Alternator voltage : min. needed?

[James Sweet]

I am adding a 12V external connection to my Volvo 92 740 to be
able to run/charge the DC system on my pop-up camper, which has a 12 V
deep cycle battery. So I got one of those three-terminal battery
isolators. The alternator output goes to the center terminal, and the
car battery and the pop-up battery go to the other two terminals. The
alternator puts out abt 14 V measured with my fluke. However, each of
the isolator diodes has about a 1 volt drop across it, so now there is
just 13 volts available to charge either battery....at the battery
terminals.
Question is, is this going to be sufficient to keep the batteries
topped off? I was expecting to have something less of a drop than what
this diode isolator has, since a silicon diode junction when forward
biased has a .6 volt drop. I have heavy gauge wire and tight
connections, so I am sure that the voltage drop is at the diode
junctions. I believe the alternator, which is OEM, has an internal
regulator, and probably no way to tweak the voltage up to compensate
for this. Do I have a problem, or not?

13V is a tad low, it ought to work, but if you could get another half
volt or so you'd be better off, especially after you deal with the drop
in the wiring all the way back to the battery in the camper. The
alternator does indeed have an internal regulator, I don't recall if
this year has a Bosch or Denso alternator, but if it's the Bosch, IPD
sells an adjustable regulator you can install. I've never looked closely
at the Denso alternator in some later cars so I don't know the details
of the regulator. Either way, you should avoid going above about 14V at
the fuse box in the car or things like lamp life will start to suffer.

 

[geronimo]

I am adding a 12V external connection to my Volvo 92 740 to be
able to run/charge the DC system on my pop-up camper, which has a 12 V
deep cycle battery. So I got one of those three-terminal battery
isolators. The alternator output goes to the center terminal, and the
car battery and the pop-up battery go to the other two terminals. The
alternator puts out abt 14 V measured with my fluke. However, each of
the isolator diodes has about a 1 volt drop across it, so now there is
just 13 volts available to charge either battery....at the battery
terminals.
Question is, is this going to be sufficient to keep the batteries
topped off? I was expecting to have something less of a drop than what
this diode isolator has, since a silicon diode junction when forward
biased has a .6 volt drop. I have heavy gauge wire and tight
connections, so I am sure that the voltage drop is at the diode
junctions. I believe the alternator, which is OEM, has an internal
regulator, and probably no way to tweak the voltage up to compensate
for this. Do I have a problem, or not?

 

[ransley]

   I am adding a 12V external  connection to my Volvo  92 740 to be
able to run/charge the DC system on my pop-up camper, which has a 12 V
deep cycle battery. So I got one of those three-terminal battery
isolators. The alternator output goes to the center terminal, and the
car battery and the pop-up battery go to the other two terminals. The
alternator puts out abt 14 V measured with my fluke. However, each of
the isolator diodes has about a 1 volt drop across it, so now there is
just 13 volts available to charge either battery....at the battery
terminals.
    Question is, is this going to be sufficient to keep the batteries
topped off? I was expecting to have something less of a drop than what
this diode isolator has, since a silicon diode junction when forward
biased has a .6 volt drop. I have heavy gauge wire and tight
connections, so I am sure that the voltage drop is at the diode
junctions. I believe the alternator, which is OEM, has an internal
regulator, and probably no way to tweak the voltage up to compensate
for this. Do I have a problem, or not?

Read at www.batteryuniversity.com 13.8 is needed to get to a peak,
14 is a peak voltage and works. 13 is not going to saturate cells but
will do if you use a seperate charger at home and this is just a
vaction set up. 12.8 is a fully stabilised charge, but it needs to
peak for a while at 13.8 or so. That is a hard load on an alternator
to run it as a charger, an easy way to burn one out is what you
propose.