It is nine in the evening, the panel says 12.6 volts, and you decide there is plenty in the bank for a film. An hour later the inverter shuts down partway through it. The panel was not lying to you. It was answering a different question from the one you thought you had asked.
Voltage is not a fuel gauge
Battery voltage is a rough, laggy, load-dependent proxy for state of charge, and it goes wrong in three separate ways that are all working on you at the same time.
Start with the sag. Take a reading while the microwave is running and the number sits well below where the battery really is. Take one while the solar is pushing hard and it sits well above. An honest figure means disconnecting everything and letting the bank rest, which is not a thing you can do while you are using it, and the moments you most want to know are exactly the moments the reading is least trustworthy.
Then there is how little of a range you are reading. On a 12 volt lead acid bank, full is around 12.7 and half is around 12.2. Half a volt is the entire measurement, top to bottom, and half a volt is comfortably inside the error of a cheap meter, a loose terminal and the run of wire between the battery and the panel.
Lithium takes that from unhelpful to nearly useless. LiFePO4 has a famously flat discharge curve: it sits stubbornly around 13.2 to 13.3 volts from roughly 90 percent charge all the way down to about 20 percent, and then falls off a cliff. For most of the useful range the voltage tells you nothing at all, and by the time it does have something to say, what it is saying is urgent.
A shunt answers the question you were actually asking. It sits in the negative cable, so every amp entering or leaving the bank has to pass through it, and it counts them. Amps in, amps out, over time. That is coulomb counting, and it is the difference between guessing from a symptom and measuring the thing itself. On lithium it is the only approach that produces an honest percentage.
Three ways people do this
1. The cheap voltage broadcaster
The small 12 volt monitors sold everywhere. They clip to the terminals and broadcast a voltage, and that is the entire product.
The Lancol is the one we support, and the reason is almost funny: it puts its voltage straight into the advertised Bluetooth name. Crude, and it works. No key, no connection, nothing to pair and nothing to configure. Worth naming the near-miss too, because people ask. The BM2, the small blue one sold under a dozen labels, keeps its voltage behind an encrypted connection with a key exchange rather than out in the broadcast, which is a different path from the passive listening the hub does, so it stays a candidate. If you are buying one specifically to use with us, buy the Lancol.
What you get for the money is voltage, and that is the whole list. No current, no state of charge, no percentage, and no way to tell a bank being charged from one being drained. On lithium it will read 13.2 volts nearly all the time regardless of what is happening, for all the reasons above.
It is still genuinely useful for one job, which is catching a chassis or house battery slowly dying while the rig sits in storage. Nothing is drawing, nothing is charging, and the bank has all the time in the world to rest. Voltage is exactly the right measurement for that, and this is the cheapest way to get it from a hundred miles away.
2. Whatever your charge controller already knows
If you have solar, the charge controller is already measuring a good deal of this. Most people never look at it, largely because looking means crouching in front of a small screen in a cupboard.
The Renogy BT-1 is the module that bridges a Renogy controller, and we talk to it over the same Modbus link the Renogy app uses. Two things are worth knowing before you turn it on. It is read-only by design, so RoamVitals never writes settings to your charge controller, and it is connection-based rather than a passive broadcaster, so readings arrive on a polling interval rather than continuously.
The limit here is one of vantage point. A charge controller sees what it is putting in and has no idea what the rest of the rig is taking out, so a controller reporting a glorious day tells you nothing about the inverter draw on the other side of the bank. Half the picture, then, and it happens to be the half you did not have to buy anything for.
3. A shunt, which is the actual answer
The Victron SmartShunt is the one we support. It installs in the negative cable between the battery bank and everything else, which is the only place in the rig that sees all of the current.
We read voltage, current, power and battery level. That last one is the point of the whole exercise: a state of charge that was counted rather than inferred from a sagging voltage. The underrated part is that the current has a sign. Positive means charging, negative means discharging, and once you have that you can start asking questions you could not previously even phrase. What does the rig actually draw overnight with everything off? Is the fridge on propane, or has it quietly been running on DC since Tuesday? Is the solar keeping up with the load, right now, while the kettle is on?
Two things to know before you order one. Victron’s Instant Readout is encrypted per device, so nothing decodes until you paste the advertisement key into the sensor in the hub. It comes from VictronConnect, under Product Info and then Instant Readout Details, and there is a short guide to finding it. Second, the battery monitors decode fully, while solar chargers and DC-DC converters in the same range are recognised but their record layouts are not decoded yet. The compatibility database says which is which, device by device, because we are not going to claim the whole Victron range on the strength of the part that works.
The costs are money and an afternoon. Fitting it means disconnecting the negative cable from the bank, which is genuinely straightforward and is also the kind of straightforward where getting it wrong matters, so it is a reasonable job to hand to somebody who does it every week.
Picking one
- Rig in storage, want to know the battery is not dying. A Lancol. Voltage is the right measurement for exactly this and it costs almost nothing.
- Shore power most of the time. A voltage monitor is fine. You are not managing a budget, you are watching for a fault.
- You boondock. A shunt, and it is not close. Knowing how much you have left is the whole activity.
- Lithium bank. A shunt, non-negotiable. The flat discharge curve means voltage will not warn you before the cliff.
- Solar already fitted. Read the controller first, since it costs nothing, then add a shunt when you want the other half.
Where to get one
The shunt first, because it is the only one of the three that measures rather than infers.
Victron SmartShunt and BMV
Victron's Instant Readout broadcast. The battery monitors decode fully. The rest of the range is recognised, and that is all we will claim for now.
We may earn a commission if you buy through this link. It does not change your price.
What is worth an alert
Not the number you are watching. Watching is what the dashboard is for, and an alert should be reserved for the thing you would want somebody to tap you on the shoulder about.
With a shunt, that is state of charge crossing a floor, and you want the floor set well above the point where the battery management system would cut you off. Set it there and it is a warning. Set it at the cutoff and it is an announcement.
With voltage only, the useful alert on a stored rig is a slow decline across days rather than any single reading. A battery that reads a tenth lower every morning for a week is failing, and that pattern is legible long before any one morning looks alarming.
Either way you are buying the same thing, which is a bit of notice. Hear about the battery on the evening you can still do something about it, rather than at the moment the inverter goes quiet and the film stops.
