Having a way to monitor the domestic batteries is useful – a way to know when to recharge, and when to stop charging if running an engine or generator. A decent battery monitor is vital for a boater who wants to look after their batteries and spends a long time away from shorepower.

The monitors I like to fit and keep in my van are the Victron BMV700/BMV712 Smart and Merlin Smartgauge. Both brands are very different, both excellent products and each have their advantages and disadvantages. Which one to fit depends on what the boater wants. I don’t like to recommend one over another and rather the customer decides what to go for.


The Smartgauge is great as it’s so simple. It displays domestic (and start) battery voltage and has a ‘state of charge’ (SOC) percentage reading which is relatively accurate. Easy to fit and simple to setup. Around £120.


The Victron BMV (starts from £140) displays more information. As well as Voltage and SOC it also shows current (Amps), power (Watts) and counts amp-hours (AH). Having a current reading is a major advantage. It’s useful to know how much various electrical items consume inside the boat and appreciate how efficient appliances are – so help to decide how and when to use them. If a large appliance such as washing machine is run off an inverter this monitor will show if the alternator output is keeping up with the power demand. State of charge can be inaccurate and misleading. The BMV is an accurate amp hour (AH) counter and uses a battery capacity AH figure manually programmed in. A big problem is knowing what battery capacity figure to enter. AH capacity quoted by the manufacturer is always higher than actual capacity often by a surprising margin, Capacity also decreases with age of batteries and is affected by temperature and the time between equilisation charge. The monitor re-calibrates to 100% state of charge when the charging current drops to a pre-set figure (tail current). Default tail current is 4% and at this figure the batteries are not actually fully charged.

If you fit a BMV700 you have to view the SOC readings with a pinch of salt. Playing around with some of the settings will improve accuracy but you have to use the voltage reading to workout state of charge (using voltage vs SOC table) and charging current readings to work out when the batteries are near to fully charged. This monitor can be used to work out actual battery AH capacity and this figure reprogramed in to get a more accurate SOC % reading.

To sum up if you understand some battery/charging theory the Victron BMV is really great. If you don’t understand or can’t be bothered to think about it then fit the Smartgauge. If you are considering Lithium Batteries in the future fit a BMV712 Smart.

Victron BMV700

Quality Construction and great design (just what you expect from Victron).

Displays Current (& Power -Watts) which can be very useful to know.

SOC figure can be misleading.

More complicated to fit as battery earths have to be rearranged so shunt is fitted in the correct place in circuit.

Battery theory & understanding necessary to workout actual SOC and when the batteries are reaching full charge.

Delicate circuit board and connector on shunt is easily damaged by moisture (such as condensation etc. in a wet engine bay).

Merlin Smartgauge

Simple – displays only voltage and state of charge.

Accurate SOC reading.

Easy to setup and no battery theory understanding is needed.

No current/power display.

Design and build quality poor in comparison to the BMV700.