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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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  • Keith on

    Hi Taylor,
    Thank you for your question. I will have to reevalulate that particular image and/or the text. It is more accurate to say "For longest life, flooded lead acid deep-cycle batteries should stay in or above the green zone. I recommend that you keep your state of charge above 50% in normal usage. Occasional discharge to 40% is not considered to cause any significant damage to a flooded lead acid deep-cycle battery by most battery manufacturers. You should recharge to 100% at intervals of less than seven to ten days for best performance and longevity. Most installations will recharge to 100% every day, less often when the weather is cloudy or a shady campsite is chosen. I don’t want to give the impression that deep-cycle batteries are going to suddenly and dramatically deteriorate when less than ideal conditions exist.
    Also, the graphic and my text seem to suggest that the target zone is 40 to 80 percent – I should say “the target (operating) zone should be from 50 to 100% in normal usage with a goal of charging to 100% whenever possible.”

  • Taylor on

    Very occasionally I like to look over your blog because there might be some interesting info there.
    While reading this page, I noted you stated the following:

    “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%.”

    When I look at the accompanying graphic, the ’ State of Charge green zone’ is shown to be between 40% and 80%. What I read when going over this page is the target zone that one should try to maintain the state of charge in ( as reflected by battery voltage at rest) is 40-80 percent SOC. This means that anything above 80 percent is not in your green zone.
    Is this an error or an easily misunderstood graphic? Or.. do I misunderstand how my battery works? My understanding is that for long battery life, you would want your green zone to be 40% – 100% state of charge.

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