Rest first, then read voltage
Battery University BU-903’s BCI table for a rested starter battery after about 24 hours is the open-circuit clue we use: about 12.65 volts at full, 12.45 at 75 percent, 12.24 at 50, 12.06 at 25, 11.89 empty. Surface charge after a run or a charger session lies high. A 12.4-volt reading two minutes after shutdown is not 75 percent on that table.
Starting batteries need a cranking look
Voltage at rest can look decent on a pack that still will not spin a hot outboard. CCA and MCA on the label are the ratings; Wikipedia’s automotive-battery article is those tests. On the boat we care whether the engine actually cranks. A conductance tester is a tool. It is not a courtroom.
Deep-cycle packs need a load over time
A trolling battery can crank a meter and still collapse after twenty minutes at the bow. Reserve capacity and amp-hours are the paper ratings (BU-105). The practical test is whether the motor makes it to lunch. History from the owner matters: sat three weeks, charger unplugged, new motor that draws more.
Chargers fake a healthy pack
A battery on a pedestal can show 13 volts and still be a hollow box. We test after a rest or under a real load. Kissimmee driveway jobs often start with “it was fine on the charger.”
What we tell you
Replace, charge and retest, or the charger is the fault. Age and sulfation (BU-802) weigh in. We would rather replace a pack that will fail next weekend than send you away on a pretty voltage.
What a cranking test cannot say about trolling
A starting battery that spins the outboard can still be the wrong answer for a bow motor. We do not use a one-second conductance number as a reserve-capacity rating. If the complaint is the trolling motor, we talk time and voltage sag, then the charger. If the complaint is no-crank, we talk rest voltage and the engine. Those are two tests, even when both batteries are group 31s.
Sources
- Battery University BU-903 — How to Measure State of Charge
- Battery University BU-105 — Battery Definitions (RC at 25 A)
- Wikipedia — Automotive battery (CCA, CA/MCA, amp-hours, BCI group size)
- Battery University BU-802 — What Causes Capacity Loss
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.