From: gerard@progress.COM (Gerard Bras)
Subject: Re: 2 batteries -- how?
Organization: Progress Software Corp.
Distribution: na
Date: Thu, 10 Sep 1992 17:00:57 GMT

kstuart@oasys.dt.navy.mil (Kenneth Stuart) writes:

>From an electronics engineer's point of view...

From the users view...

>The voltage regulator on the boat's alternator forces the output voltage
>of the alternator to be at about 14 to 14.5 volts at all times.  This
>is the voltage as measured AT THE VOLTAGE REGULATOR, not necessarily
>at the battery or other equipment.

Well, this depends on the regulator.  Most are set for a cutout of 13.8V
or so.  This is kinfd of low for real charging.  Fancier alternators vary
the cutout voltage with the perceived state of charge.

>A diode isolator will automatically "steer" the alternator's output
>to the lower voltage battery of the two.  As the voltage of the more
>discharged battery approaches that of the charged battery, the alternator's
>output is divided by the isolator as necessary to maintain the two batteries
>at the same voltage.  Since the discharged battery will be accepting
>current for charging, the current through its diode will be higher than
>that of the other diode to some extent.

This is true as far as it goes.  One thing I've been wondering about
is waht the ratios actually are.  To make an example suppose battery A
is charged about 75% and has an at rest voltage of ~12.6V.  Battery
B is pretty flat, reading around 12.2V.  The alternator is putiing
out, for the sake of the example, 100A.  Now, how many amps are
A and B getting respectively.  Is it 0/100 until they both read
12.6 at which point it's 50/50?  This appears to be the claim of
the vendors and I'd like to see it verified, or at least get the 
straight dope.

>The simple 2-diode isolator AUTOMATICALLY performs this function.  I
>know of no other isolator currently on the market which has any other
>construction than the basic 2-diode configuration.  Again, the simple
>isolator automatically gives the lions share of the alternator's output
>to the battery with the lower charge state.

Again, can you quantify what the lions share is?  I'm not trying
to corner you, I just want to know.  I grant that whatever it is that
these islators do, they do it automatically.

>With the voltage regulator connected on the alternator side of the
>isolator, there is no way that an isolator can "cook" either battery
>unless the voltage regulator itself malfunctions.  

True, but this is the wrong place to hook it up.  It needs to sense
actual battery voltage if it's going to fully charge the bugger.

>In fact, the isolator,
>as mentioned by others in this group, has a certain loss (as much as
>one volt) which may PREVENT fully charging both of the batteries, which
>is an undesirable situation.  Schottky diodes will decrease this loss,
>but I doubt that the current crop of isolators make use of these devices.

Some do, some don't.  The cheap ones don't and I doubt their forward 
voltage drop is under 1V at rated load.  BTW, this is the reason I feel
it's wrong to hook up the regulator sense line to the alternator output.

>The main purpose of the isolator is to prevent an old battery with a
>high self-discharge rate from discharging the other battery when the
>boat is sitting for any length of time.  Simply flipping the battery
>switch will do this quite effectively. Also, it is supposed to allow
>charging of both batteries without flipping the battery switch to the 
>"both" position.

The isloator will, barring failure, do a great job of the first task.
Not quite as well as a manual switch, but credible none the less.
It's the second task I'm unsure of.

>In my opinion, this gadget is a waste of money and alternator/engine
>power.  Any boater with salt water in his veins can certainly rotate
>the knob of a battery switch to allow parallel (or alternate, as
>suggested by one post in this group) charging.  By not installing this
>gadget, proper battery charging is assured to the battery/batteries
>switched on line, there is one thing less to go wrong on the boat, the
>alternator does a better job of supplying power, and money is saved.

Amen.  I agree entirely.  

regards,
gerard