Gray R. Riddick of Chocowinity, North Carolina, asks:
"I know you have discussed using a multimeter to check a battery’s voltage. What does the reading mean? Will it tell me whether or not a battery should be replaced? My two house batteries, for example, read 12.82 and 12.36 volts."
Nigel Calder replies:
A specific voltage reading won’t tell you much about a battery’s condition unless the battery has been rested for some hours; resting allows the voltage to stabilize. If the battery is in a stable state, there will be a correlation between voltage and its state of charge, but it will vary depending on the battery type—wet cell, gel cell, or absorbed glass mat (AGM.) In addition, wet-cell readings will also be affected by the concentration of electrolyte when the battery was made. To make any judgment about your two readings, I would have to know the battery type, whether the readings were taken after the batteries had been rested, and what the discharge rate was. Tracking a state of charge is much easier when you have a systems monitor installed; it can measure discharges and recharges and display the state of charge.
A voltage reading can be useful in checking the health of a charging system because you see how much voltage is reaching the batteries, rather than the voltage at the alternator or battery charger. Often there’s a voltage loss somewhere in the charging circuit that impairs performance—and ultimately battery life. A voltage reading across a battery’s posts when it is being charged should rise and then stabilize for a while at somewhere between 14.2 and 14.4 volts, depending on the type of battery and the system’s programmed parameters. Wet-cell batteries, for example, will typically be charged at 14.4 volts and AGMs at 14.1 or 14.2. As a battery comes up to full charge, voltage should drop to a float level, which can vary from a low of 13.2 volts (for many AGM batteries) to a high of 13.8 volts (for some wet-cell batteries). See my Boatowner’s Mechanical and Electrical Manual, 3rd edition, for more details.