Solving the Power Puzzle

When is a 12-volt battery considered dead?


I want to be able to tell how discharged my batteries are in order to maintain them properly (not let them discharge below 50 percent of their capacity). Im dealing with deep-discharge batteries (“house” and for trolling motors).

Relying on sources such as Nigel Calders books, Im told that the approximate voltages corresponding to 100 percent, 75 percent, 50 percent, 25 percent, and 0 percent of a batterys capacity are 12.65 volts, 12.45 volts, 12.25 volts, 12.1 volts, and 11.9 volts, respectively.

However, contradicting those figures, the standard for measuring ampere-hours (Ah) is based on how long it takes a battery, providing a standard flow of current, to drop to 10.5 volts. This would mean that the battery is 1.4 volts lower than when its considered discharged to 0 percent (according to Calder). Whats the difference in meaning between the 11.9 and 10.5 volts?

Because the “Calder scale” indicates a 0-percent battery at 11.9 volts, and the Ah test brings it to 10.5 volts, Im guessing that Calders scale from 12.65 to 11.9 volts is really a scale covering only the upper 50 percent of a batterys capacity.

The second possibility is that Calders scale does represent the full capacity, and-despite the Ah standard-to avoid dropping below 50 percent, I should not let my battery drop below 12.25 volts, the 50 percent mark on Calders scale?

So, what really is the voltage at the 50 percent mark? The difference between the right and wrong answers could make a big difference in how much battery I have to carry.

Kenneth Crossner
Somerset, N.J.

Nigel Calder is referring to a no-load voltage test, which provides a fair estimation of charge. Voltage testing under a specific load and chemical testing of the electrolytes specific gravity are more precise means of measuring the charge level.

Battery voltage measurements can be very confusing. Is a battery dead at 10.5 volts or 11.7 volts? It depends on the context. Most U.S. companies define a battery as dead when it cannot sustain 10.5 volts under load at the industry standard C20 discharge rate (5 amps), at a temperature of 80 degrees. For a 100-amp-hour Group 27 battery, this works out to a 5-amp load for 20 hours before reaching the 10.5-volt level. Once the load is removed, the battery voltage immediately begins to climb. With no load on the battery, one can measure open-circuit voltage-a reasonable and easy way to determine a batterys charge, and the only way for sealed batteries.

A battery discharged to 10.5 volts at C20 should bounce back to approximately 11.7 volts, open circuit. When access to the cell electrolyte is possible, a good hydrometer is the most accurate way to check the state of charge and cell imbalances.

For an accurate open-circuit voltage measurement, the battery must have had no loads for a minimum of several hours (24 hours is best). According to the industry group Battery Council International, the open-circuit voltage, state of charge, and specific gravity of a lead-acid battery are as follows: 12.6+ volts, 100 percent, specific gravity 1.265; 12.4 volts, 75 percent, 1.225; 12.2 volts, 50 percent, 1.19; 12 volts, 25 percent, 1.155; 11.7 volts, 0 percent, 1.12.

When estimating battery capacity-to prolong the life of your battery and, therefore, maximize your cost efficiency-you should try to estimate charging and battery requirements based on the assumption that you will not discharge your batteries below the 50 percent mark. This is an ideal scenario, and it inevitably involves some trade-offs because of the size and weight of batteries required to meet the 50-percent level and avoid constant recharging.

Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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