for your reference,

供参考

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Note added at 18 mins (2014-02-10 15:27:57 GMT)
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or in-vehicle power supply controller,

or in-vehicle power controller

http://dict.cn/on-board power supply

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Note added at 1 hr (2014-02-10 16:11:31 GMT)
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或者，更准确些，用 vehicle-borne power supply controller

Without context or know what type of car it is, it is pretty hard to say, but you'll rarely find that in English we use a phrase of more than 3 words to say something, otherwise it sounds very confusing.

You'll find 'system', 'dashboard' or 'panel' more often than 'unit' if you're talking about user controls. 'Controller' sounds odd. Perhaps just "power controls".

You don't need to translate '车载' every time it comes up if you're obviously talking about a car.

However, without any hint of context this remains an educated guess




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Note added at 18 hrs (2014-02-11 10:02:52 GMT)
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***without context or knowing***