The Monster Marine Lithium "Sea Lion" is a 12V 300Ah dual-purpose lithium (LiFePO4) battery that both cranks your outboard engine and powers a full day of onboard electronics from a single battery. It is built for offshore and freshwater anglers who run heavy electronics packages and want to replace a traditional 12V AGM or lead-acid starting and house battery with one lithium unit.

With 300 amp-hours of capacity and 3,840 watt-hours of energy, the Sea Lion is designed for boats running multiple graphs, forward-facing sonar, 360 imaging, bait tanks, pumps, shallow-water anchors, and lighting. Its large capacity is intended for anglers who want multi-day runtime between charges rather than recharging after every trip. On the starting side, Monster Marine Lithium rates the battery at 1,500 cold cranking amps to turn over high-output 2-stroke and 4-stroke outboards.

Built with Grade A LiFePO4 cells and an advanced Battery Management System (BMS), the Sea Lion is engineered for the demands of the marine environment. Bluetooth monitoring through the MMLithium app lets you check voltage, state of charge, and real-time performance from your phone. At 56 lbs, it weighs less than a comparable AGM battery bank, helping reduce overall boat weight.

The Sea Lion is a strong fit if you want a single high-capacity battery that starts your engine and runs a large electronics load, and you have a battery compartment that can accommodate its size (confirm fitment in the Compatibility tab before ordering).

Key Features

Here is what the Sea Lion offers and why each feature matters for your boat:

• Dual-Purpose Design: Built for both engine cranking and deep-cycle use, so one battery can handle starting and running your electronics instead of two separate batteries.
  
  
• 300Ah / 3,840Wh Capacity: A large energy reserve for running multiple graphs, sonar, bait tanks, pumps, and lights, supporting longer runtime between charges.
  
  
• 1,500 CCA Starting Power: Manufacturer-rated cold cranking amps to start high-output 2-stroke and 4-stroke outboards. CCA is provided as a starting-power reference for lithium.
  
  
• Bluetooth Monitoring: The MMLithium app shows voltage, state of charge, and performance on your smartphone, so you can track how much power you have left on the water.
  
  
• Advanced BMS Protection: Protects against overcharge, over-discharge, over-temperature, short circuit, and includes single-cell protection for safer, more stable operation.
  
  
• Grade A LiFePO4 Cells: Lithium iron phosphate chemistry chosen for stability and long cycle life, rated by the manufacturer at 4,000+ cycles.
  
  
• Lightweight Construction: At 56 lbs, it weighs less than a comparable AGM setup, reducing boat weight and easing installation.
  
  
• Top-Post & M8 Bolt Terminals: A combination terminal type that gives you flexibility for different cable and connector setups.

Compatibility & Fitment

The Sea Lion is a 12V dual-purpose lithium battery built to both crank an outboard or sterndrive engine and run a heavy onboard electronics load on the same battery. It is designed for anglers and offshore boaters who want to replace a traditional 12V AGM or lead-acid starting and house battery with a single lithium unit. Before ordering, confirm the fitment points below to make sure it works with your boat, your engine, and your charging setup.

Will it fit my battery compartment?

The Sea Lion measures 13.5 in (L) x 7.25 in (W) x 9.75 in (H) and weighs 56 lbs. This is slightly larger than a standard BCI Group 31 footprint, so it will not always drop into a molded Group 31 tray or battery box. Measure your battery compartment, tray, and hold-down before purchasing, especially on bass boats and center consoles with tight factory battery locations. The battery uses a top-post and M8 bolt terminal combination, so check that your existing cable ends are compatible or plan for the correct ring terminals.

Engine compatibility

According to Monster Marine Lithium, the Sea Lion has been tested as a cranking battery on the following engines:

• Mercury Pro XS (2-stroke and 4-stroke)
  
  
• Mercury Verado 4-stroke
  
  
• Yamaha SHO and 4-stroke (legacy and current)
  
  
• Yamaha HPDI
  
  
• Suzuki 4-stroke
  
  
• Evinrude 2-stroke
  
  
• MerCruiser 4-stroke

Engine manufacturers set their own guidance on lithium starting batteries, and that guidance can change by model year. Before installing any lithium cranking battery, confirm compatibility with your engine manufacturer or dealer and review your engine warranty terms, so you know the battery is approved for your specific motor and charging system.

Suitable applications

• Offshore and freshwater fishing boats running multiple graphs, forward-facing sonar, and 360 imaging
• Bass boats and center consoles needing one battery for starting plus all-day electronics
• Boats running bait tanks, livewells, pumps, shallow-water anchors, and lighting
• Multi-day trips where recharging between outings is not always possible
• Direct replacement for a 12V AGM or lead-acid starting/house battery in a compatible 12V system

Charging compatibility

Use a lithium (LiFePO4) compatible charger with a charge voltage in the 14.4 to 14.6V range. Do not use a charger set to a lead-acid profile that exceeds this range. If you plan to charge from your engine while running, confirm your alternator's regulated output voltage is within the battery's charge range and consult your engine manufacturer's guidance on charging lithium batteries.

Specifications

The specifications below are based on Monster Marine Lithium's published information for model MML-15300JB. Where the physical battery label differs from published values, the label is the authoritative source.

Note: Monster Marine Lithium does not publish an IP rating, maximum charge current, self-heating specification, or series/parallel rating for this model. If any of these apply to your installation, contact us.

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