Mercedes-Benz CLA Operator's Manual page 312

Jump-starting
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Do not start the vehicle using a rapid charging device. If your vehicle's battery is discharged,
the engine can be jump-started from another vehicle or from a second battery using jumper
cables. Observe the following points:
The battery is not accessible in all vehicles. If the other vehicle's battery is not accessible,
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jump-start the vehicle using a second battery or a jump-starting device.
You may only jump-start the vehicle when the engine and exhaust system are cold.
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Do not start the engine if the battery is frozen. Let the battery thaw first.
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Only jump-start from batteries with a 12 V voltage rating.
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Only use jump leads that are not damaged and have a sufficient cross-section and insulated
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terminal clamps.
Make sure that the two vehicles do not touch.
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Make sure that:
the jumper cables are not damaged.
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when the jumper cables are connected to the battery, uninsulated sections of the terminal
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clamp do not come into contact with other metal sections.
the jumper cables cannot come into contact with parts which can move when the engine is
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running, such as the V-belt pulley or the fan.
Secure the vehicle by applying the electric parking brake.
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Shift the transmission to position P.
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Turn the SmartKey to position 0 in the ignition lock and remove it.
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Switch off all electrical consumers, e.g. rear window defroster, lighting, etc.
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Open the hood.
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Position number B identifies the charged battery of the other vehicle or an equivalent jump-
starting device.
Press together cover : of positive clamp ; and slide it back.
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Connect positive terminal ; on your vehicle to positive terminal = of donor battery B
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using the jumper cable, beginning with your own battery.