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Old 11-05-2013, 03:36 PM   #11 (permalink)
Karmann Eclectric
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Karmann Eclectric - '71 VW Karmann Ghia Electric Conversion

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Originally Posted by paulgato View Post
My hope is that by feeding in a sufficient current at a voltage just above the voltage at which the alternator regulates its output (14.4v?)
Someone correct me if my logic is wrong here! What have I missed?
I like this idea. Main flaw I see is that your input voltage will have to be tightly regulated to avoid dumping a lot of charge current into the starting battery. The dc-dc converters I have used in my EV conversion swing pretty wildly according to load, and that's even with a 12V AGM battery for buffer. Even so, if the dc-dc is set for say, 14.5V, it won't pass many amps into the starting battery, and the alternator should kick in to level off the voltage any time you exceed the dc/dc capacity. Measurement of the energy removed from your supplemental battery will be much more precise and immediate than discerning the MPG effect.
I had hoped that simply replacing the SLI battery with Lithium Iron Phosphate would do, given the higher resting voltage of four lithium cells, but that would require detuning the alternator to not kick in until approx 13.8V or less.

Searching for such a product lead to several interesting sites, but mainly products designed to increase alternator load rather than the opposite. Here's one that might be fooled into doing what we want...

Advanced Alternator Regulator

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