A pontoon with lithium batteries has a problem the old lead-acid boat never had. Lithium's voltage barely moves across most of its capacity, so the voltmeter that used to tell you roughly how much was left now tells you almost nothing until the bank is nearly empty. Owners on the boating boards ask the same thing in the same words: which SmartShunt, and where does it go.
This is the pontoon-specific answer. The general install is covered in our SmartShunt installation guide; this page is about the decisions that are different on a boat.
What a shunt does that a voltmeter cannot
A shunt sits in the battery's negative lead and counts every amp that goes in and out. From that it calculates state of charge as a percentage, plus time remaining at the current draw, and reports it to your phone over Bluetooth.
On lithium that is the only reliable way to know what you have left, because the voltage curve is flat. A pontoon that runs a trolling motor, a livewell pump, a stereo and a fridge off one bank needs to know when to head in, and "12.9 volts" does not tell you.
The one wiring rule that decides whether it works
Every negative connection on the boat goes to the load side of the shunt. Nothing else touches the battery negative post.
This is the rule pontoon installs break most often, because a boat accumulates negative connections over the years: the trolling motor's own ground, a fish finder wired straight to the battery, an onboard charger, a bilge pump someone added. Any load or charger whose negative bypasses the shunt is invisible to it, and the state-of-charge reading drifts from the first day.
Trace every black wire back before you start. On a pontoon they often run the length of the deck under the seating, and the ones you cannot see are the ones that bypass the shunt.
A busbar on the load side of the shunt is the clean way to do it: one cable from shunt to busbar, every negative on the boat to the busbar.
Which SmartShunt
The SmartShunt comes in 500, 1000 and 2000 amp ratings. The rating is the maximum continuous current it can carry, not a measurement resolution, and it needs to exceed the largest current the bank will ever see.
For most pontoons the 500 amp unit is correct. A trolling motor draws tens of amps, a livewell pump and accessories a few more, and even a modest inverter stays well inside 500.
Go to 1000 amps if the bank feeds a large inverter, an electric outboard, or a high-draw trolling motor on a 24 or 36 volt system where the currents at the battery are substantial.
Owners on the bass boat boards ask about a smaller 300 amp unit for a trolling-motor bank; sizing is the same logic, and the 500 has enough headroom for almost any trolling setup while leaving room to add an inverter later.
Where it goes on a pontoon
As close to the battery as the cable allows. Accuracy depends on the negative run between battery post and shunt being short, and a pontoon's battery compartment under the deck usually gives you a few inches.
Somewhere the Bluetooth reaches. The SmartShunt has no display; your phone is the display. Aluminium pontoons and deck framing attenuate Bluetooth more than a fibreglass hull does, so if the battery compartment is a metal box under the deck, expect to be standing near it to read the shunt rather than at the helm. If that matters, plan for a display or a GX device later.
Dry, and above any water that gets into the compartment. The SmartShunt is not rated for immersion. Mount it high on a bulkhead, not on the compartment floor.
Setting it up for lithium
Three settings matter and the defaults are for lead-acid:
- Battery capacity in amp-hours, from the battery's own specification.
- Charged voltage, set just below your charger's absorption voltage, so the shunt recognises full. Lithium sits higher than lead-acid here; use the figure your battery manufacturer publishes.
- Peukert exponent and charge efficiency, which for lithium are close to 1.05 and 99 percent respectively rather than the lead-acid defaults.
Then charge the bank fully and synchronise the shunt to 100 percent once. It counts from there.
Charging on a pontoon, since you are in the compartment anyway
Two charging sources are common on pontoons and both have a lithium-specific point.
The outboard's alternator. Small outboard charging systems are modest, and a lithium bank will pull everything they offer. If you charge the house bank from the engine, a DC-DC charger between the starter and house batteries limits what the bank can demand and gives it a proper lithium profile. The Orion XS does this and shares its data with the SmartShunt over Bluetooth.
The onboard shore charger. Check it has a lithium profile. A lead-acid charger will undercharge lithium in float and, if it has an equalisation mode, must have it disabled.
Fusing a lithium bank on a boat
Lithium delivers far more short-circuit current than the lead-acid it replaced, and a boat is a place you do not want an electrical fire. Put a fuse with an interrupt rating suited to lithium close to the battery positive, and protect the trolling motor and accessory circuits with resettable breakers such as the 200 amp DC breaker, which is waterproof and resettable by hand. Our cable and fuse sizing guide covers the interrupt-rating rule that matters for lithium.
What to buy
- One SmartShunt, 500 amp for most pontoons
- A busbar for the load-side negatives
- Cable for the short battery-to-shunt run, sized to the bank's maximum current
- A DC-DC charger if the engine charges the house bank
Send us the battery capacity, the trolling motor's rating and whether the engine charges the bank, and we will confirm the shunt size and the fusing. Everything ships from Upland, California.