Battery: Description and Operation
TWO BATTERY SYSTEM
OVERVIEW
The 1999 750iL is equipped with two batteries. Each battery has a dedicated circuit:
- Starter Motor and Electric Catalytic Converter Heating Coils
- Vehicle Power Distribution and Generator
The two battery system is necessary due to the addition of the E-CATs. If a single battery system were used, the E-CATs would be provided operating power from the single battery just after the cold engine start-up when the battery is at its lowest capacity.
This would increase engine loads due to charging and cause an increase in injection "on" time during the crucial cold engine warm-up period. This would result in unnecessary increased tailpipe emissions.
Providing separate battery systems for the starting circuit and the vehicle circuits also minimizes the possibility of a discharged starter battery ensuring reliable engine starting.
TWO BATTERY SYSTEM COMPONENT LOCATIONS
The starter battery (110 Ah) is located in the right rear trunk well. It is connected to the starter motor and (via the E-CAT module) to the heating coils of the E-CATs.
The vehicle circuit battery (55 Ah) is located above the starter battery on a swing- out mount. It provides operating power for the balance of the vehicle's electrical requirements.
The batteries are connected by an electronically controlled Battery Isolation Switch that opens and closes the circuit between both batteries based on monitored conditions.
The battery isolation switch is never used to boost a discharged starter battery with the voltage of the vehicle circuit battery to start the engine. The battery isolation switch can only withstand a maximum current flow of 60A.
BATTERY PART NUMBER INFORMATION
Note From BMW:
Replacement batteries are available through the Deta Douglas Battery Program. Use the Part numbers as listed in the EPC for ordering through Deta Douglas and for Warranty Claim Part Identification.
Reference Information Only! -- 55 Ah Battery - Deta Douglas P/N = 8377 137.
The 55 Ah battery label contains part numbers that are not recognized by our Parts and Warranty Systems.
Do Not Use These Part Numbers.
BATTERY ISOLATION SWITCH
OVERVIEW
The battery isolation switch is open when the vehicle is not in use (ie: key off and parked). In this situation, the vehicle circuit battery is the only power source connected to power distribution. Therefore the starter battery charge remains intact even when the vehicle is not used over extended periods of time ensuring starting reliability.
When the vehicle is driven, the battery isolation switch opens and closes to maintain the starter battery charge. The battery isolation switch operates under the following programmed modes of operation.
STARTING MODES
As soon as the electronic isolation switch recognizes KL 15 (via K Bus), it determines the voltage of the vehicle circuit battery.
- If the vehicle battery is sufficiently charged, it proceeds with the normal starting mode described below.
- If the vehicle circuit battery is not sufficiently charged, it proceeds with the safety starting mode described below.
NORMAL STARTING MODE: As soon as the engine starts and the battery isolation switch recognizes the "TD" signal (K-Bus) it immediately opens the battery isolation switch.
Within 0.5 seconds of engine start-up the E-CAT module energizes the E-Cat Heating Coils (program dependent) and simultaneously signals the Battery Isolation Switch that the ECATs are on via signal "KATON" (low signal).
When the heating period is completed, the E-CAT module signals that the E-CATs are off on the same signal line (high signal).
Under normal voltage conditions, the battery isolation switch is not closed before the heating procedure is finished.
SAFETY STARTING MODE: In response to the discharged battery condition, the battery isolation switch closes to momentarily boost the vehicle circuit battery and all systems via power distribution with the starter battery voltage. This function only occurs for a maximum of 30 seconds or until the engine is started (TD signal via CAN).
With the engine running, the switch is immediately opened for the duration of E Cat heating (same function described under Normal Starting Mode).
If the engine is not started within these 30 seconds, the battery isolation switch opens. It remains open until KL 15 is recognized on the next start-up cycle.
DRIVING MODE
The vehicle circuit battery is permanently connected in parallel to the generator and is charged when the engine is running. When the battery isolation switch is closed, the generator simultaneously charges the starter battery.
As the vehicle is operated, the battery isolation switch cycles between open and closed based on the following:
- Voltage values of both batteries
- Current transfer between both batteries (monitored through switch)
- Internal temperature of the battery isolation switch.
It responds by either opening or closing the switch to stabilize the monitored conditions ensuring continued vehicle operation and long battery service life.
Conditions causing switch to open while driving:
- When the starter battery becomes adequately charged, the battery isolation switch opens disconnecting it from the generator charge supply.
- If the vehicle systems are causing a period of high current demand (ie: rain storm - lights, wipers, defroster/blower on) current will flow from the starter battery to meet the vehicle system's electrical requirements. The battery isolation switch opens when the monitored current flow through the switch exceeds 0.5A.
However, there is an exception to this rule, see "Conditions causing switch to close while driving" below.
- As soon as the isolation switch internal temperature exceeds the programmed maximum value the switch opens.
Conditions causing switch to close while driving:
- When the monitored voltage in the vehicle circuit battery exceeds that of the starter battery by +0.7 volts.
- If the voltage in the vehicle circuit battery drops below 9 volts three times within one minute, the switch closes as a safety feature ensuring continued vehicle operation.
BATTERY CHARGING & THE BATTERY ISOLATION SWITCH
When the vehicle is in the workshop it may be necessary to charge the batteries when coding or programming a control module. Use the appropriate B+ terminal as shown below.
CHARGING MODE
The battery isolation switch also monitors the starter battery voltage when the ignition key is off. This feature continues to be active when the vehicle (including the battery isolation switch) is in sleep mode.
If the starter battery voltage exceeds 13.8 volts as it's being charged by a workshop battery charger, the battery isolation switch pulls itself out of sleep mode to close the isolation switch.
This causes the starter battery voltage to bleed off to the vehicle circuit battery simultaneously charging it as well. The battery isolation switch remains closed until the next key on cycle.
SLEEP MODE
As soon as the engine is switched off the battery isolation switch goes into sleep mode. The current draw of the battery isolation switch drops to below 1 mA. The isolation switch comes out of sleep mode as soon as:
- KL 15 is recognized
- Starter battery voltage > 13.8 volts (charging mode)
SYSTEM RESPONSE TO FAULTED SIGNAL "KATON"
- It signal "KATON" is interrupted by an open or short to B+, the battery isolation switch detects the signal fault and closes the switch for 4 seconds after start and the ID signal has been recognized.
- If the signal is shorted to ground, the switch is opened 60 seconds after the ID signal has been recognized (K-Bus).
DIAGNOSIS
The battery isolation switch has a dedicated diagnosis program. The DIS/MoDiC communication interface is via TXD to the IKE. The IKE serves as the gateway to the Battery Isolation Switch via the K-Bus.