I had a neat little gadget on my 300SD for a while, it was a digital compass, gave inside/outside temp, and showed volts. I think it was made by ice warner? anyway, it hooked up to the same power wires as my radio, and after i learned the guage, i could tell you every time when i shut off the car if i could start it again(had a bad alternator), basically, what i learned is that about 13volts when running was normal, if it dropped to 11, i had problems, and if it spiked to 14 i had REAL problems( i think it was surface charging or something?). Just another thought to look into, you may be able to get som other neat fetures with the guage if you shop around
Ryan

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83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.