Short answer: a voltmeter shows the battery's voltage, which is only a rough hint at how full it is. A shunt-based battery monitor measures every amp going in and out and keeps count, so it can tell you the state of charge as a percentage and how long you've got left. If you've spent money on a decent battery, a shunt is how you find out what it's actually doing.
Why isn't battery voltage a good guide?
Because voltage moves with the current you're drawing and the temperature, not just with how full the battery is. Victron's Energy Unlimited says voltage only drops quickly once the battery is almost flat, at 80% to 90% discharged.
You can get a rough reading from voltage, but only after the battery has sat for several hours with no charging or discharging. With a fridge running, that never happens.
And by the time voltage tells you something, it's late. Victron's line: "To start charging once the 'voltage drops' is simply too late."
Those points come from Victron's lead-acid chapter. For lithium it's simpler still: Victron's Lithium Battery Smart has no state-of-charge monitoring built in, and their manual says to add a battery monitor such as a BMV or SmartShunt (or use a Lynx Smart BMS).
A voltmeter still has a place. It's a handy glance on a switch panel, and since voltage drops quickly near empty, it can warn you when you're nearly there. It just can't tell you that you've got 62% left.
How does a shunt monitor work?
The shunt sits in the battery's negative cable, so every amp in or out passes through it and gets measured. From voltage and current, the monitor works out the state of charge and the time to go.
The counting is plain arithmetic. Victron's example: 10 A for 2 hours takes 10 x 2 = 20 Ah from the battery. The monitor keeps a running total, and also allows for discharge rate, temperature and charge efficiency. Victron call this the most practical and accurate way to track a battery's state of charge.
The figure on the screen is always the net: current in minus current out. If the solar is putting in 15 A and the fridge is taking 4 A, you'll see about +11 A.
How do I wire a shunt?
One rule matters more than the rest: the shunt must be the only thing on the battery negative. Victron's SmartShunt manual is clear: "Any loads or chargers connected here will be excluded from the battery state of charge calculation."
So: battery negative to one side of the shunt, and everything else to the other, including the negatives of all DC loads, inverters, battery chargers, solar chargers and other charge sources. The BMV-712's shunt is even labelled for it: "BATTERY ONLY" on one side, "LOAD AND CHARGER" on the other.
The easy one to miss is a small earth left on the battery terminal, say a USB socket or a fridge lead. It works fine, but the monitor never sees it, so its count is wrong. Our fusing guide covers the positive side.
Two other install notes: the SmartShunt is IP21, not waterproof, and has to be mounted somewhere dry. The BMV-712's head unit is IP55 but not meant for outdoor use.
What settings does a lithium battery need?
Victron's factory defaults suit lead-acid. For LiFePO4, the SmartShunt manual says to change these, unless your battery maker advises otherwise:
- Peukert exponent: 1.05.
- Charge efficiency: 99%.
- Discharge floor: somewhere between 10% and 20%.
- Tail current: also on Victron's list to change for lithium; check your battery maker's figure.
And set the battery capacity to what you actually have. For how much of a lithium battery is really usable, see our lithium vs AGM guide.
Why does my monitor show "---" or drift?
A shunt counts, and small errors add up. To stop it drifting, it has to re-sync with the battery's true state of charge regularly. A sync resets it to 100%.
It does this on its own when the battery is genuinely full. Victron's 12 V example: the voltage is above 13.2 V, the charge current has dropped below 4% of capacity (8 A for a 200 Ah battery), and both have held for 3 minutes. If your battery rarely gets to full, the reading will drift over time.
Three dashes ("---") mean the monitor isn't synchronised yet. That's normal on first install, or after its power supply has been interrupted. Charge the battery to full and it sorts itself out.
How much power does the monitor itself use?
Victron's SmartShunt manual gives two different figures: under 12 mA in its warning section, and under 1 mA in the technical data. Either way it's small, but if the vehicle is stored for months it adds up. Our arithmetic over 30 days:
- 12 mA x 24 h x 30 days = about 8.6 Ah.
- 1 mA x 24 h x 30 days = about 0.7 Ah.
The same section warns that a lithium battery left flat can be damaged by small loads like this, and says to isolate the battery when you leave the system unattended. (Our sizing guide has more on hidden loads.)
SmartShunt or BMV-712?
They measure the same way and to the same accuracy (current ±0.4%, voltage ±0.3%). The difference is how you see it and what else it can do:
| SmartShunt | BMV-712 Smart | |
|---|---|---|
| Display | None: your phone is the display | Display and four buttons, plus the phone app |
| Alarm | No programmable visual and audible alarm | Yes |
| Relay | No programmable relay | Yes, 60 V / 1 A |
| Shunt sizes | 300 A, 500 A, 1000 A, 2000 A | 500 A |
| Second-battery, midpoint or temperature input | Yes | Yes |
Victron pitch the SmartShunt for systems where you want monitoring with less wiring and clutter. The BMV-712 makes sense if you want a screen on the wall, or the relay. By default its relay switches on below 50% charge and off at 90%, which is the sort of thing used to start a generator.
The auxiliary input on both can watch a second battery or the midpoint of a battery bank. Midpoint monitoring matters if you run batteries in series: Victron warn "One bad cell or one bad battery can destroy a large, expensive battery bank", and comparing the two halves gives an early warning. Add the temperature sensor if you want battery temperature instead.
For most touring setups, the SmartShunt 300A does the job. If you want a display, the BMV-712 Smart is the one. Both, and the bigger sizes, are in battery monitors.