Two things to check:

• Charger: use one built for lithium. An old lead-acid or AGM charger could overcharge a lithium battery, or it may not have enough voltage to charge it fully.
• Terminals: use ring terminals, not clamps.

For the battery your outboard charges, choose a Millertech Dual Purpose battery. It's built to work with your outboard's alternator. If you're not sure about your setup, call our Support Team at 855-MAX-LITH.

To winterize:
• Charge the battery fully.
• Disconnect every load and cable.
• Store it with nothing attached.
LiFePO4 loses only about 1–3% of its charge per month, so it holds charge well over the off-season. Millertech’s “How to Winterize Your Boat’s Millertech Batteries” video walks through it.
How to Winterize Your Boat's MillerTech Batteries

The simplest way is one 36V battery, which needs no series wiring. If you’re wiring three 12V batteries in series, Millertech allows up to four in a string. Watch the “Series & Parallel Battery Connections 101” video. Torque terminals to 10 ft-lb.
Series & Parallel Battery Connections 101

Divide the battery's amp hours (Ah) by the charger's amps. A 100Ah battery takes 10 hours to charge from empty with a 10A charger, or five hours with a 20A charger.

Much longer, and exactly how long depends on how deeply you drain the battery each time. That's called depth of discharge (DoD): how much of the battery's capacity you use before you recharge. A Millertech LiFePO4 battery is rated for:

• 30% DoD: 10,000 cycles
• 50% DoD: 7,000 cycles
• 80% DoD: 5,000 cycles

Lead-acid gets 150 to 300 cycles and usually lasts one to two years. And on every cycle, a Millertech battery delivers 100% of its rated capacity.

The numbers add up quickly:
• A 12-inch graph pulls about 2–3A.
• Forward-facing sonar adds 3–5A.
• With lighting and device charging, a full setup typically runs 6–8A.
Millertech’s 12V 135Ah and 190Ah Dual Purpose batteries are rated to run multiple power-hungry graphs for 8+ hours.

Roughly half or more. One 36V 60Ah Sport Series battery (3660-BT) weighs 41.23 lb and replaces three Group 27 lead-acid batteries. Millertech says that cuts more than 50% of the weight. For 12V, a Millertech 100Ah AGM weighs 70 lb, while the 12V 100Ah Dual Purpose lithium weighs 31 lb.

Yes, once you look at cost per cycle and not just the price tag. A Millertech LiFePO4 battery is rated for more than 3,000 cycles, compared with 600–800 for an AGM. It can also last up to 10 times longer than sealed lead-acid. Add the 10-year Sport Series warranty and it costs less over the life of your boat.

We lean toward one 36V battery, mostly for reliability. When three 12V batteries run in series, every one of them has to work perfectly, because if one fails, the whole system goes down. That gives you three times the chance of a failure compared with a single 36V battery. One weak battery also drags the others down: if one of three 12V 100Ah batteries only delivers 75Ah, the whole bank drops to 75% capacity.

Want a backup on the water?

During your active season, yes. Keep the battery on the charger whenever you’re not using it. The BMS and the charger’s own cut-off together mean it can’t be overcharged. For storage of a month or more, follow the instructions on this video:
How to Winterize Your Boat's MillerTech Batteries

Fish finders and LED lighting, too. Fish finders need a cord sold separately.

Most Millertech marine batteries come in Group 24 or Group 31 cases:
• Group 24: 12V 100Ah Dual Purpose (heated and standard), 24V 50Ah
• Group 31: 12V 135Ah and 190Ah Dual Purpose, 24V 80Ah, 36V 50Ah and 60Ah
• Group U1: 12V 40Ah Dual Purpose
The 36V 105Ah measures 13.5 × 9.6 × 10.5 in and needs its own tray.

The Battery Management System (BMS) puts the battery into low-voltage protection (sleep mode) to protect the cells. To wake it, connect a lithium charger, a jump pack, or another battery that’s at least 50% charged. The Dark Speed and three-bank chargers have a built-in BMS wake-up. The app shows remaining capacity in real time, so you’ll see it coming.

A heated battery has a built-in heater that turns on when the charger is connected and the temperature is below 32°F. It warms the cells to 34°F, then resumes charging. That lets you charge in the cold and fish down to 0°F. If you fish late fall, winter or early spring and keep the boat outside, it’s the one to get.

Heated Sport Series models:

• 12V 100Ah and 135Ah Dual Purpose
• 12V 300Ah
• 24V 80Ah
• 36V 60Ah

A dual-bank charger charges your 12V starting or house battery and your 24V or 36V trolling motor battery at the same time, from one plug. If you run one cranking battery plus one trolling motor battery, it’s the simplest setup. Millertech offers:

• Dual-bank: 12V/24V and 12V/36V
• Three-bank: for boats that add an electronics battery
• Four-bank: 12V, waterproof, mounts on the boat