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A New Battery Monitor

Back in the 1990s when I was cruising full time and living aboard a 1964 Pearson Alberg 35, my electrical system was dirt simple. I had two 100AH wet-cell batteries, a battery selector switch, a 30-watt flexible solar panel and a multimeter. When I wanted to know how the batteries were doing, I put the

Back in the 1990s when I was cruising full time and living aboard a 1964 Pearson Alberg 35, my electrical system was dirt simple. I had two 100AH wet-cell batteries, a battery selector switch, a 30-watt flexible solar panel and a multimeter. When I wanted to know how the batteries were doing, I put the multimeter leads on the battery terminals and read the surface voltage. If ever I needed to engage in more sophisticated analysis, I got out my big eye-dropper-style hydrometer and tested the specific gravity of the electrolyte solution in the battery cells.

Much later, I thought I’d died and gone to heaven when I moved into my current boat, Lunacy, a 39-foot Tanton aluminum cutter. She was equipped with a Heart Interface inverter/charger with a matching Link 2000 control pad that not only controlled the charger and inverter, but also monitored both the (rather large) house battery bank and the separate (rather small) engine-cranking bank, providing detailed data as to surface voltage, amperage flowing in or out, and total available amp-hour capacity. About 20 years earlier (back when I was still playing with my eye-dropper and multimeter on the Alberg), this had been state-of the-art technology and Heart Interface kit (produced by a small independent company, Cruising Equipment Co., based in Seattle) was what every savvy cruiser craved.

I reveled in all the information offered by my Link 2000 monitor for some five years, until this year, when suddenly said information became very dark and grim. The monitor continually displayed “Low Battery” warnings for both banks, showed their respective amp-hour capacities as being sorely depleted, and measured amperage flowing out of the batteries at all times, even when there was no load on them. Even worse, negative amperage readings persisted when the batteries were fed power from any and all sources (wind/solar, engine alternator and shore connection). The only normal readings were for voltage, which I easily confirmed with my old friend the multimeter.

I quickly concluded that either all the batteries on the boat had abruptly died and gone to heaven themselves, or the battery monitor had gone insane. I discussed this situation with Jeff Stack, service manager at Maine Yacht Center, and he agreed the latter scenario was much more likely. In the past year or so, he said, he’d seen several formerly reliable 90s-vintage Heart Interface monitors suddenly go crazy.

I asked Jeff to look into my electrical system, and he quickly confirmed that my batteries were fine and the monitor needed replacing. Not too surprisingly, there is now no support for old Heart Interface gear, as the old company has been consumed more than once by bigger ones. The current leader of the pack in this corner of the market is Mastervolt, a Dutch company that produces a vast array of marine electrical equipment. I checked out their Masterlink BTM-III battery monitor and decided it would suit my purposes quite nicely.

Though it has many fewer buttons than my old Link 2000 monitor, it is actually a bit more sophisticated. For one thing, it can monitor three different battery banks, though one (the primary house bank) is monitored in more detail than the others. Like the Link 2000, the BTM-III can tell you how many amps are flowing into or out of a battery, how many amp-hours have been consumed, and how long a battery can feed a given load before running dangerously low on juice. It can also, however, provide oodles of interesting “historical” information: time elapsed since the battery was last fully charged; time elapsed since the last “low battery” alarm; total number of discharge-recharge cycles; number of “abusive” cycles; total number of hours the battery has spent discharged below 20 percent; total amp-hours consumed; average depth of discharge; deepest discharge; and highest recorded voltage.

With this kind of data, I expect I’ll be able to write a very melodramatic biography of my batteries. Soon to be followed by a major motion picture, action figures, and all the rest.

One interesting note: I can’t just toss the old Link 2000 unit, as still I need it to control my inverter/charger. So now the two monitors are mounted side-by-side on my distribution panel—one of them spewing nonsense and defaming my poor batteries, the other providing a sober and reliable account of their doings.

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