Why Generator Batteries Fail

Even a generator that is tested regularly may fail to start in a real outage because of its battery. Even when the fuel and the engine are in good condition, a weakened battery cannot turn the starter motor fast enough, and the set does not start.

On site, a significant share of “the generator didn’t start” complaints are caused by the battery and the starting system. A battery failure can usually be detected in advance through regular measurement; however, many facilities do not carry out this measurement.

Why does the battery go flat?

1. No battery charger, or it isn’t working. While the set is on standby, the controller, relays and some modules draw a small, constant current from the battery. Without a battery charger feeding that current from the mains, the battery drains within days or weeks. If there is a charger but its fuse has blown, it has been unplugged or it is faulty, the result is the same. This is the most common cause on site.

2. Wrong charging voltage. If the charger charges at too low a voltage, the battery never gets fully charged and the plates start to sulphate. If it charges too high, the battery heats up, loses water and its life is shortened. The charging voltage must be set to suit the battery type.

3. Temperature. The life of lead-acid batteries drops noticeably with heat. A battery inside the canopy, next to the engine or in a metal box in the sun lives far shorter than one in a cool environment. Cold is a different problem: the cranking current the battery can deliver falls, and the engine is also harder to turn in the cold. See running a generator in winter.

4. Loose or corroded connections. White-green deposits on the terminals, a loose cable lug. The resting voltage looks normal, but at the moment of cranking, with hundreds of amps flowing, a large voltage drop occurs across the connection.

5. Short runs. If the set is started once a month, run for a few minutes and shut down, the charging alternator may not put back the energy the starter used. Without a charger, the battery drains a little more with every test.

6. Age. Every battery ages. Maintenance-free batteries have a service life too.

How to test the battery

Resting voltage (typical values for a 12 V battery):

Voltage	State (approximate)
12.6–12.8 V	Fully charged
~12.4 V	Partially charged
~12.2 V	Half charged
12.0 V and below	Seriously discharged

Measure a while after charging has stopped. With the charger connected, the voltage looks high and misleads you. On 24 V systems the values are doubled; if two batteries are connected in series, measure each one separately.

Cranking voltage: How far does the voltage drop while cranking? On a 12 V system, the voltage typically falling below 9.5–10 V during cranking shows that the battery or the connections are getting weak. The values vary with the battery and the starter motor.

Battery tester: Testers that perform a conductance or load test give the battery’s cranking capacity as a number. They tell you far more than resting voltage.

Age: The battery carries a manufacturing date. Record it.

Why does it fail during cranking?

To turn a diesel engine over, the starter motor draws hundreds of amps for a short time. On large engines the figure is even higher. A battery’s internal resistance rises with age and sulphation. The higher the internal resistance, the further the voltage drops during cranking. Below a certain voltage, the controller may reset, the fuel solenoid may drop out, or the starter motor may not reach enough speed. The result is the same: the set does not start. That is why the voltage drop during cranking is the best indicator of battery health. Some controllers record this value; if the alarm history shows “low battery voltage” or “fail to start”, check the battery first.

Choosing the right battery

• The cold cranking amps (CCA) must not be below the value the engine manufacturer requires.
• In cold regions, a higher CCA rating is preferred.
• The battery type (flooded, AGM, etc.) must match the charger setting.
• The battery must be secured firmly enough to be protected from the set’s vibration.

Battery cables and earthing

The cables matter as much as the battery. The starter cable’s cross-section, length and lug tightness directly affect the voltage drop during cranking. If the battery bank has been moved away from the engine, the cable cross-section must be increased accordingly. The negative (earth) connection between the engine block and the base frame should also be checked; a loose earth cable can make even a healthy battery look useless.

Common mistakes on site

• Unplugging the charger. Cleaning, renovation work, or “who plugged this in?”. The battery goes flat a few weeks later.
• Looking only at resting voltage. A battery showing 12.6 V can collapse when cranking.
• Replacing only one battery. If one of two series-connected batteries is new and the other old, the old one wears out the new one too. They are usually replaced together.
• Never cleaning the terminals.
• Running the set at idle for a few minutes during tests, then shutting it down. A bad habit for both the battery and the engine.

Checklist

• Is there a battery charger, is it connected and is it working?
• Is the charger’s charging voltage right for the battery type?
• Are resting voltage and cranking voltage measured and recorded?
• Are the terminals clean and the connections tight?
• Is the battery age recorded?
• Is the battery’s environment excessively hot or cold?
• If there is remote monitoring, is a low battery alarm defined?

On remote sites, seeing battery voltage remotely catches the problem before an outage does: generator remote monitoring.

Next step

Checking the battery is one of the simplest but most important steps in scheduled maintenance. For the other steps, see our generator maintenance checklist and scheduled maintenance pages; for support, contact us.