Estimating Total Capacity in an Old Battery

Volt meters, ammeters, and bank monitors can all give you part of the story, but only a draw-down test, performed over 6-24 hours will give a true picture of battery capacity.

To carry out this process you’ll need some basic tools and materials that will serve you well for other on-board electrical projects: a basic multimeter (meters from Gardner Bender and Fluke fared well in our testing, see “Digital Multimeters,” April 2004), a clamp-on DC ammeter (Klein CL120, $55, is an economical choice), a DIY constant load (a 12-volt auto headlamp of known wattage works) or, preferably, an electronic adjustable resistance tester (MakerHawk has a $50 tester rated for 150W-200V-20A; www.makerhawk.com). You’ll also need a battery hydrometer or refractometer.

TEMPERATURE VS. VOLTAGE

If you only sail in the summer, and your batteries sit in the bilge, their temperature is moderated by the warm water. In 80-degree waters, you’ll get used to 12.7 volts being a full battery and 12.2 being about 50 percent, and that is all you need to know. But that changes in cold water or if the batteries are on the bridgedeck of a catamaran.

Battery voltage drops with temperature. In theory, amp-hours available remain constant, but in practice,
useful power falls considerably.

Power is measured in watts (volts x amps), so at a lower voltage it takes more amps to deliver the power required to run a windlass or start an engine. More importantly, there is a minimum voltage, about 12.1 V, below which some electronics shut down, 12V motors won’t run at full power and engines don’t start. In cold temperatures that minimum voltage will be reached when the battery still has considerable charge remaining. In effect, one-third or more of the battery capacity may become unusable in winter due to the drop in ambient temperature.

VOLTAGE VS. DRAW

If you are drawing more than a few amps, the voltage will drop. If you are charging, the voltage is boosted. If the battery was just fully charged and there is zero draw, the battery can hold what is known as a surface charge, reading over 13 volts, though that will dissipate and the battery will return to normal full charge voltage if loaded with just a few amps for 10 minutes, or by “resting” under no load (or charge) for 20 minutes. Measure battery voltage with zero load (nothing connected) and only after any surface charge has dissipated.

USING A HYDROMETER

Use a hydrometer or refractometer to estimate the sulfate ions in the electrolyte, and from that, determine whether the battery is fully charged in a chemical sense. A hydrometer does not tell you about the batteries internal condition, its capacity in amp hours (Ah) or its capacity in cold cranking amps (CCA). It will, however confirm the battery has reached a fully charged state, necessary for an accurate test of capacity. It can also double-check the before/after SOC in the draw-down test described below.

A battery that stays 1-2 volts below the Table A voltage even after extended charging, and shows full SOC by refractometer probably has a shorted cell.

ESTIMATING AH CAPACITY

1. Top off the battery water.

2. Fully charge the battery and disconnect all charging sources and loads. Allow to rest.

3. Check SOC of each cell with a refractometer.

4. Check the voltage with a multimeter. Check this result against Table A.

5. Use your constant load (or auto headlamp, or any boat equipment) to draw the battery down over 6-24 hours until you reach about 50 percent SOC as measured in volts. A longer draw-down period is better, but you may have multiple batteries to test and 6 hours is sufficient. (If you use an auto headlamp make sure it is on a surface that won’t burn.)

6. Monitor current (amps) of load during the test. If testing the whole bank, the panel ammeter can also be used for this. If testing one battery at a time, you can do this using a clamp-on DC ammeter.

7. Allow the batteries to rest and check the voltage against table A. The average current draw in amps multiplied by the time in hours is the Ah used during the test. The estimated remaining battery Ah capacity can be determined using the following formula: Total Ah drawn during the test x (100 / percent of charge at start – percent of charge at finish) = the estimated battery Ah capacity.

For example: It is 70 degrees F and we have a fully charged battery rated for 80 Ah. We apply a 5 amp load for 8 hours, so the total amps drawn is 40 Ah. We check the voltage and it reads 11.9 volts, refer to Table A and determine 11.9 volts is 40 percent of capacity at 70F. The formula 40Ah x (100/100-40) = 66.7 Ah gives us our maximum capacity (67 Ah), and tells us that this Ah capacity is about 83 percent of its original 80Ah capacity (66.7/80 x 100 = 83).

You can run the test as a bank, but you will have to retest each battery alone if you suspect that there is one bad battery. For example, the voltage is not quite up, the Ah numbers are low, and specifically, if one battery is using water faster than the others.

Remember, if you routinely discharge flooded lead acid batteries below 50 percent SOC, discharge them to nearly dead just a few times, or leave the battery below 50 percent SOC for long periods, you can severely reduce battery life expectancy. AGMs can be damaged if left below 80 percent SOC for long periods, including cruises where the battery cannot be fully charged every few days (see “Don’t Kill that AGM,” PS July 2021 for more information.)