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Old 06-02-2009, 01:09 AM   #5 (permalink)
NiHaoMike
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Join Date: Sep 2008
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And I realized that an ordinary 13.8v supply will probably not work with an auxiliary battery unless it is modified. First off, 13.8v is not enough voltage to get much charge for short trips. A timer needs to be added to put it up to 14.4v for a few minutes. Then a current limit circuit must be added so the supply will never overload. (And tie that into the timer so it starts timing once the current limit is out.) And the supply must be tolerant of voltage remaining on its output when the input is disconnected. Most supplies will discharge the battery or even be damaged by that. Then add a circuit that will light up a warning light if the voltage drops below 13v and flash it whenever it is in current limit.
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