Knowing the voltage of all your boat batteries at any given time is critical to managing your onboard electrical power system. The Low Battery Alarm from Battery Failsafe Devices (so-bfd.com) not only tells you the voltage, but also alerts you if volts fall below a pre-set minimum.
The company sent me one, and I mounted 3x4-inch instrument in the cabin, with an on-off-on toggle switch so I can monitor both the starting and house batteries. The LED display is easy to read, and setting the minimum thresholds for the alarm is about as simple as it gets. I tested it by setting the minimum relatively high, and sure enough, it emited a piercing audible alarm and flashed a red light to indicate a low battery.
The device can be used for either 12- or 24-volt systems. I chose the cabin location to install it because the instrument and its plug receptacle are not waterproof, and the flashing green “good-to-go” light can be a bit blinding at night, if mounted at the helm. The Low Battery Alarm can utilize a cigarette-lighter style plug or you can hard wire it to the battery, as I did. It sells for $57.95, plus shipping, and that includes a one-year warranty.
For complete guide to marine batteries, visit Boatingmag.com/batteries.