Battery: Description and Operation

Fig.1 Battery - Typical:

BATTERY
The Battery is a 12-volt, top post, non-serviceable unit located underneath the air cleaner box in the left front corner of the engine compartment.

The battery stores, stabilizes, and delivers electrical current to operate various electrical systems in the vehicle. The determination of whether a battery is good or bad is made by its ability to accept a charge. It also must supply high-amperage current for a long enough period to be able to start the vehicle.

The original equipment (OE) battery is equipped with a hydrometer (test indicator) built into the battery cover.

The capability of the battery to store electrical current comes from a chemical reaction. This reaction takes place between the sulfuric acid solution (electrolyte) and the lead +A plates in each cell of the battery. As the battery discharges, the plates react with the acid from the electrolyte. When the charging system charges the battery, the water is converted to sulfuric acid in the battery. The concentration of acid in the electrolyte is measured as specific gravity using a hydrometer. The specific gravity indicates the battery's state-of-charge. The OE battery is sealed and water cannot be added.

The battery is vented to release gases that are created when the battery is being charged and discharged. The battery top, posts, and terminals should be cleaned when other under hood maintenance is performed.

When the electrolyte level is below the top of the plates, Clear in the test Indicator, the battery must be replaced. The battery must be completely charged, and the battery top, posts, and cable clamps must be cleaned before diagnostic procedures are performed.

BATTERY IGNITION OFF DRAW (IOD)
A completely normal vehicle will have a small amount of current drain on the battery with the key out of the ignition. It can range from 5 to 25 milliamperes after all the modules time out.

If a vehicle will not be operated for approximately 20 days, the IOD fuse should be pulled to eliminate the vehicle electrical drain on the battery. The IOD fuse is located in the Fuse Block. Refer to the Fuse Cover/End Cap for proper fuse.

BATTERY VOLTAGE - PCM INPUT
The direct battery feed to the PCM is used as a reference point to sense battery voltage.

In order for the PCM to operate, it must be supplied with battery voltage and ground. The PCM monitors the direct battery feed input to determine battery charging rate and to control the injector initial opening point. It also has back-up RAM memory used to store Diagnostic Trouble Codes (supply working DTCs). Direct battery feed is also used to perform key-OFF diagnostics and to supply working voltage to the controller for OBDII.

If battery voltage is low the PCM will increase injector pulse width (period of time that the injector is energized).

Effect on Fuel Injectors
Fuel injectors are rated for operation at a specific voltage. If the voltage increases, the plunger will open faster and conversely, if voltage is low the injector will be slow to open. Therefore, if sensed battery voltage drops, the PCM increases injector pulse-width to maintain the same volume of fuel through the injector.

Charging
The PCM uses sensed battery voltage to verify that target charging voltage (determined by Battery Temperature Sensor) is being reached. To maintain the target charging voltage, the PCM will full field the generator to 0.5 volt above target then turn OFF to 0.5 volt below target. This will continue to occur up to a 100 Hz frequency, 100 times per second.