Why does my back-up generator fail to start, and what can I do about it?

Your back-up generator fails to start for three reasons: a flat or worn starter battery, stale or wet diesel, or an engine that only idles. Maintenance fixes much of that, but a start can still fail. A business battery carries the first seconds and hours of a cut and gives the generator a second chance.

Tested at delivery, the generator in the yard runs a few minutes now and then. On a winter night the power fails: the starter clicks, or the engine dies after a minute. A generator that waits is ageing.

In Maasmechelen a generator waited for emergencies. With our semi-off-grid system the customer now chooses grid or generator as the charger (24 kVA, 30.8 kWh).

What are the causes when a back-up generator fails to start?

Most start failures come down to the starter battery, the fuel, an engine that never works under load, or settings left in the wrong position.

The Belgian maker Grupel lists the battery, air in the fuel system, leaks, carbon deposits and tripped breakers. Most failures, it says, come from the battery. The American emergency power standard NFPA 110 asks that engine and starter battery stay warm enough for a cold start.

Our guide to emergency power supply for your business compares UPS, generator and battery; here we look at the generator that must start.

Why is the generator starter battery the first thing to fail?

It must deliver a heavy current at the one moment you need it, after months of doing nothing. A small lead-acid battery hates that life, yet it is the cheapest part to check.

Grupel explains that lead sulphate builds up on the plates until the battery can no longer turn the starter. A charged 12 V starter battery shows 12.4 to 12.6 V; below 12 V it is unreliable. Heat, wrong use and a lack of maintenance shorten its life.

It only charges when the engine runs or a mains charger keeps it full. NFPA 110 therefore asks for an automatic charger besides the engine-driven one, and a battery that cranks through two complete cranking cycles.

A charger is no guarantee. An off-grid farmhouse owner in Wales kept his generator battery on a good conditioner, yet after about four years it failed to start in midwinter. A worn battery can show a decent voltage at rest and collapse when the starter pulls.

What happens to stale diesel in a generator tank?

It ages. A Cummins training on NFPA 110 says diesel stays stable for about 12 to 16 months in ideal conditions. Most generator tanks are not ideal.

A biodiesel blend can cut stability to around six months, attracts water and helps microbes grow. Low-sulphur diesel also protects less against microbes.

Microbes live where fuel meets water at the tank bottom and grow faster when warm. Water can damage the injection pump and freeze in winter. Standard diesel starts to gel at around minus 8 °C, biodiesel already at 2 to 15 °C.

NFPA 110 asks for a tank sized so the fuel is used within its storage life, or a plan to clean or replace stale fuel. A huge tank that never empties is not extra safety.

Does your tank need a permit? Read our article on permits and inspection for a back-up generator for the tank rules per region. For fuels such as HVO, our article on a generator with a battery compares the options.

What maintenance does a standby generator need?

Short regular checks, a real test run with load and one full service a year. Short runs without load do more harm than good.

Without load a diesel engine never burns all its fuel. The rest ends up as black, oily residue in the exhaust: wet stacking. It dilutes the oil, wears the engine and costs power, and Grupel names these carbon deposits as a cause of start failures.

The Dutch supplier Bredenoord advises at most five minutes without load at the monthly inspection, as longer mainly makes soot. For the monthly generator test run, NFPA 110 means at least 30% of rated power, or the exhaust temperature the maker asks for.

The table combines the sources; your maker's manual comes first. In Belgium the AREI counts, and for critical installations the operator's supervision also covers the auxiliary equipment.

Plan that test with our article on manual or automatic transfer switches. The legal inspection is in our article on permits and inspection.

How does a battery take the first hit when the generator does not start?

It carries your loads from the first millisecond, so the generator is no longer the first line. A Victron Quattro takes over in less than 20 milliseconds, fast enough for computers to keep running.

Without a battery, a failed start is a blackout. With a battery in front, it is an alarm, and the loads keep running while someone deals with the generator.

The Victron GX starts the generator only on conditions you set, such as state of charge or load. Its alarm fires when no generator power reaches the inverter input. Victron says that points to problems such as lack of fuel or a mechanical or electrical fault.

The time the battery buys depends on its size and your load, but it lets someone check battery, fuel or settings. Many short cuts end before the generator is needed, and the GX stops it when the grid returns. Read what downtime costs in what a power cut costs your company.

Can the battery system keep the generator fit as well?

Yes. The controller that starts the generator also tests it, counts its hours and warns you. Many causes from the table then show up before a cut.

• Periodic test run. The GX starts the generator at an interval and time you choose, and can run it until the battery is full, so the engine works under real load.
• Warm-up and cool-down. The inverter connects the generator after a warm-up time and moves the loads back to the battery before the engine stops.
• Run time and service interval. The GX counts running hours and warns you when a service is due.
• Alarm when auto start is off. After 10 minutes disabled, the GX raises an alarm; Victron notes this catches a technician who forgot it after servicing.
• Engine data. With digitally connected controllers such as Deep Sea or ComAp, the GX shows starter battery voltage, fuel tank level and engine temperatures.

On normal days the battery stores solar power and cuts your peaks, while a fixed reserve stays locked for the cut. Our article on how long a battery keeps a cold room cold works out the hours. On a livestock farm, read about barn ventilation during a power cut.

Can you keep the generator you already have?

Usually yes, and it gets a smaller, better job. Several of our projects started with a generator already on site.

On a home battery in Maaseik, an existing generator got an off-grid mode with a generator connection (9 kVA, 15.4 kWh). The same logic works on a business site.

A farm in St-Georges-sur-Meuse without a street connection ran entirely on its generator. With solar panels and a 30 kVA, 46.2 kWh battery, it now only serves as back-up. Our semi-off-grid system combines grid, battery and generator the same way.