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Keeping track of battery charge levels


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What is the issue?
Most boats have analogue voltmeters to indicate battery charge levels / state-of-charge. The needle readout on analogue voltmeters are 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?
Analogue voltmeters are not readable at this level of detail. It is very important 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 lead-acid battery to zero volts, just once, will render a battery unusable.

How to address this?
Replace or augment the standard analogue voltmeter with a digital panel meter as presented in figure 1.

The presented 5 cm x 2 cm of display we implement provided us with a large two decimal point reading. Placed in a conspicuous location it was very easy to see the exact battery status at a glance and it revolutionised our battery control.

Here are the state-of-charge readings with an open circuit voltage or float voltage (battery totally disconnected from any load). After a full charge the terminal voltage will drop quickly to 13.2 V and then slowly to:

Float Voltage State-of-charge

12.65 V 100 %

12.45 V 75 %

12.24 V 50 %

12.06 V 25 %

11.89 V 0 %

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.


With thanks to:
Michael Harpur, Yacht Obsession.



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