What a Battery Test Measures

Batteries rarely give much warning. One morning in December the starter turns slowly, and the next morning it clicks. A battery test is the way to see that coming. It takes very little time in the bay and is the single best way to avoid a dead car in a parking lot in Minden in January. Here is what the test is actually measuring and how to read the result.

At a glance

• Voltage shows charge; a load or conductance test shows health.
• Cold cranking amps (CCA) is the rating that matters most in winter.
• The Car Care Council advises testing after three years and before every winter.
• Summer heat often does the damage that winter cold reveals.

What does the voltage reading tell you?

A fully charged 12-volt battery at rest reads about 12.6 volts or a little higher. Around 12.4 volts is roughly three quarters charged, and 12.0 volts or lower is close to discharged. Voltage is a state-of-charge reading, not a health reading. A battery can show full voltage and still collapse under load, and a healthy battery can read low simply because the car sat for two weeks at Reno-Tahoe airport parking.

Typical values for a standard flooded lead-acid battery at rest. AGM batteries read slightly higher.

What is cold cranking amps?

Cold cranking amps, or CCA, is the rating printed on the battery label. It is the current the battery can deliver at 0 F for 30 seconds while staying above a minimum voltage. Cold thickens engine oil and slows the chemical reaction inside the battery, so starting takes more current just when the battery has less to give. That is why CCA is the number that matters in a place where Minden averages about 188 freezing nights a year and Reno about 122 (NOAA 1991-2020 normals).

How does a battery tester measure health?

Most shops now use electronic conductance testers. They send a small signal through the battery and estimate how much cranking power it can actually deliver, then compare that with the CCA rating you enter. The result usually reads good, good but recharge, charge and retest, or replace. Older carbon-pile load testers do the same job by drawing a heavy current and watching the voltage drop. Many testers also check the alternator's charging voltage and the starter's draw, which helps separate a battery problem from a charging problem.

• Good: the battery delivers close to its rating
• Good, recharge: healthy but low on charge
• Charge and retest: too low to judge; charge it first
• Replace: it can no longer deliver enough current

Why do Northern Nevada batteries wear out?

Heat starts it. Reno averages about 65 days a year at 90 F or hotter (NOAA 1991-2020 normals), and under-hood temperatures run far higher. Heat speeds up corrosion inside the battery and evaporates electrolyte. The damage builds quietly over the summer, and then the first cold snap in November asks for full cranking power from a battery that no longer has it. Short trips add to it, since the alternator never gets time to fully recharge. The Car Care Council suggests testing a battery once it is three years old and before every winter.

What we do about it in the bay

A battery check is part of the maintenance check at service centers that offer it; call your service center to confirm. The technician tests the battery, looks at the terminals for corrosion and checks that the hold-down is tight, then tells you the result and what it means. If the battery tests good, you hear that. If it tests weak, you decide what to do next.

Fall is the best time. The first freeze checklist and our cold weather battery guide cover the rest. Drive in to Mae Anne Avenue, Longley Lane, College Parkway or US 395 in Gardnerville.