity, ~220 cycles down to 60% of nominal capacity,

- 50% DoD: ~360 cycles down to 80% of nominal capacity, ~470 cycles down to 60% of nominal capacity,

- 30% DoD: ~840 cycles down to 80% of nominal capacity, ~1150 cycles down to 60% of nominal capacity.

So, if possible, don't discharge your lead-acid batteries too low.


Mighty Max ML55-12 battery comes with F5 terminals, with the right terminal on the left side of the battery.

Also, the battery comes with a pair of carrying handles, allowing the user to carry the battery to shorter distances.

The nominal voltage of the Mighty Max ML55-12 battery is 12V, and the battery is labeled as a "55 Ah" battery. But, the nominal 20h capacity is 58 Ah and the 10h capacity is 55 Ah.

When the fully charged battery is stored at 68°F (20°C), the capacity is lost over time:

- 3 months: down to 90% capacity,

- 6 months: down to 80% capacity,

- 12 months: down to 60% capacity.

During storage, it is recommended not to let the battery capacity drop below 80% - store the battery in the cold area, fully charge it and if possible, keep it connected to the good AGM battery maintainer.

Depending on the type of use, the charging voltage should be (@77°F/25°C):

- float use: 13.6 - 13.8 volts,

- cycle use: 14.4 - 14.7 volts.

Note that the maximum charging current should be 16.5 Amps, preferably less.

The fully charged Mighty Max ML55-12 battery features internal resistance of 8mΩ and can provide up to 550 Amps for 5 seconds if needed.

Note: although 550 Amps for 5 seconds can crank larger gas and even diesel engines, this battery is not intended for such use. It can be used to crank smaller engines found at portable power generators, small lawn tractors, small marine inboard and outboard engines, and similar, but that is all since anything stronger can ruin this (or any similar) deep cycle battery.

Mighty Max ML55-12 AGM Battery Constant Discharge Charts

The following charts show Mighty Max ML55-12 battery constant current and power discharge rates (@77°F, 25°C).

Constant current discharge (values are given in Amps):

Cell End Point Voltage Battery End Point Voltage 5 min 10 min 15 min 30 min 1h 3h 5h 10h 20h
1.60 V 9.6 V 172 130 98.0 58.2 35.2 14.5 9.98 5.69 2.98
1.65 V 9.9 V 162 123 94.0 56.5 34.4 14.3 9.88 5.65 2.97
1.70 V 10.2 V 152 115 89.5 54.5 33.6 14.0 9.75 5.60 2.95
1.75 V 10.5 V 141 108 84.4 52.9 32.7 13.7 9.60 5.55 2.93
1.80 V 10.8 V 128 100 80.5 51.0 31.6 13.4 9.43 5.50 2.90

Constant power discharge (values are given in watts):

Cell End Point Voltage Battery End Point Voltage 5 min 10 min 15 min 30 min 45 min 1h 2h 3h 5h
1.60 V 9.6 V 1920 1482 1146 690 530.4 428.4 255.0 182.4 116.4
1.65 V 9.9 V 1860 1398 1098 678 348.4 418.8 250.2 177.0 114.6
1.70 V 10.2 V 1746 1320 1056 666 511.8 409.2 244.8 173.4 112.8
1.75 V 10.5 V 1626 1242 1008 654 500.4 399.6 238.8 169.8 112.2
1.80 V 10.8 V 1542 1152 954 642 487.2 389.4 232.8 166.2 111.0

For example, Mighty Max ML55-12 can provide 31.6 Amps or ~390 watts continuously for one hour, with the voltage not dropping below 10.8 volts.

Also, it can provide 172 Amps for 5 minutes with the voltage drop down to 9.6 volts - again, feel free to crank small internal combustion engines that require 100-150 starting Amps.

How to Charge Mighty Max ML55-12 Battery

If the battery is used within the device or vehicle that features the built-in battery charger suitable for AGM battery like Mighty Max ML55-12 battery, let that charger charge the battery.

If the battery is charged directly with the charger, an intelligent AGM battery charger is recommended.

Such battery chargers analyze the battery first and then charge the battery according to its condition (deep recovery, for example), temperature (temperature compensation required), etc.

Although the maximum charging current is 16.5 Amps, smaller battery chargers are recommended, up to 10 Amps are perfectly fine:

When the ML55-12 battery is discharged down to 50% DoD, a 10 Amps battery charger can recharge it in ~3 hours, if everything is alright with the battery. If there are issues that require cell normalization, charging will last longer, but not always, of course.

Mighty Max ML55-12 Battery as Trolling Motor Battery

Mighty Max ML55-12 battery is often used as a lightweight lead-acid battery for powering smaller trolling motors.

When trolling motor draws up to 30 Amps, Mighty Max ML55-12 can be used to power such motor for 1 hour, with some energy left in the battery, for safety reasons.

Also, Mighty Max ML55-12 can be used to power even larger trolling motors - for example, it can power 50 Amps trolling motor for up to 30 minutes continuously.

And if such motor is not pushed to the limit all the time, the battery can last longer, sometimes much longer.

Mighty Max ML55-12 Battery as Wheelchair Battery

Mighty Max ML55-12 is commonly used as a wheelchair battery, either one battery or two batteries, depending on if the electric system is 12V or 24V.

When a single Mighty Max ML55-12 battery is used to power 2x250 watts motors, a new fully charged Mighty Max ML55-12 at 77°F (25°C) can power such motors up to 45 minutes, with the voltage not dropping below 10.5 volts.

When two Mighty Max ML55-12 batteries are connected in series (24V), they can power 2x250 watts motors for almost two hours.

Since wheelchairs are not used continuously and especially not all the time at full power, the Mighty Max ML55-12 battery can last longer than it is given in these two examples.

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Mighty Max ML55-12 battery comes with a 1-year warranty.

Long Story Short: If You are looking for a Group 22NF battery that features ~55-60 Ah capacity and is a maintenance-free, spill-proof AGM battery, consider this model.

Mighty Max ML55-12 is well designed and built battery, very popular in its class thanks to its features and affordable price.

For more reviews and recommendations, feel free to check the Mighty Max ML55-12 12V 55Ah AGM Deep Cycle Battery Amazon link (link opens in the new window).

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