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Battery charge monitoring

May 10, 2017
You can keep tabs on the Victron BMV 7 battery monitor via Bluetooth on your smart device.

You can keep tabs on the Victron BMV 7 battery monitor via Bluetooth on your smart device.


Battery charge is one critically important function that appears to have been left out of the race to integrate monitoring on today’s pleasure craft. In time this will change, but for now we must continue to assess battery status through a separate set of devices.

The old standard for giving a quick reading on battery health, and still popular with many sailors, is a separate analog voltmeter gauge for each bank, commonly mounted on the switch panel. The switches themselves may wear out from time to time, but the needle gauge voltmeter is likely to outlive the crew. The problem with needle gauges in general, though, is precision. After a while, you acquire a “feel” for what the meter is saying — knowing that when the gauge reads 14 volts, your digital multimeter could be reading 13.8 volts.

To keep up on every measurable aspect of a battery’s status, we can take advantage of the most recent battery monitoring technology. Some systems are small stand-alone devices offering a surprising level of comprehensive data on a battery, and some may be configured for two banks.

At the most basic level is the humble Nasa Marine Clipper BM-2 battery monitor from the U.K. This small monitor features an LCD screen showing volts and amps discharged with a bar scale of current charge level. The device is limited to one battery bank, so at least two will be needed for both the cranking and house banks. Also, the BM-2 is designed for use only on flooded lead-acid batteries, which covers a lot of smaller cruising vessels. The BM-2 measures up to 16 volts DC and a maximum current of 199 amps on its 200-amp shunt.

The Xantrex LinkPRO and LinkLITE LCD monitors indicate battery charge for a dual battery bank, letting you “read your battery bank like a fuel gauge.” The LinkPRO, the direct replacement for the old Xantrex XBM battery monitor, handles up to 10,000 amps DC and monitors two battery charges simultaneously.

The lower-priced LinkLITE is rated at 1,000 amps. Both monitors display voltage, amperage, consumed amp-hours, remaining charge and time remaining in the battery bank. The two units also feature a 500-amp shunt, enabling them to measure a high range of amperage. Unlike the larger rectangular footprint of the Xantrex Link 2000, each monitor is encased in a round bezel.

Xantrex has expanded the utility of its LinkPRO with its battery monitor communications interface, intended for use with a laptop computer. The separately sold monitor system consists of an interface box, a proprietary serial cable and PC-compatible software on a CD-ROM. Before ordering this interface, contact Xantrex to ask about the latest developments regarding software compatibility.

Victron markets a comparable gauge, also with a round bezel roughly the same size as the Xantrex pair. The Victron BMV 700 measures a voltage range of 6.5 to 95 volts and has a shunt selection capacity of up to 10,000 amps through its 500-amp quick-connect shunt. A big advantage offered by Victron is its BMV 702 Color Control GX unit, which brings together charge monitoring data from up to four BMV 700 monitors through CAN Bus (NMEA 2000), Ethernet cable or USB. Bluetooth capability allows you to received real-time charging data through your smartphone or tablet. For additional communication ability, you can add a Victron Global Remote, a modem that sends updates and alarms to your cellphone via text messages.  

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May 10, 2017 03:09 pm
 Posted by  Fred

These may be what is available for battery monitoring but they aren't really accurate enough to give a good indication of the state of charge. For 12v wet cells you must stay between 12.0 and 12.6v if the meter is only 5% accurate how can you know where you are in the discharge cycle? You need at least 100th volt readings to be reasonably sure of what it's telling you. This would not be a cheap "marine" panel meter.

May 18, 2017 10:47 am
 Posted by  AlainB

Fred, pure and precise instantaneous battery voltage is not an exact indication of the charge remaining in the bank, more so with deep discharge batteries. E.g.: with some level of charge, my meters reads 12.5V, after I run the fridge for 20 minutes the voltage will fall at about 12.3V and 10 minutes later the voltage will be 12.4V or even 12.5. Now exact readings when battery is at rest, 10 minutes after the fridge as stopped is still not an precise indication of the charge, a full battery will read 13.7 or about, after consuming 10% your voltage will drop to less than 13V. The voltage will slowly drop for the next 50%-70% of the charge down to 12V, the remaining charge will force voltage below 12V which can damage batteries at some point. The voltage vs charge is not linear. Battery manufacturers do not rely on voltage, the look at acid specific gravity; most do not quote full charge voltage.

Please do not pick on the above number, 1- I did not use my exact numbers, 2- I do not know exact theoretical numbers, 3- your battery is not the same age and the same make as mine, and 4- a 125 pound 4D is not the same as a 90 pound battery.

The only way to have a good (at an affordable price) indication of the remaining battery charge is with a battery monitor that measure charge and discharge in real time. This monitor must be intelligent, e.g.: to not integrate tickle charge current.

May 25, 2017 12:36 am
 Posted by  PAKoenig

You should look at the Balmar SmartGuage to get a better indication of state of charge for lead acid batteries. I used it for the Pacific Cup race to Hawaii in 2016 and for the return - 4800 nm in 6 weeks. I used it conjunction with the Xantrex and they did have different indications of state of charge. The Xantrex provided great and highly accurate load information, but the Balmar was the go to instrument for battery state of charge.

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