Re: Battery replacement: 1995 A6

[DeWitt Harrison <de@aztek-eng.com>]

At 09:15 AM 12/18/98 -0500, David Kroth wrote:
>DeWitt,
>
>> Buy any old unregulated
>> 12 VDC, 300mA 
>
>This is exactly what I ended up going.  But I didn't have
>to buy the power supply.  I had four "junk" ones in my basement:
>every time an appliance dies I throw the power supply in a box
>in the basement saying, "someday that will come in handy."  And
>something in there finally did!
>[ ... ]
>> DO NOT switch
>> on any electrical equipment while in this mode since 300 mA
>> or whatever isn't going to go far. 
>
>Keeping all the equipment OFF was actually a challenge.  Opening
>any of the doors causes the interior lights to come on.

I open one or more doors and wait for the interior lamp relay to time
out. Then the dome light, etc., remain off as long as at least one door
remains open at any given time. If a fellow knew where to clip
a ground onto a door switch connection, he wouldn't have to
play this door game.

>An interesting note that I have no explanation for:  The no
>load voltage of my power supply was 13.5 actual.  It was rated
>at 12V on the side of the case.  I don't remember what the mA
>rating was, but when I connected it to a dome light bulb (3W??)
>it was dragged down to about 9.  Anyway, when I connected the
>power supply and disconnected the battery, the voltage across
>the power supply actually INCREASED to 14.5 volts.  It dropped
>back down to 13.5 when I got the new battery back in. [ ... ]

These little supplies have a fairly high internal resistance which
means there is a voltage drop inside the unit which depends on
the output load.  A supply marked 12V, 300 mA is usually closest
to a true 12V at the output terminals when all 300 mA are being
drawn. Draw less and the output voltage will rise and vice versa.

A charged car battery has a very low internal resistance when
supplying current but a relatively high resistance when being charged.
When you connect the small auxiliary supply to the battery through
the car's wiring, the battery is basically being trickle charged
allowing the battery voltage to rise a bit to 13.5 volts.

When you disconnected the battery, the little supply was free
to rise to its true no load output of 14.5V. This is not unlike a
lot of Bosch alternators I've had the misfortune to know.

DeWitt Harrison
Boulder, CO
88 5kcstq