In short. The first energy a standby generator uses in a mains failure comes from its battery, and most failed starts begin there. A battery that reads 12.6 V at rest can still collapse under the starter’s load, so the meaningful test is voltage measured during cranking. Starting-system service tests the battery under load, checks the charger float voltage, measures voltage drop across every cable and terminal, and confirms the starter and the controller’s start logic.
The starting chain and where it breaks
A diesel generator starts when the battery pushes enough current through the cables, solenoid and starter motor to turn the engine fast enough for compression heat to ignite the fuel. On a standby set that happens once a week for a few seconds, which is exactly the duty that lets batteries sulphate, terminals corrode and solenoid contacts pit unnoticed. The chain runs from battery through terminals and cables, the controller’s crank output and start relay, the solenoid, the starter motor and the engine’s own cranking resistance, and a fault anywhere is reported as “the generator did not start”. The service task is to find which link gave way.
Reading the symptom
The behaviour at the moment of the start attempt points to the link, and recording it before anyone touches the set saves most of the diagnosis.
| What happens on the start command | Most likely cause | First measurement |
|---|---|---|
| Nothing, no click | Controller not issuing crank, blown crank fuse, open start relay, dead battery | Voltage at the crank output and at the solenoid terminal during the attempt |
| Single click, no crank | Battery voltage collapsing under load, corroded terminal, failed solenoid contacts, poor engine earth | Battery terminal voltage during the click; voltage drop across each terminal and cable |
| Rapid clicking | Discharged or sulphated battery, high resistance in cables or terminals | Battery voltage during the attempt; drop across positive and negative cables |
| Slow cranking, no start | Weak battery, undersized or heated cables, worn starter, cold engine with failed preheat | Cranking voltage and cranking speed; starter current with a DC clamp meter |
| Cranks well, no start | Fuel, air, compression or controller running-signal fault, not the starting system | Move to engine and controller diagnosis |
Slow cranking deserves emphasis. A diesel needs a minimum cranking speed, typically 150 to 250 rpm depending on the engine, to generate enough compression temperature and atomise fuel. An engine turning slowly may have fuel and compression and still never fire, so a crank-no-start should not be blamed on the fuel system until cranking speed and voltage have been measured.
Testing the battery properly
Open-circuit voltage tells you the state of charge, not the capacity. A sulphated battery sits at a healthy voltage and falls to 7 or 8 V when the starter engages. The test that matters is terminal voltage during an actual crank, recorded with a meter clipped on or with a recording instrument. As a rule of thumb for a 12 V lead-acid battery, cranking voltage should stay above about 9.5 to 10 V, and on a 24 V system above about 19 to 20 V, while the engine turns at normal cranking speed. A conductance tester gives a quick indication between visits, and a load test against the cold cranking amps rating settles a marginal result. On flooded batteries, electrolyte level and specific gravity in each cell complete the picture. The manufacturer’s limits take precedence over these figures.
Replace batteries on condition and age together. In an acoustic canopy in Hyderabad the battery compartment often exceeds 40 °C, and a battery that has passed three or four summers should be tested more closely even if it still cranks. The batteries and chargers page covers the types and sizing.
The charger decides whether the battery is ready
A standby battery is only as good as the charger that maintains it between runs. Service measures the charger’s output with the battery connected and with it disconnected, checks the float voltage against the battery type (around 13.5 to 13.8 V for a 12 V flooded battery, and the manufacturer’s value for sealed or gel types), checks for excessive ripple that cooks the battery, and confirms the charger alarm actually reaches the controller. A failed charger is silent: the battery shows 12.4 V a week later, 12.0 V a month later, and the first sign is a failed start three months on. The engine-driven charging alternator and its belt are checked at the same time.
Cables, terminals and earths
Starter current on a medium generator can be several hundred amperes, and at that current a few milliohms of corrosion matter a great deal. With a helper cranking, measure the voltage drop across each terminal connection, along each battery cable and across the engine-to-frame earth strap. A drop of more than about 0.2 to 0.3 V across any single connection, or more than about 0.5 V along a cable, indicates a connection that needs cleaning and re-torquing or a cable that is undersized or damaged. A corroded engine earth strap is a particularly common cause of a single click with no crank, and because it also disturbs every resistive sender it often shows up first as odd controller readings.
Starter, solenoid and controller logic
With battery and cables proven, the starter’s current draw is measured with a DC clamp meter during a crank and compared with the manufacturer’s figure; a starter drawing far above it has worn bushes or a dragging armature, and one that spins without engaging has a failed pinion. The controller’s crank attempts, rest time and crank-disconnect speed are checked against the engine data. More crank attempts to mask a weak battery only burn the starter, and a crank-disconnect speed set too high keeps the starter engaged after the engine fires. See controller diagnostics for the logic checks.
Keeping a standby set ready
- Record charger voltage and battery rest voltage at every weekly check, so a drift is seen before it becomes a failure.
- Run a genuine automatic start test at least monthly and record the cranking voltage during it.
- Test each battery of a bank individually; one weak battery in series hides at rest and fails under load.
- Replace ageing batteries before failure, and recycle through authorised channels.
These checks belong in a preventive maintenance routine.
Related photographs and videos
Common mistakes
- Declaring the battery healthy from its rest voltage, so a sulphated battery that collapses under load is left in a standby set until the next mains failure.
- Replacing the starter for a single-click fault that was a corroded earth strap, which costs a starter and leaves the fault in place.
- Increasing the controller’s crank attempts to get a weak battery to start the engine, which burns the starter and leaves the real fault untouched.
- Never checking the charger, so a failed charger lets a good battery discharge silently over weeks until the set fails to start.
What to send us
Send the generator nameplate, the battery type, voltage and capacity, the charger make, and a description of exactly what the set does on a start command: nothing, a click, rapid clicking, slow cranking or cranking without firing. Battery voltage at rest and during cranking, if you can measure it, tells us most of what we need. Use the service and AMC enquiry form, or the spare parts enquiry if you have already confirmed the battery or charger has failed.
Questions engineers ask
The battery shows 12.6 V but the generator will not crank. Why?
Rest voltage shows state of charge, not the ability to deliver several hundred amperes. A sulphated battery or a corroded connection holds voltage with no load and collapses the moment the starter engages. Measure the voltage at the battery terminals during the start attempt; if it falls below about 9.5 V on a 12 V system the battery or a connection has failed under load.
How long should a generator battery last?
In a hot canopy with a weekly standby duty, three to four years is typical for a flooded lead-acid battery, and less if the charger has been overcharging or the battery has been allowed to discharge. Condition testing matters more than calendar age; a battery that fails a cranking-voltage or conductance test is replaced regardless of age.
Can I use a vehicle battery in a generator?
Yes, if its voltage, capacity and cold cranking amps meet the engine manufacturer’s requirement and the charger float voltage suits the battery type; many generator batteries are heavy-vehicle construction. Sealed or deep-cycle batteries need a charger set for them.
Why does the generator start on manual but not on automatic?
Manual start usually bypasses part of the control logic. If the engine cranks and starts on manual but not on an automatic start command, the fault is in the start signal path from the ATS or mains sensing, the controller’s auto-mode permissives, or a timer, not in the battery or starter. Check the controller event log for what it saw when the automatic start was requested.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.