Battery Monitors with a Shunt: Installing, Setting Up and Reading State of Charge

Practical Sailor puts it bluntly: measuring a battery's voltage while it's in use is nearly worthless for telling how much charge is left. Voltage sags under load and rises while charging. It's only meaningful after the battery has rested with no load and no charging for a good while: Practical Sailor says about 30 minutes for a rough reading and an hour for real accuracy.

Lithium (LiFePO4) batteries make it worse. Their voltage stays almost flat from nearly full to nearly empty, so a voltmeter that reads "fine" can hide a battery that's close to its cutoff. Counting amp-hours with a shunt avoids that problem.

Write down:

• Battery type (flooded, AGM, gel or lithium) and nominal voltage (12, 24 volts).
• Capacity in amp-hours. For lead-acid, Victron recommends entering the 20-hour (C20) rating from the label or data sheet. For several batteries in parallel, add their amp-hours together.
• Your charger's absorption voltage and, for lithium, the battery maker's recommended settings.
• The biggest load on the bank (inverter, windlass, trolling motor) to size the shunt.

Open the battery compartment and photograph every cable on each battery's negative post and on any negative bus bar. On many boats, the battery negative has several cables: the engine ground, a cable to a negative bus, maybe a charger or solar controller. A shunt only works if all of those go through it.

Also note any negative battery switch (common on European sailboats) or any place where two banks share a negative. If your system has several banks with a common negative, or the start battery and house bank share one cable, sketch it out. That's when an electrician's half hour is money well spent.

Mount the shunt within a short cable length of the battery's negative post, where you can reach its terminals. It must be dry: Victron notes the standard SmartShunt is rated IP21 and isn't waterproof (a sealed IP65 version exists for damp spots). Keep it out of the bilge and away from spray. Screw it to a solid surface so cables don't hang on it.

Turn off shore power, chargers, solar controllers, inverters and every battery switch. Then remove the negative cable(s) from the battery you're monitoring, negative first. Cover the bare post with a rag or a boot so nothing can touch it while you work.

The shunt has two large studs. Victron labels them BATTERY MINUS and SYSTEM MINUS (older units say LOAD MINUS). Run one short cable, the same size as your existing main negative, from the battery's negative post to the battery side of the shunt.

The rule that makes or breaks the installation: nothing else goes on the battery post or on the battery side of the shunt. Victron warns that any load or charger connected there is left out of the state-of-charge calculation.

Torque every stud to the value in your monitor's manual, not by feel.

Now connect all the other negative cables (engine ground, negative bus, charger, solar controller, inverter, trolling motor bank feed) to the system side of the shunt, or to a negative bus that is fed only from that side. Victron's instruction is that the negatives of all loads and all charge sources connect "after" the shunt.

If the stud gets crowded, run one heavy cable from the shunt's system side to a negative bus bar and land the smaller cables there. Label each cable as you go.

Most monitors take a thin red wire with an inline fuse from the battery's positive post to a small terminal on the shunt (Victron calls it Vbatt+). That wire lets the monitor read voltage and powers it. Keep the fuse close to the battery, as supplied.

If you want to watch the start battery's voltage too, the second fused wire goes from the start battery positive to the auxiliary terminal. Victron notes the start battery's negative must also be on the system side of the shunt for this to work.

Reconnect, put the boots back over the positive terminals (the Coast Guard's battery rule requires that metal objects can't touch them), turn on the battery switch, and look for the monitor's status light or Bluetooth connection.

Open the app or display menu and set:

• Battery capacity in amp-hours. Victron's default is 200 Ah; it's almost never right for your boat.
• Charged voltage. Victron recommends about 14.0 volts for a 12-volt lead-acid bank and 13.9 volts for 12-volt lithium; it should sit just below your charger's absorption voltage. Confirm with your battery and charger makers.
• Auxiliary input (start battery voltage, midpoint or temperature), if used.
• Change the Bluetooth PIN from the default if your model has one.

Victron's factory defaults suit lead-acid (AGM, gel, flooded). For other brands, look for the same settings under similar names.

Victron lists five settings to change for lithium iron phosphate:

• Charged voltage: about 13.9 volts for a 12-volt bank.
• Tail current: higher than the current at which your lithium charger stops.
• Peukert exponent: 1.01 unless the battery maker says otherwise.
• Charge efficiency: about 98%.
• Discharge floor: 10 to 20% instead of the lead-acid default of 50%, unless your battery maker advises otherwise.

The battery maker's numbers always win over these generic ones.

A new monitor doesn't know where the battery starts. Charge the bank fully on shore power or a long run. The monitor resets itself to 100% when three things happen together: voltage above the charged voltage, charge current below the tail current (Victron's example is 4% of capacity, so 8 amps on a 200 Ah bank), and both holding for the charged detection time (3 minutes by default). Until then it may show dashes instead of a percentage.

Victron explains the two ways this goes wrong. Set the charged voltage too low and a half-full battery on a fading solar charge can be marked 100%. Set it too high and the battery never reaches it, so the monitor never resets and the percentage slowly drifts.

Once it's synced, these are the readings that matter:

• State of charge (%): the one to watch. Victron calls it the best way to monitor the battery.
• Current (amps): negative means the battery is supplying loads; positive means it's being charged. With everything "off," a steady small negative number is a parasitic draw worth chasing.
• Consumed amp-hours: how much you've taken out since the last full charge.
• Time remaining: an estimate that jumps around with changing loads. Treat it as a rough guide only.
• Voltage: still useful for spotting a charging problem or a weak battery.

Practical Sailor also notes that these monitors are only as accurate as their setup, and that occasional tweaking keeps them honest.

Turn on a load you can estimate (a cabin light, the fridge) with chargers off, and confirm the current reads negative and roughly what you'd expect. Then plug in shore power and confirm the current turns positive. If a charger or load doesn't show up, its negative is still on the battery post or the battery side of the shunt.

The monitor itself draws a small current all the time (Victron rates the SmartShunt under 12 milliamps). That's nothing on a weekend, but over a long layup it adds up. Victron warns that a lithium battery left deeply discharged with even a tiny draw can be damaged permanently, and advises cutting the monitor's positive supply when the boat is left for long periods. Because the monitor's sense wire usually goes straight to the battery post, your main battery switch may not turn it off. Ask your installer how to isolate it, or pull its fuse for winter. See our parasitic drain guide for measuring standby current.