The Millertech 12V 135Ah Dual Purpose is a LiFePO4 (lithium iron phosphate) battery built to do two jobs from a single Group 31 box: deliver the high burst of current needed to start your outboard, and provide the steady, stable deep-cycle energy that today's electronics-heavy boats demand. It carries a 1500 CCA cranking rating and 135Ah of storage, with a Battery Management System (BMS) that handles over-charge, over-discharge, over-current, and temperature protection automatically.

For most bass, bay, and multi-species anglers, this is the "one battery" answer, it cranks the motor and runs the console and bow electronics off the same source, which is why owners routinely report replacing a pair of lead-acid batteries with a single Millertech and shedding weight in the process.

Standard vs. Heated, which variant?

This page offers two versions. The Standard (12135-DP) is the right choice for most boaters, especially in warm climates. The Heated (12135-DPH) adds smart self-heating for cold-weather charging: when it is connected to a charger and the internal temperature drops to 32°F, the heater automatically warms the battery to 34°F before charging resumes, so freezing temperatures don't stop you from topping off. Note the heater runs only while connected to a charger, it is a cold-charging feature, not a discharge warmer. If you launch, store, or charge in sub-freezing conditions, choose Heated.

Is this the right battery for you?

Choose this battery if you:

• Run a Mercury outboard that permits lithium starting batteries and want one battery to crank the motor and run your electronics.
• Fish with forward-facing sonar and multiple graphs and keep pulling your AGM too low by the afternoon.
• Want to cut weight by replacing one or two lead-acid batteries with a single 33 lb lithium.
• Fish tournaments or long days and need stable voltage from launch to weigh-in.
• Want Bluetooth monitoring so you know your charge before you leave the ramp.

Consider a different setup if you:

• Need a dedicated 24V or 36V trolling motor bank use a Millertech 24V/36V trolling battery for that job.
• Run an engine whose maker prohibits lithium cranking batteries and you don't want to add a DC-DC charger pair an AGM cranking battery with a lithium house battery instead.
• Only need light, occasional electronics power on a small boat, a smaller-capacity battery may be enough.

Popular with:

• Tournament bass anglers and weekend fishermen
• Bay boats, multi-species, walleye, crappie, and musky rigs
• Electronics-heavy boats running forward-facing sonar
• RV, off-grid, solar, and tiny-home 12V systems

Why upgrade from AGM

Serious anglers move off AGM and flooded lead-acid for a handful of concrete reasons:

• Voltage stays flat. Lead-acid sags as it drains, which is exactly when forward-facing sonar starts dropping out. LiFePO4 holds a stable voltage deep into the discharge, so your graphs stay lit and connected.
• More usable capacity. You can safely use nearly all of a lithium battery's rated capacity, versus roughly half of a lead-acid battery, so 135Ah of lithium does the work of a much larger AGM.
• Weight. At 33 lb, it is a fraction of a comparable lead-acid starting battery, and consolidating two batteries into one compounds the savings.
• Faster charging and no memory effect. Charge it at any state of charge without harming capacity.
• Long, maintenance-free service life. Millertech rates the Sport Series for many times the cycle life of lead-acid, backed by a 10-year warranty.

Applications & Compatibility

Common applications

• Bass boats, single cranking + electronics battery.
• Bay boats, multi-species, and center-console rigs, as a starting battery, house/electronics battery, or both.
• Forward-facing sonar setups, a dedicated electronics battery for FFS and networked graphs.
• RV and campers, dual-purpose starting and house power.
• Off-grid, solar, and tiny homes, 12V storage with a rugged, sealed case.
• Golf carts and utility vehicles, 12V applications needing both cranking and cycling.
• Emergency and backup power.

Boat, engine & electronics compatibility

Boats it's commonly installed on

As a Group 31 dual-purpose battery, it drops into the standard trays used across most fishing platforms, Skeeter, Ranger, Phoenix, Bass Cat, Nitro, Triton, Caymas, Vexus, Blazer, Falcon, Xpress, SeaArk, Lund, Alumacraft, Crestliner, and Tracker among them. If you're searching for a battery for a specific hull (for example, a Ranger Z520 or a Phoenix 921), the fit comes down to your Group 31 tray and your engine's charging requirements, confirm both before ordering.

Engines

Millertech states this battery meets the cranking and charging requirements for Mercury 4-stroke outboards up to 250 hp (Pro XS, FourStroke, and Verado families), consistent with Mercury's service bulletin permitting lithium starting batteries on qualifying engines. Owners have run them since 2016.

For other brands: Yamaha (SHO, VMAX), Suzuki, Honda, Tohatsu verify with the engine manufacturer first. Several have not approved lithium cranking batteries, or require specific conditions. When in doubt, run a dedicated AGM cranking battery alongside a lithium house battery with a DC-DC charger. Always confirm against your engine's current manual before converting.

Electronics it powers

This battery supplies stable 12V power to essentially any marine electronics you can wire to it. Because LiFePO4 holds a flat voltage deep into the discharge, voltage-sensitive gear stays lit and connected far longer than on a sagging lead-acid battery. Commonly powered gear includes:

• Forward-facing sonar (FFS): Garmin LiveScope (LVS34, LVS62), Humminbird MEGA Live and MEGA Live 2, Lowrance ActiveTarget and ActiveTarget 2.
• Chartplotters and graphs: Garmin, Humminbird, and Lowrance multifunction displays, including networked NMEA 2000 setups.
• Trolling motors (as a house/electronics source, not the trolling bank): Minn Kota Ultrex, Quest, and Ghost; Garmin Force and Force Pro; MotorGuide Tour Pro; Rhodan.
• Accessories: Power-Pole shallow-water anchors, livewell and bilge pumps, aerators, lighting, stereo, and USB charging.

Electronics are electrical loads, not compatibility restrictions any 12V device can be powered. Naming a brand here does not imply endorsement by that manufacturer.

Real-World Performance

Boaters have shared the setups they power from a single 12V 135Ah Dual Purpose. Individual results vary with your loads, run time, and how you fish, but these give a realistic picture of what one battery handles:

Estimated runtime by setup

Estimates only, based on typical 2-3A draw per graph or forward-facing sonar module and 135Ah of capacity; engine cranking draw is negligible over a day. Real results vary with your exact electronics, backlight level, and pump duty cycle. Putting units in standby during runs extends runtime.

Charging & Installation

What charger should I use?

Use a lithium (LiFePO4) charger. This battery is built around a 14.6V max charge voltage, and a lead-acid or AGM charger set to higher absorption or equalization voltages can trip the BMS or stress the cells. Equalization is not recommended turn it off if your charger has it. Millertech's recommended charge current is about 30A, and the battery accepts up to 135A maximum. Matching chargers below.

Can I use a NOCO or my existing onboard marine charger?

Yes, provided it has a true lithium/LiFePO4 mode. Many marine chargers (including NOCO Genius models) offer a selectable lithium profile. Avoid charging on a lead-acid-only profile with high equalization voltage.

Can I charge it from my outboard's alternator while running?

This is the most important compatibility question for a lithium starting battery, so here is the straight answer. Because lithium has low internal resistance, it can pull a high charge current, and a marine alternator running at full output for long stretches generates heat. Millertech states this battery meets the cranking and charging requirements to work with Mercury 4-stroke outboards up to 250 hp, and reports customers running them since 2016. Mercury has issued a service bulletin permitting lithium starting batteries on qualifying engines when the battery meets their specifications.

Always confirm against your own engine's manual or service bulletin before installing. Yamaha, Suzuki, and Honda have generally not approved lithium starting batteries at the time of writing; if you run one of these engines, verify with the engine manufacturer first, or run a dedicated AGM cranking battery with a lithium house battery and a DC-DC charger.

Storage and winterizing

LiFePO4 stores well. Charge it, disconnect it, and keep it in a cool, dry place; it does not need a constant trickle charge. Self-discharge is under 4% per month.

Cold weather & low-temp protection

Every version of this battery includes built-in low-temperature charging protection. The recommended charge temperature range is 25°F-131°F, and the BMS blocks charging when the battery is too cold to prevent the permanent damage (lithium plating) that occurs if a LiFePO4 cell is charged below freezing. The battery still delivers power in cold conditions, but its rated operating range is 32°F-130°F and usable capacity drops as temperatures fall; the low-temp protection specifically prevents charging below freezing.

The Heated (12135-DPH) version adds a smart heater for cold-weather charging: connected to a charger, if internal temperature drops to 32°F the heater warms the battery to 34°F, then charging resumes automatically. The heater runs only while a charger is connected. If you fish, store, or charge in freezing conditions, the Heated variant removes the guesswork.

Trolling motors (24V / 36V)

This battery is designed as a 12V cranking and house battery. For a 24V or 36V trolling motor, the cleanest setup is a dedicated Millertech 24V or 36V trolling battery sized to your motor, kept separate from the starting battery. That is how most owners run it: one Dual Purpose for the boat, a separate higher-voltage battery for the trolling motor.

Some anglers do wire two 12V 135Ah Dual Purpose batteries in series to build a 24V bank. If that is your plan, confirm the exact series/parallel configuration and limits with Millertech before wiring, and always use identical batteries (same model, capacity, and age) with proper fusing. Contact us and we'll confirm the right configuration for your motor.

Installation

• Fitment. This is a true BCI Group 31 (13″ x 6.75″ x 9″) and drops into standard Group 31 trays. A matching tray is available if you need one.
• Terminals. Heavy-duty M8 bolts and washers in 18-8 stainless. Torque terminal connections to 10 ft-lbs. If you are stacking multiple lugs, you may need to remove a washer or use a slightly longer bolt to fully seat into the copper pillar.
• Orientation and mounting. Seat the battery in the tray and secure with hold-down straps; the case has molded strap grooves to keep straps in place.
• Wiring. Use correctly sized cables and fuse/breaker protection appropriate to your loads. Connect electronics and accessories to the battery through a fused distribution point.
• App. Download the Millertech Bluetooth app to monitor charge percentage, voltage, current, and temperature.

Specifications

Specifications per Millertech published data sheets. Confirm against the current manufacturer manual for your exact SKU.

AGM vs. Millertech lithium

Frequently Asked Questions

Why Buy From Us

Choosing a $900+ lithium marine battery is about more than picking specs, it's about matching the right battery to your boat, engine, electronics, and charging system, and knowing someone has your back if a question comes up. That's where we come in.

We help you choose the right battery

No two boats are set up the same way. Before you order, we can walk through your boat make and model, engine and horsepower, tray dimensions, number of displays, forward-facing sonar, trolling motor voltage, existing charger, and climate. Our goal isn't to sell you the biggest battery, it's to match the battery to your application, and to tell you plainly if a different configuration fits better.

Support before and after the sale

Reach out before ordering and we'll review your setup. We can help with choosing the right model, Standard vs. Heated, charger compatibility, Group 31 tray fitment, charging questions, basic installation guidance, product registration, and warranty support. Most questions come after installation, when you add another graph, upgrade a charger, or plan future electronics and we're here for those too.

Genuine, warranty-backed products

Every Millertech battery we sell is a genuine, current-version product with full manufacturer-backed warranty coverage and original documentation. Millertech's 10-year warranty requires no proof of purchase and covers shipping both ways.

Fast, secure shipping

Your battery ships securely packaged with tracking provided once your order is processed. Shipping is free.

Still not sure it's the right fit?

Contact us before you order and we'll review your boat, engine, charger, electronics, sonar, trolling motor, climate, and any planned upgrades, then recommend the configuration that fits your setup. We'd rather help you get the right battery than sell you the wrong one.

What's included

• Millertech 12V 135Ah Dual Purpose battery (Standard or Heated variant selected)
• Terminal hardware (M8 18-8 stainless bolts and washers)
• Bluetooth app access (iOS and Android)
• Product manual and warranty documentation