I've always wondered about the charge control for auto batteries.
I suspect that there is no curent control on the alternator, only a
voltage regulator. If this is true then the only thing regulating the
current is the voltage drop along that red charging wire (to the starting
battery). More current = more voltage drop = lower battery voltage until
some charging equlibrium
is acheived. That being said putting in a larger alternator will not
increase your charging rate unless you run a larger cable direct from the
alternator stud. If you connect the aux battery via the charging wire on
the starting battery (through a relay or isolator, by the way the
isolators drastically reduce the charging current by inserting the .7V
diode drop.) it should not overload the charging system because of the
effect described above.  Assuming that your aux battery is of the group 24
varity (almost fits under drivers seat) charging current should be kept
below
100AH(approx size)/10 or below 10A anyway but no one seems to use a
current regulator to prevent over currents to battery.!? This being said I
would be careful about running that heavy gauge wire directly from the
alternator and stick with the stock length of red wire that runs to the
passenger seat. 14V from a 65A or 90A alternator through a heavy gauge
wire to an empty 100Ah size deep cycle battery would result in some very
dangerously large charging currents.....I think.
I think my points are:
1) You don't need a heavy duty alternator to recharge a 100Ah battery that
only requires a 10A charge current
2) Becareful about connecting empty deep cycle (RV and marine) batteries
to the alternator through a low resistance wire and no charge controller
(curent regulator), currents could be dangerously high.
3) If all your loads combined exceed 65A then of course you need a larger
alternator.

I have my aux battery under the drivers seat and use a relay and the
charging wire from the starting battery. I have been monitoring charge
currents and everything works fine with no need for the charge controller.
I don't experience any charge currents above 15A even when the battery is
charged from completely dead.

That reminds me, install a fuse on the aux charge wire. 20A is good for
group 24 size batteries. This could save your alternator against large
battery charge demands.

Hope this makes some kina sense.

ANdys
Vancouver

On Fri, 28 Jan 2000, Ken Hunter wrote:

> I would like to increase the output of the alternator on my 82 diesel Westy
> for 65A to 90A to help charge the aux battery. I've heard this was an easy
> upgrade but no one knows the particulars that I've talked to. Advice?
>