Power Outlet Minimum Voltage Shutdown
10-02-2011, 05:56 PM
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Power Outlet Minimum Voltage Shutdown
I've been doing a bit of work wiring an extender from my cigarette lighter (switched power outlet) for my GPS. Along the way, I've looked into a couple options and had the chance to learn about the battery condition resulting in shutdown of the other power outlets in the car.
From the Manual:
"An emergency shut-off feature ensures that the vehicle’s electrical voltage does not fall below a minimum level. If the voltage drops to this minimum level, the power outlets are automatically switched off. This ensures that enough power remains to start the engine."
On my 2008 W164, the shut-off seems to kick in at 12.50 volts. After turning off the engine, I can monitor the voltage from the outlet. When it falls below 12.50 volts, there's a click (a relay?) and all the power outlets shut down.
Looking into Wikipedia for lead acid batteries, this level corresponds to a 90% charged battery in an open circuit. From what I can read, and as we'd expect, the set point is quite conservative and doesn't allow the power outlets to sap much of the main battery before they are shut-off.
