Mercedes-Benz B-Class Electric Drive W245 Operator's Manual page 272

Jump-starting
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The drive system cannot be started if the 12 V battery is discharged. This is not dependent on
whether the high-voltage battery is charged or not. The vehicle cannot be jump-started if the
high-voltage battery is discharged. The high-voltage battery must be charged first.
The drive system can be started using another vehicle.
Only use batteries with an equal nominal voltage (12 volts).
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Make sure that the battery of the donor vehicle does not have a significantly lower capacity
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than the discharged battery.
Use jumper cables with a sufficient cross-section and insulated terminal clamps from a
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qualified specialist workshop, e.g. an authorized Mercedes-Benz Center.
Make sure that the two vehicles do not touch.
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Route the jumper cables so that they cannot be caught by rotating components in the engine
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compartment.
Do not disconnect the discharged battery from the vehicle's electrical system.
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Switch off the engine of both vehicles.
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Shift the transmission to position P.
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Switch off all electrical consumers.
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Remove the battery cover.
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Connect positive terminal ; on the vehicle with the flat battery to positive terminal = of the
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donor vehicle using the red jumper cable. Begin with the flat battery.
Connect negative terminal ? of the donor vehicle to negative terminal A of the vehicle with
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the flat battery using the black jumper cable, beginning with donor vehicle's battery B.
Start the engine of the donor vehicle and run it at idling speed.
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Switch on the ignition in the vehicle with the flat battery.
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If the traction drive cannot be activated immediately, wait for approximately 60 seconds
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between starting attempts.
If the drive system does not start, call a breakdown service.