The switch to lithium on small boats has one side effect nobody mentions in the sales pitch. The little voltage gauge on the trolling motor, or the "battery level reader" on the kayak box, stops telling you anything. Owners on the duck-boat and kayak boards describe it the same way: full bars all day, then the motor quits with no warning.
That is not a faulty battery. It is how lithium works, and the fix is a different kind of monitor.
Why the voltage gauge lies on lithium
Lead-acid voltage falls steadily as the battery empties, so a voltage bar graph is a rough but honest fuel gauge. Lithium iron phosphate holds almost the same voltage from 90 percent down to about 15 percent, then falls off a cliff. A gauge that reads voltage shows "full" for most of the day and "empty" a minute before the BMS shuts the battery off.
The only honest measurement on lithium is counting the current in and out. That is what a shunt does. Every amp the trolling motor draws passes through it and is subtracted; every amp the charger puts back is added. The result is a real state of charge in percent, plus the time remaining at the current draw, which for a trolling motor is the number you actually want.
The SmartShunt on a small boat
The Victron SmartShunt is a shunt with Bluetooth built in and no display to mount. It bolts into the battery's negative lead, and your phone is the gauge.
Three things make it suit trolling-motor banks specifically:
- It works on 12, 24 and 36 volt banks with one unit. Its input range runs from 6.5 to 70 volts, so a 36 volt bank made of three batteries in series is monitored by one shunt on the bank's main negative. You do not need one per battery.
- It has no display to get wet. The shunt lives with the batteries. The only thing exposed to the boat is your phone.
- It shares data. If you charge from a Victron charger or an MPPT, the shunt's state of charge appears alongside them in the same app.
The 500 amp model is the one for any trolling-motor or kayak bank; the larger sizes exist for big house banks and are not needed here.
Wiring it in
The rule is one rule: everything that connects to the battery negative connects to the shunt's load side, not to the battery post. The shunt's battery side goes to the battery negative and nothing else. If the trolling motor's negative, or the charger's negative, or the fish finder's negative bypasses the shunt to the post, that current is invisible and the percentage drifts.
On a multi-battery series bank, the shunt goes on the negative of the battery at the bottom of the string, the one whose negative is the bank's negative. On paralleled batteries it goes on the common negative after the batteries are joined.
A small power wire from the shunt to the battery positive runs its electronics; it comes with an inline fuse. Fit it.
The kayak battery box question
Kayak anglers on the boards ask for a box with a breaker and a level reader. Replace "level reader" with a SmartShunt inside the box and you have a box that tells the truth. The breaker is still right: a waterproof resettable breaker on the positive lead protects the wiring and doubles as a disconnect, and it is rated for spray.
The standard SmartShunt itself is not sealed against water. In a closed battery box on a kayak that is fine; on an open duck boat with the batteries in the bilge, mount it in a dry compartment or use the sealed IP65 version, which Victron makes and we do not currently stock.
For a fish-finder battery that has to last sixteen hours, the shunt answers the sizing question directly: it shows the finder's actual average draw on the water, and sixteen hours times that number is the capacity you need, with margin.
Charging and the percentage
A shunt-based monitor is only as accurate as its full-charge reference. It resets to 100 percent when it sees the charger hold the battery at full voltage with the current tapered down, so the battery has to actually reach full periodically. A lithium charger with a proper profile, such as the Blue Smart IP65, which is sealed and made for exactly this, gets it there every time. A charger that never finishes the absorption stage leaves the monitor guessing.
If the trolling-motor bank charges from the outboard while running, the same rule applies to that charger. Set the SmartShunt's charged-voltage parameter to match the battery maker's figure, which for most 12 volt lithium is around 14.2 to 14.4 volts.
The disconnect
A quick-disconnect on the battery positive, such as an Anderson-style connector, lets you pull the battery for charging or storage without tools, which matters on a boat you trailer home after every trip. Put it on the load side of the shunt like everything else.
What to buy for a duck boat or kayak
- A lithium battery sized to the day, not the label; a 50 to 100 amp-hour battery is the usual range for a trolling motor, and the shunt tells you within one outing whether you chose right. The Victron 12.8 volt Smart range covers 50 to 200 amp-hours.
- A SmartShunt 500 amp on the main negative.
- A sealed breaker on the positive.
- A lithium charger that actually reaches full.
Our pontoon SmartShunt guide covers the same monitor on a bigger house bank, and our SmartShunt install article walks through the wiring step by step. Everything ships from Upland, California.