Keeping your world up and running.®

Taking charge of genset battery health

By Jack Smith

December 2011

Fluke 87V

Check the charger’s dc output using a multimeter such as this Fluke 87V.

In my last column, I wrote about standby generator starting batteries and some of the problems that can keep them from performing at peak potential. This column will offer information on proper genset battery charging and ways to avoid battery and charger failure.

Usually, batteries fail because of failing to replenish water when necessary and charging them improperly. Although conventional lead-acid batteries require less than 2 ounces of make-up water for 30 hours operation, if lost water is not replenished the battery plates will eventually go dry, which causes permanent capacity loss.

Improper charging intensifies the effects of not keeping battery water replenished. While some water loss inevitably occurs during charging, overcharging accelerates water loss. Undercharging causes sulfation, which appears as crystalline lead-sulfate deposits on the plates. Undercharging also causes permanent loss of battery capacity.

More than one way to charge a battery

Each cell of a charged lead-acid battery contains lead and lead-oxide electrodes, or plates, submerged in an electrolytic solution of around 33.5 % sulfuric acid. During discharge, both plates turn into lead sulfate and the electrolytic solution becomes primarily water.

Fluke 87V

With a ScopeMeter® portable oscilloscope you can both see and measure the ac ripple waveform.

Battery chargers supply dc current to the battery. Recharging the battery changes the lead sulfates back into lead and lead oxide by driving the sulfate from the plates back into the water, converting it back into an electrolytic solution. To avoid charger and battery damage, the polarity of the charger must always match the polarity of the battery.

You can charge genset starting batteries using an alternator or solid-state battery charger. Engine-driven alternators are very similar to those used in gasoline-powered automobiles. The operating principle is identical: the alternator recharges the battery when the genset is running. This is the primary disadvantage of using alternators to recharge genset batteries - especially if gensets are used infrequently or have multiple, short start/stop cycles.

Gensets that supply standby power for mission-critical operations should not rely solely on engine-mounted alternators for battery recharging. Standby gensets require a utility-connected solid-state battery charger to keep the battery charged while the genset is not in operation. In contrast, a genset that provides primary power runs for longer periods, which allows more time for the alternator to adequately charge the battery.

Trickle and float

There are two basic types of solid-state battery chargers: trickle and float. A trickle charger is typically an open-loop unit, which means that it continues to charge, regardless of whether or not the battery is fully charged. An open-loop charger can alternately undercharge and overcharge a battery. Also, a trickle charger must be disconnected from the battery during the cranking cycle, which can be accomplished using a relay.

Float chargers typically have closed-loop regulating circuits that prevent battery overcharging. Float voltage is the constant voltage that’s continuously applied to a battery to ensure it’s fully charged. When the charger senses that the battery float voltage is at the appropriate level, it temporarily stops charging, but maintains the charging current at a very low level. The charger keeps the charging current near zero until it senses a drop in battery output voltage due to a generator start, at which time it resumes charging.

Ambient temperature has a considerable effect on battery float voltage. A float charger can remain connected to the battery without damaging either the charger or the battery as long as the charger can provide proper temperature compensation. A charger with current-limiting and closed-loop regulating circuits but without battery temperature compensation can still overcharge and undercharge a battery as the seasons change.

Float-type chargers are typically more expensive than trickle chargers. Applications that demand long battery life, such as standby genset starting, typically use float chargers. Typical float charger features can include line and load regulation, current limiting, complex charging algorithms, temperature compensation, and monitoring functions with alarm capabilities.

Both trickle and float chargers typically use silicon-controlled rectifiers (SCRs) for rectification and to control current. However, SCRs are notorious for producing harmonics. These harmonics can cause an electronic governor on a genset to act erratically. This can be avoided by connecting the battery charger directly to the battery or the battery terminals on the genset starter.


While charging system components can be maintained expediently by replacing them with new units, troubleshooting at the genset’s location is required to isolate the offending component. Charging systems that use alternators are diagnosed just like automobile charging systems.

You can ensure that ac power is reaching the battery charger as well as check the charger’s dc output using a digital multimeter (DMM). AC ripple on the output of the charger should be kept to a minimum. For maintenance-free batteries, ripple should not exceed 100 mV rms with the battery connected. You can use a DMM to check for ac ripple. However, using a ScopeMeter® portable oscilloscope to check for ripple allows you to both see and measure the ac ripple waveform.

You can diagnose solid-state chargers by following the troubleshooting guidelines typically included with chargers from most manufacturers. Once you isolate a specific module or circuit, most manufacturers encourage module/circuit replacement because doing so is more cost-effective than troubleshooting to the component level. Some manufactures also provide detailed instructions for checking SCRs using the ohmmeter function of a good quality DMM.

Until next time, keep standing on “Solid Ground.”

Additional information

Battery Council International: »
Electrical Generating Systems Association: »
Fluke Batteries, Chargers and Adapters »