Why isn't my alternator charging my 100 Ah battery?
Why isn't my alternator charging my 100 Ah battery? I expected it to recharge with the engine running, but it still seems low afterwards. What should I check before buying another battery or alternator?
Answer
A healthy alternator can charge a 100 Ah battery; capacity alone will not prevent charging. Check the belt, battery connections, earth connections and charging fuses first, then compare voltage at the battery and alternator with the engine running to find where charging stops.
Check the belt and connections
On a conventional 12 V installation, common faults include a slipping belt, corroded terminals, a failed fuse or a poor engine-to-battery negative connection. Check these before replacing the alternator: any one can interrupt an otherwise healthy charging system.
Stop the engine and isolate the batteries before removing connections. Keep probes and clothing clear of moving belts during running tests, and never disconnect a battery while the engine is running.
- Belt: look for cracks, glazing, black dust and loose adjustment. Set tension to the engine or alternator manufacturer's specification; excessive tension damages bearings.
- Positive connections: inspect the battery posts, alternator B+ terminal and any starter-terminal junction, battery switch or split-charge device.
- Negative connections: check the battery negative cable, engine earth stud and alternator earth connection where fitted.
- Fuses: check both the heavy charging cable protection and any small regulator-supply fuse. Test continuity with power isolated; appearance alone tells you little.
Clean and tighten suspect connections. Replace heat-damaged terminals; do not simply tighten a burnt connection or fit a larger fuse.
Measure battery and alternator voltage
A basic digital multimeter is enough for the first checks. Measure directly on the battery's lead posts, rather than on the cable clamps.
- Engine stopped: after several hours without charging or substantial loads, a fully charged lead-acid battery typically reads about 12.6 to 12.8 V. Around 12.2 V suggests roughly half charge, although temperature and battery type affect the reading.
- Engine running: raise engine speed to roughly 1,500 to 2,000 rpm, within the engine's permitted unloaded range. Many alternators deliver little current at idle.
- Check charging voltage: a conventional lead-acid system commonly rises to about 13.8 to 14.6 V, depending on regulator setting, temperature and charging stage. A reading that stays around 12.2 to 12.6 V usually means little or no charging.
- Compare at the alternator: measure B+ against its designated negative return or casing earth, as applicable. Around 14.2 V there but only 12.5 V at the battery points to the wiring, switch, fuse or split-charge equipment.
Under meaningful charging load, measure voltage drop from alternator B+ to battery positive, then across the negative return. A drop above about 0.2 V on either side needs investigation. Older diode splitters can introduce a larger positive-side drop unless the regulator compensates for it.