By Jack Smith
The BC7217 Battery Charger
During a utility power outage, what would happen if the emergency standby generator at a mission-critical facility failed to start? Mission-critical facilities include hospitals, data centers, military bases, and emergency response systems.
Most mission-critical facilities have an uninterruptible power supply (UPS) system that provides ride-through power to accommodate critical loads until the standby generator starts and stabilizes. When the generator is ready to accept loading, the automatic transfer switch (ATS) connects the generator to the facility’s electrical infrastructure.
We take our utilities and power grid for granted - until that outage, which usually happens at the most inconvenient time. For many customers, such as typical residential users, resetting digital clocks on microwave ovens and alarm clocks can be annoying. But the stakes are much higher for hospital patients on life support and to surgeons in operating rooms. There must be power. Period.
There are very few reasons why standby generators fail to start. Assuming that those responsible for doing so performed the normal testing and commissioning tasks when installing the standby generator system, typical failure-to-start causes are pesky circuit breaker trips and generator controls, or an ATS that is not in automatic mode. However, the single largest cause is battery failure. More than 80 % of standby generator starting failures are caused by weak or dead batteries.
More than one way to start a generator
The prevalent standby generator starting methods are pneumatic, hydraulic, and electric. Pneumatic starting systems use compressed air stored in pressure tanks, which are recharged by an air compressor. Pneumatic starting systems can be recharged quickly. However, air storage tanks are susceptible to condensation issues and must be protected from corrosion and/or freezing, depending on storage tank location.
Hydraulic starting systems use hydraulic oil stored in vessels under high pressure. A hydraulic pump keeps the high-pressure vessel recharged. Hydraulic starting is typically used where electrical connections could pose a safety hazard. Hydraulic recharging time is fast. However, high hydraulic system pressure requires special pipes and fittings. Hydraulic connections must be extremely tight. Although oil lost due to leakage can be replaced easily, recharging hydraulic accumulators requires special equipment because of the high pressures involved. Also, because of these high pressures, accumulators must be protected from potential perforation, as they contain a considerable amount of stored mechanical energy.
Obviously, electrical starting systems use batteries to operate an electric starting motor. Although compressed air starting systems are generally considered to be more reliable than electric starting, electric starting is the most economical and convenient starting system to use.
Boosting battery reliability
Electric starting is the most widely used standby generator starting method. Lead-acid batteries are used most frequently for electrical standby generator starting because of their relatively low upfront cost, maintenance needs, and easy replacement. Nickel cadmium batteries can be useful when infrequent operation is the norm. Nickel cadmium batteries maintain nearly constant-voltage output during discharge cycles and can tolerate extended overcharge intervals better than lead-acid batteries. However, lead-acid batteries are typically chosen for their lower cost. Consider system lifecycle costs before specifying standby generator starting battery type.
Standby generator starting battery maintenance is extremely important. Although keeping them maintained and fully charged is not enough - lead acid batteries deteriorate over time; they must be replaced when they can no longer hold a proper charge.
Routine lead-acid battery maintenance includes testing, cleaning, and checking specific gravity and electrolyte level. When testing standby generator batteries, simply measuring their voltage doesn’t gauge their starting capability. Internal resistance increases as batteries age. Battery voltage must be measured under load to ensure accuracy.
A load tester measures a battery’s ability to rebound from starting a standby generator by simulating the load on a battery when cranking a cold generator engine. Although a nearly-dead battery can produce its rated voltage, it won’t have the reserve cold-cranking amps required to start the generator engine. The load tester reveals weak battery cells.
Some standby generator suppliers as well as third-party solution providers offer sophisticated system monitoring packages that include load-based battery and charging system testing options. However, these systems can add significant upfront costs to generator systems.
A less expensive - and highly accurate - alternative is using a battery load tester along with a high quality digital multimeter and current clamp meter, such as those available from Fluke. Simply measure the battery voltage and current while the load tester loads the battery.
If the battery is new and the genset is in good operating condition, record how low the voltage drops each time the genset is started. For example, if the system uses a 24 Vdc battery bank, a typical voltage reading is 19 Vdc, or 80% of the battery bank voltage. If this is consistent, that’s your baseline. Battery bank voltage that starts to drop below your baseline during cranking is the first indication of a weakening battery. Note, however, that temperature affects battery life, battery performance, and the amount of cold cranking amps to start a genset in colder weather. Also, if the genset is not used or exercised very often, the battery discharge voltage technique is of little value.
Since standby power for critical applications must be relied upon to start when needed, waiting until the batteries are no longer reliable to do something about them is too late. Gensets must be tested to ensure they operate properly, the batteries will start the genset, and that maintenance is performed appropriately. For gensets that see very infrequent use, one rule-of-thumb is to replace the batteries every three years to ensure they will be able to start the genset and to accept a charge. The genset manufacturer will have detailed information regarding specific battery maintenance procedures and capacity parameters.
Cleanliness is very important to battery health. Wipe batteries with a damp cloth to remove dirt. If the terminals are corroded, remove the battery cables and wash the terminals with a water and baking-soda solution. Don’t allow the solution to enter the battery cells. After washing, flush the batteries with clean water and ensure they are thoroughly dry. Replace the connections and lightly coat the terminals with petroleum jelly.
Using a battery hydrometer, check the specific gravity of the electrolyte in each cell. If the specific gravity of any cell is lower than 1.215, charge the battery. The specific gravity of a fully-charged battery is 1.260.
The electrolyte level should be checked at least every 200 hours of operation. Since this can be difficult to determine on standby generator batteries, an alternate rule-of-thumb is checking electrolyte level monthly, as well as the aforementioned inspections and tests. Fill low battery cells to the bottom of the filler neck with distilled water.
Healthy batteries take charge
While weak or undercharged standby generator starting batteries are to blame for most failure-to-start situations, they represent only half the story. Behind every reliable generator starting battery is a good charging system.
Batteries can cause enough standby generator starting problems - especially if they haven’t been properly maintained. Charging systems can cause their share of problems as well. In next month’s “Solid Ground” column I will discuss some of these pesky charging problems and how to correct or avoid them.
Until next time, keep standing on “Solid Ground.”