Battery Monitor, who is he? The super-expert explains it to you


Give or treat yourself to a subscription to the print + digital Journal of Sailing and for only 69 euros a year you get the magazine at home plus read it on your PC, smartphone and tablet. With a sea of advantages.

battery-monitorIn the last installment we learned how to roughly calculate daily electricity consumption. Today we see instead how the same thing can be achieved absolutely precisely and automatically with a simple electronic tool: the Battery Monitor (BM). This tool provides us with valuable information related not only to electricity consumption, but also to the state of charge of the batteries. In essence, the BM performs the function of an energy meter, just as Enel’s meter does. With one difference from this: in addition to counting the energy we consume-that is, the energy that comes out of the batteries-it also calculates the energy we provide during charging. The former has a conventional negative value (discharge), while the latter takes a positive value (charge), and the two add up algebraically.

To be used profitably, therefore, this instrument must be reset with fully charged batteries. By doing so, on its display we will be able to read at any instant the exact indication of how much these have discharged, i.e., how many Ah have been delivered, net of any energy input from external sources (solar panels, engine alternator, etc.). Suppose we reset the meter when we leave the port, with fully charged batteries. If after one day we consumed 100 Ah and during the same period the solar panels and alternator provided a total of 60 Ah, the Battery Monitor will report a discharge of 40 Ah. By knowing the total capacity of the installed batteries, we will therefore have a very accurate indication of the remaining range.

Screenshot 2016-05-04 at 12:05:54 p.m.The photo on the right is an example of how the instrument installed on board works: the numbers indicate that 32 Ah have been consumed since the meter was reset when the batteries were charged. Fifteen minutes before the survey it was 45 Ah, then the engine was turned on and in 15 minutes the alternator provided 13 Ah, bringing the energy deficit to 32 Ah. When properly set up with the capacity of the battery bank, this meter will also provide the state of charge in percentage remaining, making the reading even more immediate.

Another useful indication provided by the Battery Monitor is the instantaneous charge or discharge current. It basically tells us how many amps are coming out or going into the battery at a certain instant. This allows us to find out how much is the real consumption of a certain instrument, or a light bulb, etc., or to know how much your alternator charges when the engine is idling, or solar panels, wind, etc. In the side photo, for example, the display shows that the alternator is charging the batteries with a current of 24 amps. A precision digital voltmeter completes the set of measuring instruments contained in the Battery Monitor.

Installing a BM is not complicated and is certainly within everyone’s reach. The most difficult thing is to get the cables from the instrument-usually installed at the charting station-to the batteries. This requires an electrician’s probe and a little patience. All cables connected to the negative terminal of the battery should then be disconnected, to which the “shunt,” which is a small metal cylinder supplied with the BM, should be connected instead. The previously disconnected cables should then be connected to the other end of the shunt. The price is around 150 euros.



Roberto Minoia is involved in software design in his life, but as soon as he can he devotes himself to sailing in the Tyrrhenian Sea aboard his Dehler 41 CR. His passion for sailing and technology, combined with a penchant for popularization led, in 2008, to the birth of the Blog Della Vela ( where, consistent with his free time, he writes technical articles, reviews boating products that he himself uses, and publishes diaries of his sailings.



Leave a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Check out the latest issue

Are you already a subscriber?

Ultimi annunci
Our social

Sign up for our Newsletter

We give you a gift

Sailing, its stories, all boats, accessories. Sign up now for our free newsletter and receive the best news selected by the Sailing Newspaper editorial staff each week. Plus we give you one month of GdV digitally on PC, Tablet, Smartphone. Enter your email below, agree to the Privacy Policy and click the “sign me up” button. You will receive a code to activate your month of GdV for free!

Once you click on the button below check your mailbox



You may also be interested in.

Now you have the incentive if you buy an electric motor

  Incentives are coming for the purchase of a marine electric motor. After years of vain waiting, bureaucratic delays and the feeling of being practically “invisible” in the eyes of the government in that much-ballyhooed race for “ecological transition,” a

Here are the right self-inflating jackets to sail safely

Self-inflating jackets are personal protective equipment that ensure the safety of the entire crew. Just as we wear helmets when we ride motorcycles, when we are sailing life jackets and can mean the difference between life and death. By wearing

VIDEO New Generation Parasailor. We tried the supersail

The cruising world is increasingly witnessing the demise of spinnakers. There is little that can be done, no one uses them (almost) anymore. And perhaps rightly so, because, outside the regatta, it is an “uncomfortable” sail. The solution is often


Sign in