From: fletch@spf.trw.com (Fletcher Riggs)
Newsgroups: rec.boats
Subject: Re: 2 batteries -- how?
Date: 2 Sep 92 15:17:44 GMT
Distribution: na
Organization: TRW Sensor Data Processing Center, Redondo Beach, CA

In article <19861@fritz.filenet.com> daver@fritz.filenet.com (Dave Richards) writes:

>As I see it, there are several options:
>
>(A) Run the output of the alternator directly to the isolater and the
>two isolator outputs to the two batteries.  Problem: If the selector
>is left on "1", battery 1 charges fine through the selector switch,
>bypassing the resistance of the isolator, but #2 never charges up all
>the way because of the .8V diode drop.
>
>(B) Same as (A) only I readjust the alternator output to .8V higher.
>Now selector is left in the "off" position at all times while the en-
>gine is running and both batteries charge equally to the correct vol-
>tage through the isolator.  Problem:  The switch may unintentionally
>be left on "1" or "2", and the higher voltage will cook the battery(ies). 
>
>(C) I forget about the isolator altogether and just hook the alternator
>output to the selector switch common terminal.  Only the selected battery
>will be charged.  Is it OK to leave the selector switch on "both" while
>the engine is running, to charge both batteries in parallel? (That's
>what the switch documentation says, but I have doubts.)
>
>(D) Same as (C) only the switch isn't left on "both".  The skipper has 
>to remember to periodically alternate the switch between 1 and 2 while
>the engine is running to charge both batteries.
>
>(E) Any other options I haven't thought about?

Your proposed solutions suggest that the alternator is connected directly 
to your ship's electronics in addition to being connected to the batteries
via the isolator.  Solution (E):  Connect the alternator only to the batteries
via the isolator.  Connect the ship's gear only the the batteries through
the switch.  When the alternator is running it will supply the ship (provided
it is turned on) and, if there is enough output, charge the batteries at the
same time.  When the alternator is not running the ship will simple draw from 
the batteries via the switch.  

There is one small snag with this approach.  If the regulator senses voltage
directly off of the aternator outout line, then it will fall .8V short of
reaching the desired level at the battery.  If the output level is adjustable
(as you suggest is (B)), then it's no problem since all output goes through
the isolator.  If the output level is not adjustable but the regulator has a
sensor circuit separate from the output line, then run that sensor circuit
directory to one of the batteries (the one you think will be discharged or
used the most).  A slight modification of this last approach will give you
full control over your changing system:  run the regulator sensor return
though a rheostat on the dash so you can manually control the alternator
output.  CAUTION:  you must monitor voltages yourself.  Although you can
"cook" you battery this way if you don't pay attention, it does allow you to
recharge you batteries in a shorter time.  The reason some peaple take this
apprach is that alternators/regulators are designed to supply your vessel
with power on demand, not to recharge batteries.  When simply charging a
battery the voltage reaches its shutoff point (14V or so) long before the
battery is fully charged.  volt1

-- 
fletch@spf.trw.com 
Newsgroups: rec.boats