How many times have you heard that electricity is mysterious, unfathomable, beyond the ken of the average human? The difficulty in understanding electricity is derived from the fact that we can’t see it. We can see its effect in a light bulb, hear its effect when a radio is turned on, or feel its effect when we operate a fan. We can understand the work being done, but not the electricity itself.

For most of us, a great deal of the mystery surrounding electricity would be removed if we could see electricity as it was produced, directed into our batteries, and used by our various appliances. Fortunately, sophisticated yet affordable system monitors are now readily available that let us do just that, but before we review the gear presently on the market and how it operates, let’s go over what information we would ideally like to know to track the operation and general health of our electrical power system.

Knowing how much electrical current each of our charging sources is producing at any given time allows us to determine how well they are performing in various conditions (levels of sunshine for solar panels, wind speeds for wind generators, boat speeds for water generators, and engine rpm for high-output alternators). From this we can estimate how much energy the charging sources are producing over time (ie., if a device is producing 10 amps for 5 hours it has produced 50 amp-hours). Without an ammeter to read charging source output, you simply don’t know how or even if it is performing.

Knowing how much electricity is being used by our appliances at any given time, and over periods of time, helps us track energy use and maintain a balance between energy production and energy consumption. Having an accurate ammeter on the load side of our power system also tells us instantly how efficient our appliances are.

Monitoring battery state of charge is also essential to good system operation. In a lead-acid battery (wet or gel), battery state of charge is proportional to voltage readings, making an accurate voltmeter function desirable. Since only a single volt distinguishes between a full and a discharged battery, it’s best to have the voltage displayed like a fuel gauge (from empty to full) or digitally to at least a tenth of a volt accuracy. Unfortunately, voltage readings serve only as an indictation of battery health, since voltage can be temporarily affected by charging or discharging sources and by temperature.

More accurate information about how much energy is available in a battery bank would be helpful. Amp-hour gauges do just that by counting amp-hours going into or out of a battery. A single glance tells you how much energy remains and how long before it’s time to recharge.

Modern digital system monitors supply all the information described above and more in a single, simple-to-use package, allowing you to assume the role of system manager of your own small utility company. System monitors use a shunt or loop current measuring device installed in-line with a specific battery bank, usually the house bank since it’s not necessary to count amp-hours in a starting battery. Current is displayed as the sum total of all charging or discharging gear at any given moment. To read an individual charging source output, you must turn off all other chargers and all appliances. To read an individual appliance load, turn off all other appliances and all charging sources.

Models designed to monitor a single house bank include the Heart Interface Link 10, Cruising Equipment’s E-Meter, and Ample Power’s BatMon. The Link 20 from Heart provides complete monitoring functions for two battery banks, while Ample Power’s EMON Energy Monitor II monitors one house bank, displays voltage only for an engine starting bank, and as an option amps and amp-hours produced by renewable charging sources. A more sophisticated approach is taken by the Systems Monitor from SALT, which can monitor up to four battery banks or charging sources, uses either standard shunts or unique current loops (no cable splicing necessary), has up to twenty programmable alarm functions, and can connect to a PC.

Installation of system monitor consists of connecting a current shunt or loop in the main negative cable of each battery bank to be monitored, then running a handful of low-current wires from various points on or near the battery bank back to the system monitor display. The display can be mounted in any convenient location onboard, typically near the nav station.