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Keeping track of battery charge levels

What is the issue?

Most boats have analogue voltmeters to indicate battery charge levels / state-of-charge. The needle readout on analogue voltmeters is scarcely readable at any distance and small shifts are imperceptible. Yet the battery state-of-charge can traverse from a 100% full charge to 0% complete discharge in less than the movement of a single volt on the meter.

Why address this?

Volt Meter
Photo: Public Domain

The moment the shore-power cord is unplugged and a boat moves away from the dock, almost all energy the boat uses comes from the batteries. Standard analogue voltmeters are not readable at this level of detail.

It is critical to know what power capacity is available to the vessel at any time for correct operation of the vessels electrics. Careful management is just as important for the life of the batteries. Discharging a battery even slightly below its fully discharged voltage shortens its serviceable life. Fully discharging a lead-acid battery to zero volts a few times can destroy a battery.

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How to address this?

Digital LCD Voltmeter
Photo: Public Domain

Install a low-cost digital LCD display panel with two decimal points in a conspicuous position where it can be glanced at in passing throughout the day. This will transform the understanding of the battery charge status.

The voltage of a 12V battery bank should ideally be between 12 - 13 volts. A battery should rest a minimum of 30 minutes after charging to allow the surface charge to dissipate before a reading is taken. A fully charged 12V battery should register about 13.5 volts when there is no load on it, and this value should drop slightly when there is a load on it. Around 12.7 or 12.8 volts is a good average. The voltage of a 24V bank of batteries should be maintained at 24 – 26 volts offload and approximately 24V on load. The charging voltage should be more than 24V, usually 25 – 30 Volts, depending on the condition of the batteries which should be checked daily.

Another useful approach can be to plug in an off-the-shelf car charger socket voltmeter into a socket that can usually be found in the vicinity of the navigation station on most yachts. As it happens our last yacht had a digital voltmeter as part of the boat console. But it required some exploring through menus and its fine print needed close up inspection, with glasses in my case. Permanently keeping a car voltmeter inserted, pictured below, had the house battery data brightly lit so I could see it everytime I passed. It also provided the advantage of two 2.4A charging ports for the ever-present tablet and smartphone.

Voltmeter with dual USB charging ports
Photo: Public Domain

The problem with the float voltage is you have to completely disconnect any load from the battery and wait about 20 minutes for the battery to stabilize to take this reading. Add just the slightest load and a 12V battery will drop about 0.3 volt. Add more and it will fall further. This makes it difficult to know the precise capacity available depending on what loads are being imposed.

However with some experience of the various loads such as GPS, Navigation Lights etc, in conjunction with the aid of an accurate digital panel meter, you will in time find it is easy to get a good view of battery capacity in operation.

If you can spend a little more money it may be worth considering installing enhanced battery monitors from companies Victron, see video below, Simarine Pico, Trimetric or Maretron. These provide Volts, Amps, Amp Hours, State of Charge, Time To Go and have many programmable alarms, warnings, relay settings and can detail various charging sources etc. See also care and maintenance of batteries.

With thanks to:

Michael Harpur, Yacht Obsession.

Installing a Victron battery monitor

NASA Clipper BM-1 Battery Monitor Review

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