[Vwdiesel] Alternator - Battery - jumpstart questions

Sandy Cameron scameron at compmore.net
Wed Jun 19 23:19:21 EDT 2002

I'm getting confused(easy for me), as to who wrote what to whom on the van
battery thread, but my 2 cents worth:

I agree with all afore said on this.

The charging circuit  works thus:

The output from the alternator goes direct to the isolator. Inside the
isolator are 2 diodes which direct curent to the 2 batteries, but prevent
either of them from draining into the other. the price paid for this trick
is the loss of about 1/2 a volt across each diode. The diode in the isolator
can easily be blown by incorrectly reverse connecting booster cables. it
perishes instantly like a fuse.

A voltmeter test with the engine running shouild show about 14 volts at the
alternator output/isolator input. About 13 to 13.5v at the isolator outputs
to the batteries.

My bet is on a blown diode in the isolator preventing the alternator from
charging the starting battery.

The camper battery could be charged by using a relay operated only when the
"ignition" is on, and doing away with the isolator, connecting the starting
battery directly to the alternator, and the camper battery throught the relay.

That would make the full output voltage of the alternator available to both..

In dual battery boat systems, there is a switch that permits using either
battery for starting, You should only need to jummper the positive posts
together for an emergency start, if the ground wire on the camper battery is
a heavy battery type cable. Otherwise, if the negative of the camper battery
is wired with wimpy wire, you will have to jumper both positive an negative
to avoid flames when starting


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