Battery Monitors

Battery monitor battery voltage state of charge

bogart monitor and shunt

The Bogart Trimetric battery monitor uses current and voltage readings to track the power in and out of a battery. A precision resistive shunt is installed between the negative terminal of the battery and all the loads. All current going into the battery (charging) and all current leaving the battery (discharging) is measured. Electronic circuitry and a microcontroller record battery voltage and battery current.

Because the Trimetric keeps track of current leaving or entering the battery, a very good approximation of the state of charge of the battery can be determined and displayed.

Less Expensive Battery Monitor

A battery monitor could be as simple as a voltmeter and an ammeter, permanently installed so the user can keep tabs on the battery at any time. If the user has an understanding of the properties of deep-cycle batteries, a great deal of information can be observed and recorded.

analog voltmeter  analog ammeter

The battery voltage is displayed on the voltmeter. The ammeter is connected such that all the current going in and out of the battery passes through the meter. Observing the ammeter will tell the user ‘how heavy’ the load is that is powered by the battery or how much current the charging system is providing.

State of Charge Related to Battery Voltage with No Load

Voltages shown are for a 12-volt battery system at 25 degrees C (77 degrees F) . These voltages are for batteries that have been at rest for 3 hours or more and not being charged. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity with a hydrometer, but in some battery installations, this is difficult or impossible.

Approximate State of Charge 12 Volt lead acid battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.4 2.07
70% 12.3 2.05
60% 12.2 2.03
50% 12.1 2.01
40% 11.9 1.98
30% 11.8 1.96
20% 11.6 1.93
10% 11.3 1.89
0 10.5 1.75

table showing approximate state of charge for an unloaded lead acid 12V battery.

It is very important to remember that the voltages shown in this table represent a 12-volt deep-cycle battery that is at rest - there is no load and there is no charger connected to the battery.

For longest life, batteries should stay in the green zone. I recommend that the battery state of charge not be allowed to drop below 50%. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life. Lead acid batteries should be fully charged at least every 7 days.

If the user has time and records battery and current readings regularly, a profile of the battery may be used to keep the battery state of charge in the recommended zone. Most people however greatly benefit from a quality battery monitor that is designed to do this work for you!


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