Check three things in order: battery open-circuit voltage after a rest, voltage at the battery while cranking, and charging voltage at fast idle. A healthy battery reads about 12.6 volts at rest, should not fall below roughly 9.5 to 10 volts while cranking, and the alternator should hold about 13.8 to 14.4 volts at fast idle with loads on. Terminals and ground straps cause more failures than cells do, so clean and tighten them before condemning any component.
How to confirm it
- Inspect and clean the battery terminals
Disconnect negative first, clean both posts and both cable ends to bright metal, refit and protect. Note the condition of the cable under the insulation, not just at the visible end.
- Measure battery voltage at rest
With the machine off and rested, a healthy battery reads about 12.6 volts. Around 12.2 volts indicates partial discharge and 12.0 volts or below indicates a battery that needs charging or replacing.
- Measure voltage while cranking
The reading at the battery posts during cranking should stay at about 9.5 to 10 volts or above. A reading that collapses below that points to the battery or the cables, not to the starter.
- Measure voltage drop across cables and grounds
From battery negative to engine block with the engine cranking, the drop should be a fraction of a volt. A higher figure identifies the cable or strap that needs attention.
- Measure charging voltage at fast idle with loads on
Expect about 13.8 to 14.4 volts. A low reading means the charging circuit is the problem; check the belt and the sense wire before replacing the alternator.
- Check belt condition and tension
A glazed or slack belt produces the same symptoms as a weak alternator, at a fraction of the cost. This takes two minutes and should precede any alternator decision.
- Record the readings and the battery date
Batteries and alternators fail gradually. A written record of rest voltage and charging voltage at each service shows the trend, which is what allows a replacement to be planned rather than suffered.
What to do about it
Battery that will not hold charge: charge it and re-test. If it will not hold above about 12.4 volts after a full charge and a rest, replace it. Do not replace an alternator to cure a battery fault.
Cranking voltage below specification with a charged battery: inspect and replace cables and ground straps before the starter. Cable faults are cheap, common and easy to miss.
Charging voltage low at fast idle with a good belt: test the alternator on the machine, then check the sense and excitation circuits. Replace only after the circuit has been eliminated.
Intermittent no-start that only appears in damp or cold conditions: this is almost always a connection problem, not a component problem. Work systematically through terminals, grounds and connector blocks, cleaning and refitting each one.
For long-term storage: disconnect the battery and charge it periodically, or fit a maintenance charger. Recover a sulfated battery is rarely possible, and prevention costs a fraction of the replacement.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Battery | Genuine; OEM-quality. Match cold cranking amps and physical hold-down to the machine, not just the voltage. |
| Battery cable and terminal kit | Genuine; OEM-quality. Length, lug size and insulation grade all matter. |
| Fan and alternator belt | Genuine; OEM-quality. Match the section and length precisely. |
| Starter motor | Genuine; OEM-quality new; remanufactured exchange. |
| Alternator | Genuine; OEM-quality new; remanufactured exchange. Mitsubishi Electric units are not covered by the Mitsubishi Heavy Industries engine-parts authorization. |
| Engine ground strap | Genuine; OEM-quality. Frequently the cheapest fix on the list. |
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
Should I replace the battery and alternator together?
Only if testing shows both are out of specification. Replace both by instinct and you may fit a new alternator to cure a cable fault, or a new battery to cure a charging fault, and be back at the same problem in a month. Measure first; the tests are quick and they separate the two cleanly.
My machine starts fine in summer but not in winter. Is that the battery?
Usually it is the charging circuit or the cables, and the cold weather is simply exposing a fault that has been present all year. Cold increases cranking current demand at the same time as it reduces what a battery can deliver. Test charging voltage and voltage drop before assuming the battery is finished.
How long should an excavator battery last?
It varies enormously with duty cycle, temperature and how the machine is stored, so any single figure is misleading. The useful approach is to record rest voltage and charging voltage at every service. A battery whose rest voltage is drifting down service by service is telling you the end is coming, and that is a planned replacement instead of a breakdown.