Why a voltmeter is not enough
You can stick a $12 voltmeter on any battery and get a number. The number will look reassuring. It will also be wrong — or at least wrong in the way that matters most, which is when you're trying to decide whether to run the inverter for another hour or whether the fridge is about to murder the bank overnight at anchor.
The problem is that voltage is a noisy and lagging indicator of state of charge. Under load, a healthy lead-acid bank reads several tenths of a volt low. Off load, it takes 30 to 60 minutes to settle to its true resting voltage. And lithium — the chemistry that's quietly become the default house bank for any serious cruiser — has a famously flat discharge curve. A LiFePO4 bank reads roughly 13.2 to 13.3 volts across almost its entire usable capacity. The same 13.25V can mean 85% full or 25% full. A voltmeter cannot tell the difference.
Coulomb counting: the only honest answer
A real battery monitor doesn't watch voltage. It watches current. Every amp that goes into the bank from the alternator, solar, charger, or wind generator gets added to a running total. Every amp that comes out to feed the fridge, the inverter, the chartplotter, the autopilot gets subtracted. The running total — multiplied by time — is amp-hours, and amp-hours relative to bank capacity is your state of charge.
That's the technique. It's called coulomb counting (a coulomb is an amp-second of charge), and every monitor we recommend in this guide does it the same way: by measuring every amp into and out of the bank through a precision low-resistance bar called a shunt, installed on the negative side of the bank between the bank and every load.
Coulomb counting tells you three numbers that voltage alone cannot:
- State of charge (%) — what's actually left in the bank, accurate to within 1–2% on a properly synchronized monitor.
- Time-to-empty — at current draw, how many hours of run-time you have left. Critical at anchor when you're deciding whether the inverter is worth running.
- Net current (amps in/out) — instantaneous balance of what's charging versus discharging. If solar is producing 12A and loads are pulling 8A, you're net +4A and the bank is filling.
These three numbers, available in real time on a panel display or a phone app, change how you think about boat power. They're also the foundation for anyone managing a serious solar install — our solar power for smart boats guide assumes you have a coulomb counter on the negative bus, because otherwise the math doesn't close.
Before we get to picks, here's how the three units we recommend line up against the most common alternatives.
| Monitor | Method | Wireless | NMEA 2000 | Banks | Price (approx) |
|---|---|---|---|---|---|
| Victron BMV-712 Smart | 500A shunt | Bluetooth | Via VE.Direct gateway | 2 (1 SoC + 1 voltage) | ~$229 |
| Victron SmartShunt | 500A shunt | Bluetooth | Via VE.Direct gateway | 2 (1 SoC + 1 voltage) | ~$135 |
| Balmar SG200 | 500A shunt | Wired panel + N2K | Native | 3 | ~$439 |
| Renogy 500A monitor | 500A shunt | None (panel only) | None | 1 | ~$80 |
| AiLi 350A monitor | 350A shunt | None (panel only) | None | 1 | ~$45 |
Shunt-based vs Hall-effect — accuracy, install, cost
Every battery monitor measures current somehow. There are two ways to do it, and the choice has real consequences for accuracy and install effort.
Shunt-based monitors
A shunt is a precision low-resistance bar — typically a thick chunk of manganin alloy bolted between two terminals. Current passes through it, and Ohm's law (V = IR) means a known resistance produces a tiny voltage drop proportional to the current. The monitor reads that millivolt-level signal and converts it to amps.
The Victron 500A shunt drops 50mV at 500A — a resistance of roughly 100 microohms. That's small enough that the shunt doesn't heat up significantly at normal currents, but large enough to give a clean, repeatable signal that the monitor's ADC can read with a fraction of a percent of accuracy.
Shunt-based monitoring is what the Victron BMV-712, SmartShunt, and Balmar SG200 all use. It's the gold-standard technique for permanent installations because it's accurate from milliamps to hundreds of amps, doesn't drift with temperature, and has no moving parts. The install cost is one: you have to break the negative cable and run it through the shunt. On a boat where the negative bus is a single bolted bar behind the helm, that's a 30-minute job. On a boat where the negative cable runs through inaccessible bilges, it's an afternoon.
Hall-effect monitors
A Hall-effect sensor clamps around a cable and measures the magnetic field produced by current flowing through it. No broken cable, no shunt to install — you slide the sensor over the existing negative cable and you're done. That sounds appealing, and for some applications (RV battery monitors, removable test gear) it is.
For a primary marine battery monitor it is not. Hall sensors drift with temperature in a way that's hard to compensate for, they're typically less accurate at the low currents that dominate a boat at anchor (fridge cycling, a few LED lights), and they're more susceptible to interference from nearby cables carrying high current. The math at the end of a weekend at anchor — where you might draw 60Ah over 48 hours mostly at 1–3A — is precisely where Hall sensors are weakest.
The practical recommendation is simple: shunt-based for any primary house-bank monitor. Hall sensors have their place — clamp meters for diagnostics, removable monitoring for a tender or a trailer — but they don't belong on the negative bus of a permanent install.
Three top picks, in detail
We compared specs and operator feedback across five current shunt-based monitors. The three below are the picks we'd actually recommend to friends; the cheaper Renogy and AiLi monitors are fine for trailer-boat owners on a strict budget, but they don't have the Bluetooth, NMEA 2000, or sync-detection sophistication that matters on a cruising boat. Pricing was checked against Amazon and vendor websites at time of publication.
Victron BMV-712 Smart — the default cruiser pick
The BMV-712 Smart is, for most cruisers, the right answer and the end of the conversation. It's a shunt-based coulomb counter with the same Bluetooth radio that runs the rest of the Victron ecosystem, a small backlit panel display you mount at the nav station, and a second-bank voltage input that lets you watch the starter battery alongside the house bank. The VictronConnect app on iOS or Android pairs in seconds and shows state of charge, time-to-empty, voltage, current, and lifetime statistics (deepest discharge, average discharge, total Ah cycled, days since last full charge). For NMEA 2000 integration with a Garmin/Raymarine/Simrad/B&G plotter, you add the VE.Direct to NMEA 2000 cable for roughly $60 — Victron's gateway protocol is well-supported and the data shows up on the plotter without configuration. The only meaningful argument for spending more is if you want native N2K without a gateway, or if you want temperature-compensated lithium-aware monitoring, both of which point you at the Balmar.
Strengths
- Shunt-based — accurate from milliamps to 500A
- Bluetooth + panel display — both phone and helm access
- Second-bank voltage input — watch starter alongside house
- VictronConnect app is genuinely good — lifetime statistics included
- VE.Direct gateway puts data on NMEA 2000 plotters
- Part of the broader Victron ecosystem — pairs with MPPT, Cerbo GX
Trade-offs
- NMEA 2000 requires the separate ~$60 VE.Direct gateway cable
- Second bank is voltage-only, not coulomb-counted
- No temperature sensor included — buy the temp sensor separately
- Panel display is small — fine at the nav station, hard to read across the cabin
Victron SmartShunt — same data, $90 less
The SmartShunt is the BMV-712 with the dedicated display removed. That is the entire functional difference. Same precision shunt, same microprocessor, same Bluetooth radio, same VictronConnect app, same second-bank voltage input, same VE.Direct output for the NMEA 2000 gateway. If you're comfortable reading state of charge on your phone, your tablet, or your chartplotter — and most cruisers under 45 are — the SmartShunt saves you roughly $90 for identical core capability. It's the unit we'd recommend to anyone building a fresh power system around a phone-first workflow, anyone whose nav station already has too many panel-mount instruments, or anyone running a serious Home Assistant or Signal K install where the monitor is a data source rather than a display. The trade-off is honest: if you're at sea overnight and your phone is dead, you have no way to read state of charge. A small panel display sometimes earns its $90.
Strengths
- Same internals as BMV-712 — same accuracy, same features
- $90 cheaper than the BMV-712 for identical core capability
- Smaller install footprint — shunt fits anywhere, no panel cutout
- Phone-first workflow suits most modern boats
- Works with Signal K, Home Assistant via the Victron VRM bridge
Trade-offs
- No dedicated display — you need a phone or plotter to read it
- Phone dead = no readout (rare but real on long passages)
- Second bank still voltage-only, not coulomb-counted
- Temperature sensor still sold separately
Balmar SG200 — lithium-aware, NMEA 2000 native
The SG200 is Balmar's premium answer, and the right pick in two specific cases: you already run a Balmar charging regulator (the MC-614 or 618) on the alternator, or you have a lithium house bank and want temperature-compensated state-of-charge calculations done properly. The SG200 is native NMEA 2000 — no gateway, no proprietary cable, just plug it into the backbone and the chartplotter picks up the battery data alongside everything else. It supports three banks (typically house, starter, thruster), includes a temperature sensor in the box, and has Balmar's lithium-aware charging-curve detection that handles the deceptive flat-voltage region of LiFePO4 chemistry better than monitors designed primarily for lead-acid. The color panel display is genuinely useful — large numbers, time-to-empty, and a simple charge/discharge indicator that's readable across the cabin. The price is the friction. At $439 it's roughly twice the BMV-712 and three times the SmartShunt. If you don't already own a Balmar regulator, and you're not building a serious lithium house bank where the temperature compensation matters, the Victron answer is the smarter buy.
Strengths
- Native NMEA 2000 — no gateway, no proprietary bus
- Temperature-compensated, lithium-aware SoC algorithm
- 3-bank monitoring (house, starter, third) out of the box
- Color panel display readable across the cabin
- Temperature sensor included in the box
- Integrates with Balmar MC-614/618 regulator data
Trade-offs
- ~$439 — roughly twice the BMV-712, three times the SmartShunt
- No Bluetooth — phone access requires an NMEA 2000 Wi-Fi gateway
- Smaller third-party ecosystem than Victron
- Configuration UI feels more dated than VictronConnect
Two budget alternatives we considered
The Renogy 500A monitor (check price) is a $80 shunt-based monitor with a backlit panel and no wireless. It's accurate enough for a trailer-boat or a small daysailer with a single 100Ah AGM bank and no charging complexity. The Victron ecosystem (MPPT integration, lifetime statistics, app, VRM cloud) is what justifies the price jump, but if none of that matters to your boat, the Renogy is honest hardware at an honest price.
The AiLi 350A monitor (check price) is the $45 Amazon special — a no-name shunt-based monitor with a small LCD panel. Accuracy is acceptable for a lead-acid bank under 200Ah, the build quality is utilitarian, and there's no support if it dies. It's the right answer for a tender, a jonboat, or a winterized backup install where a failure costs you a $45 part and an afternoon. Not the right answer for a primary cruising-boat install.
Multi-bank monitoring — house, starter, thruster
Most cruising boats have at least two battery banks: a house bank that runs everything that isn't engine cranking, and a starter bank dedicated to the engine. Many add a third — a thruster bank, a windlass bank, or a dedicated inverter bank — and the question of how to monitor all of them with one system is the most common follow-up after "which monitor do I buy."
What "multi-bank" actually means
There's an important distinction: a monitor that coulomb-counts two banks needs two shunts. A monitor that watches voltage on a second bank needs only a wire to its positive terminal. The Victron BMV-712 and SmartShunt both do the second — one shunt on the house bank (full SoC math), plus a voltage input to a second bank (voltage and presence/absence of charge). That's the standard architecture, and for most boats it's exactly what you want.
The argument: your starter battery is a simple beast. It needs to be at or near 12.7V when you turn the key, it should be charging when the engine runs, and it should not be discharged by anything other than cranking. You don't care about a precise state-of-charge percentage on it — you care that it's there and it's full. A voltage reading every few seconds tells you both. The same logic applies to a thruster or windlass bank.
If you genuinely need amp-hour accounting on two separate banks (rare — most often this is a misunderstanding of how boat power works), the answer is two SmartShunts. Each is a complete monitor; both report to the same VictronConnect app; each gets its own shunt on its own bank. Two SmartShunts cost roughly the same as one Balmar SG200 and give you twice the coulomb-counted data.
The Balmar three-bank advantage
The Balmar SG200 supports three banks natively, with the second and third as voltage inputs. If you have a true three-bank vessel — house, starter, and thruster — and you want all three on the same display without three separate apps, the SG200 is the cleanest single-box answer. The price difference versus a SmartShunt + a second SmartShunt + a third voltage source is small, and the integration is genuinely tidier.
If your "house bank" is really four batteries wired in parallel, the monitor sees the bank as a single unit — current sums through the shunt, voltage is shared. That's the correct architecture. What you cannot do is wire two physically separate banks in parallel sometimes (with a combiner or solenoid) and expect the shunt to track them honestly — current going into the combined bank doesn't all come from the source the monitor thinks it does. Keep banks isolated, with charge sources feeding the house bank and the starter charged from a DC-DC converter or an alternator-managed split. Our no-subscription monitoring guide covers the broader topology question.
NMEA 2000, Bluetooth, and chartplotter integration
A monitor is only useful if you can see it. There are three ways the picks above get their data in front of you: Bluetooth to a phone or tablet, a panel-mount display at the helm or nav station, or a network protocol that puts data on a chartplotter or larger boat-wide system.
Bluetooth (Victron's default)
Both the Victron BMV-712 and SmartShunt pair with VictronConnect on iOS or Android. Range is roughly 10 meters in an open cabin, less through bulkheads. The app is genuinely good — fast pairing, no account required, lifetime statistics shown alongside live data, and a clean history view of voltage and current over the last 30 days. For a sailor at anchor checking state of charge on their phone before turning the inverter on, Bluetooth is the right answer.
The limit is range and visibility. Bluetooth doesn't reach the cockpit on most boats, and it doesn't get data onto the chartplotter at the helm. For that you want NMEA 2000.
NMEA 2000 (the network answer)
The Balmar SG200 is native NMEA 2000 — plug into the backbone, the plotter discovers it, and battery state of charge appears as overlay data on the chart or as a dedicated instrument page. Modern Garmin, Raymarine, Simrad, B&G, and Furuno plotters all decode the SG200's PGNs automatically. If you're not familiar with the protocol, our NMEA 2000 gateway guide covers the basics.
The Victron BMV-712 and SmartShunt get to NMEA 2000 through the VE.Direct to NMEA 2000 cable — a ~$60 accessory that bridges Victron's proprietary VE.Direct bus to standard N2K. Once installed, the plotter sees Victron data as standard battery PGNs and the integration is identical to the Balmar's. The cable is the entire difference. If you're building a fresh boat with a single N2K backbone and no Victron gateway elsewhere, the Balmar saves you the gateway purchase; if you already have a Cerbo GX or VE.Direct devices, the Victron path is the natural fit.
Wi-Fi, MQTT, and the DIY path
If you've built an Home Assistant marine setup or an open-source chartplotter on a Raspberry Pi, the Victron units are the easier integration. The Victron VRM bridge and the open-source victron-mqtt projects pull data directly from VE.Direct over serial or Bluetooth, and surface it as MQTT topics. From there, Home Assistant, Signal K, or AvNav all consume it. The Balmar SG200 can be read off NMEA 2000 with a USB or Wi-Fi N2K gateway, but the ecosystem of off-the-shelf integrations is smaller.
Install: shunt wiring, sense lead, fusing
This is the part of the project where a $135 SmartShunt turns into a $300 install if you skip the math. The work itself isn't hard — it's 90 minutes for someone comfortable around 12V — but every wire matters and a missed connection ruins the accuracy of every reading.
Where the shunt goes
The shunt installs on the negative side of the house bank, between the bank's negative bus and every load and charge source. Every amp into or out of the bank has to pass through the shunt — otherwise the math is wrong, the SoC drifts, and you spend the next month wondering why your monitor says 80% when the bank is clearly nearly empty.
The physical layout: house bank negative posts → one heavy cable → large stud on the shunt (often labeled "BATTERY ONLY"). The other large stud on the shunt (often labeled "LOAD & CHARGE") → the boat's negative bus or distribution block → everything else (loads, charger negatives, solar charge controller negative, alternator negative if it doesn't return through the engine block). The only things connected to the battery-side stud are battery negative cables. Everything else is on the load-side stud.
This single rule is what people get wrong. A bilge pump wired directly to the battery negative post bypasses the shunt, so the monitor never sees its current draw, and after a season of bilge cycles the SoC reading is meaningfully off. Trace every existing negative connection back to the bank and re-route them all to the load side of the shunt.
Wire gauge
Use the same wire gauge as your existing main negative cable. For a 200Ah+ house bank, that's typically 2/0 AWG. For a smaller bank, 4 AWG. The shunt is rated to 500A continuous (on the Victron and Balmar units) so it's not the limit — your existing cable is. Torque both bolts to the spec on the shunt's data sheet (typically 11–14 N·m), and use star washers or a small drop of thread locker to keep the bolts from loosening over time and vibration. A loose shunt bolt is the single most common cause of an inaccurate monitor reading.
The voltage sense lead
Every shunt-based monitor needs a small wire from the shunt's positive terminal to the battery positive (or to the closest fused positive bus). This is how the monitor measures bank voltage — it can't measure voltage from the shunt itself because the shunt only sees current. The lead is 20–22 AWG and carries milliamps; fuse it at 1A with a small inline fuse right at the battery end. If the lead chafes through and shorts, you do not want the battery bank dumping into a 22AWG wire.
Programming
Once wired, the monitor needs to know your bank's capacity (typically in Ah at 20-hour rate), the battery chemistry, and the full-charge detection parameters (charged voltage, tail current, charged detection time). Victron's defaults are reasonable for most lithium and AGM banks; if you're running flooded lead-acid, bump the charged-voltage parameter up to 14.4V. The monitor learns its calibration over the first few charge cycles — give it a week of normal use before judging accuracy.
Who should buy what
The short version, by profile
Cruiser with a serious house bank (lead-acid or lithium), phone-first workflow: Victron SmartShunt. $135 gets you everything the BMV-712 does except the panel display, and most modern boat owners are happier reading state of charge on a phone than on a small backlit panel at the nav station. Add the VE.Direct to N2K gateway for ~$60 if you want the data on a chartplotter.
Cruiser who wants a dedicated helm/nav-station display: Victron BMV-712 Smart. Same shunt and same data as the SmartShunt, plus a backlit panel for the times you don't have a phone in hand. The right pick on long passages where a glance-able permanent display matters. ~$229.
Lithium house bank with a Balmar charging regulator already installed: Balmar SG200. Native NMEA 2000, temperature-compensated SoC algorithm tuned for LiFePO4, three-bank monitoring out of the box, and integration with the Balmar MC-614/618 alternator regulator. ~$439 — twice the price of the BMV-712, but the right tool if the ecosystem is already in place.
Trailer boat or jonboat under 100Ah with no charging complexity: Renogy 500A monitor at ~$80, or the AiLi 350A monitor at ~$45 if you really want to keep it cheap. Both are shunt-based and accurate enough for the duty cycle. You give up Bluetooth and NMEA 2000, but on a boat that small, you don't need either.
The general rule: Victron SmartShunt if you read your boat on a phone, Victron BMV-712 if you read it on a panel, Balmar SG200 if your boat is already Balmar and lithium. Everything else is a niche answer.
Frequently asked questions
Why is a voltmeter not enough to tell me my battery state of charge?
Voltage is a noisy and lagging indicator of state of charge. A lead-acid battery under load reads several tenths of a volt low even when nearly full; a lithium battery sits at roughly 13.2–13.3V across most of its usable capacity, so the same voltage can mean 80% full or 20% full. A proper battery monitor counts amp-hours in and out of the bank through a shunt, calculates net charge, and gives you a percentage that reflects what's actually left rather than what the voltmeter happens to read at the moment.
What is the difference between a shunt-based monitor and a Hall-effect monitor?
Both measure current. A shunt-based monitor passes all bank current through a precision low-resistance bar and reads the millivolt drop across it — accurate, well-understood, and the basis for the Victron BMV, SmartShunt, and Balmar SG200. A Hall-effect sensor clamps around the cable and measures the magnetic field from the current. Hall sensors are easier to install (no breaking the negative cable) but drift with temperature and are typically less accurate at very low currents. For a primary state-of-charge monitor on a house bank, shunt-based is the right answer.
Can one monitor track my house bank and starter battery at the same time?
Yes — with the right model. The Victron BMV-712 Smart and Balmar SG200 both support a second battery input that reads voltage on a starter (or thruster, or windlass) bank in addition to the primary shunt monitoring on the house bank. The second bank is voltage-only, not coulomb-counted, but for a starter battery that's usually enough — you mostly want to know it has not been discharged and is being recharged. The SmartShunt supports the same second-bank voltage input through its app.
Does the Victron SmartShunt do anything the BMV-712 does not?
No — and that is the point. The SmartShunt is functionally the same monitor as the BMV-712 but without the dedicated panel-mount display. Both have the same shunt, the same microprocessor, the same Bluetooth, the same app, and the same accuracy. The BMV-712 adds a backlit display you mount at the nav station. If you're happy reading state of charge on your phone or chartplotter, the SmartShunt saves you roughly $90 for identical core capability.
Do I need NMEA 2000 to see battery data on my chartplotter?
Yes if you want it on a Garmin, Raymarine, Simrad, B&G, or Furuno chartplotter. The Balmar SG200 has NMEA 2000 native — plug it into the backbone and the plotter discovers it. The Victron BMV-712 and SmartShunt need the Victron VE.Direct to NMEA 2000 cable (sold separately, around $60) to bridge their proprietary bus onto N2K. Bluetooth alone reaches your phone and tablet but not the chartplotter at the helm.
Where does the shunt go and what wire size do I need?
The shunt goes on the negative side of the house bank, between the bank's negative bus and every load and charge source. Every amp into or out of the bank has to pass through it — otherwise the math is wrong. Use the same wire gauge as your existing main negative cable (typically 2/0 AWG for a 200Ah+ bank, 4 AWG for smaller banks) and torque both bolts to the shunt's spec. The small voltage-sense lead from the shunt's positive terminal to battery positive should be fused at 1A with a small inline fuse.