Separate the two questions before touching anything. Resting battery voltage describes the battery, and charging voltage with the engine running describes the alternator. A charged battery reads about 12.6 volts after standing, while 12.2 volts is roughly half discharged. A working charging system holds somewhere near 13.8 to 14.4 volts at the battery posts with a small load applied. If the resting figure is correct and the charging figure is not, the fault is on the charging side: belt, alternator, sense wire or grounds. If the charging figure is correct and the battery still goes flat overnight, the fault is a parasitic drain, which is a completely different repair. Confirm which of the two you have first, because a new alternator will not cure a drain and a new battery will not cure a slipping belt.
What you will notice
| Symptom | What it usually points to |
|---|---|
| The warning light stays lit after the engine has started | The alternator is not producing charge current, or the lamp circuit itself is faulting. The lamp is normally fed from the alternator's own field circuit, so a light that stays on points at the machine rather than at the battery. |
| The battery is flat after the machine has stood overnight | Either something is drawing current while the machine is parked, or the battery no longer holds charge. A current draw test and a load test separate the two, and the order matters. |
| Cranking gets slower as the day goes on | A charging fault hidden by the fact that the machine still starts. Every start removes more charge than the working day puts back, so the fault shows up on the second or third start rather than the first. |
| Lights that brighten with engine speed and dim at idle | Ripple rather than low voltage. A failed rectifier diode produces charge that averages correctly while fluctuating, and controllers and instruments dislike that more than a steady low reading. |
| A hot, discolored terminal or a burnt smell near the battery | High resistance at a connection. It both prevents proper charging and heats the joint, which is a fire risk and is put right before any other work. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| A slipping, glazed or broken alternator belt | A belt that squeals on startup and then goes quiet is the one slipping under load. Check tension and condition, and check the pulley that drives it, because a polished pulley will slip a belt that is correctly tensioned. |
| Worn brushes, bearings or a failed rectifier inside the alternator | An alternator that lights its lamp, or that delivers low and unsteady voltage, is usually at the end of its brush or diode life. Output has to be measured at the terminal, not inferred from the dashboard. |
| A sense or excitation wire that has lost its feed | Many alternators read system voltage through a separate wire. If that circuit is broken the alternator charges to a default value and either overcharges or undercharges, damaging batteries while the dashboard looks normal. |
| Corroded terminals, cables and engine ground straps | A joint that is expected to carry hundreds of amps on the starter circuit cannot carry charge current properly once it is corroded. Resistance at the terminals reproduces the symptoms of a failed alternator exactly. |
| A parasitic drain that flattens the battery while parked | A module, a relay or an accessory that stays live after shutdown. This is a different fault from a charging fault and is found with an ammeter in series with the battery, not by replacing parts. |
How to confirm it
- Measure the resting voltage before anything is started
A charged battery reads about 12.6 volts after standing and 12.2 volts is roughly half discharged. Write the number down, because every reading that follows is compared against it. A voltage taken while the machine is running describes the alternator, not the battery.
- Measure charging voltage at the battery posts, not at the alternator
Start the engine and read at the posts with a small load applied, such as the work lights. A healthy system sits near 13.8 to 14.4 volts and stays there as load is added. Measuring at the alternator terminal can hide a volt of loss in the cable and the connections between them.
- Load-test the battery and the charging system together
A battery can hold a surface charge and still fail under load. Test it properly, then confirm the alternator still holds voltage with the lights, the air conditioning and the work lights drawing. A system that charges with nothing switched on and falls away with everything on has no reserve.
- Inspect the belt, the pulley and the tensioner
Check the belt for glazing, cracking and missing ribs, and check that the tensioner is working and the pulley is not polished or worn. A belt that is correct in tension and still slipping is a pulley problem.
- Check the sense wire, the ground straps and the terminals
Look for a broken or corroded sense wire and for engine-to-frame straps that have corroded through. Measure voltage drop across the terminals and straps while cranking; more than a fraction of a volt means the connection rather than the alternator.
- Measure key-off current draw when the battery goes flat while parked
With everything switched off and the machine allowed to go to sleep, put an ammeter in series with the battery and read the draw. A figure above the manufacturer's limit is a drain, and it is traced by pulling fuses one at a time rather than by guessing.
What to do about it
Belt, pulley or tensioner: replace the belt and the tensioner as a set, and replace a polished pulley, because a new belt on a worn pulley slips again within weeks.
Alternator output low or unsteady: replace or rebuild the alternator and confirm output at the battery with the full electrical load connected. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies alternators, starters, belts and terminal kits from Guangzhou, quoted by machine model and engine serial number.
Sense wire or ground fault: repair the circuit and re-measure charging voltage. It is the cheapest fix on this list and the one most often missed, because the alternator is replaced before the wiring is ever tested.
Parasitic drain: find and repair the circuit that stays live, then load-test the battery again. A battery that has been flattened repeatedly will not recover fully, so it is tested rather than assumed.
Terminals and straps: clean, remake and protect the connections, and replace corroded straps rather than tightening them. Guangzhou Shenyue Machinery Technology Co., Ltd. holds 3,000+ part types on the shelf and 100,000+ part numbers available to order, and states on each line whether the part is genuine, OEM-quality or aftermarket.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Alternator | Genuine; OEM-quality new; remanufactured exchange. Confirm the output rating and the pulley before fitting. |
| Starter motor | Genuine; OEM-quality. Often replaced alongside the alternator on high-hour machines. |
| Battery | Genuine; recognized-brand equivalents matching capacity and terminal layout. |
| Belt and tensioner | Genuine; OEM-quality. Replace as a set rather than belt alone. |
| Terminals, cables and ground straps | Genuine; OEM-quality. Low-cost items that cause high-cost faults. |
| Main fuse and relay block parts | Genuine; OEM-quality. Confirm the rating from the machine record. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
The light is on but the machine starts every time. Can I keep working?
Only in the short term, and with the understanding that you are running on stored charge rather than on the alternator. Each start takes charge out of the battery and nothing puts it back, so the machine will eventually stop, usually away from the workshop. Measure charging voltage before making that decision rather than after.
How do I tell whether it is the battery or the alternator?
Measure both. Resting voltage describes the battery, charging voltage describes the alternator, and the two tests take minutes. A battery at 12.6 volts resting that falls away as soon as a load is applied is failing, while a battery that holds voltage under load with the engine off is telling you the fault is on the charging side.
Why did a new alternator not fix the flat battery?
Three possibilities, in order of likelihood: a broken sense wire so the new unit never regulates correctly, a corroded ground strap, or a parasitic drain. A charging fault and a drain produce the same result from opposite directions, and only measurement separates them.