From: gerard@progress.COM (Gerard Bras)
Subject: Re: 2 batteries -- how?
Organization: Progress Software Corp.
Distribution: na
Date: Tue, 8 Sep 1992 20:58:26 GMT

daver@felix.filenet.com (Dave Richards) writes:

>I'm electrifying my dad's 50+ year old sailboat.  Everything was pro-
>gressing smoothly until I decided to include wiring to allow a second
>battery to be added at some later date (I don't think it's a critical
>requirement at this point).  

There are a couple of reasons to have two batteries.  One is to be able
to start your engine  if one battery gets too low.  The other is more 
debatable and has to with it being better for the batteries to rotate
them on a daily basis.  Personally, I'm unconvinced on the latter point.

In any case, there's quite a bit to learn about the care and feeding of
leead-acid batteries.  If you want I can provifde a few titles which
will give you some intro.

>Anyway, I bought a selector switch that
>allows you to select one or the other or both batteries, and I also got
>a battery isolator so they can both be charged while only one is being
>discharged.  As you may already know, the isolator is simply two diodes
>with a common anode connection.  The two cathodes are the two outputs
>that go to the batteries.  The diodes introduce about .8V drop from the
>alternator voltage (I measured it).

What was the current load when you measured?  The forward voltage drop
of diodes is not constant and increases with forward current.  An
isolator that doesn't use Schottky diodes is going to run pretty hot.
So you need to size these devices very generously.

...del

IMO, you should try to return the isolator.  There is no reasonable way
to regulate the alternator that I know of which is both safe and effective
when using one of these devices.  Unless of course you remember and are
able to keep them in an equal state of charge at all times.  Sounds
impossible to me too.

>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). 

Both A and B, IMO, are poor ideas waiting to turn into problems.
You seem to have a good grasp of the reasons why.

>(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.)

I don't like this option.  If you have a bad alternator or regualtor then 
some night both of your batteries will go flat.  If you don't believe me ask
John Hughes.  You have redundant batteries for reasons besides con-
venience and it's foolish to parallel them since it exposes them 
both to failure at the same time.

>(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.

There is an awful lot to be said for this tried and true method.  

>(E) Any other options I haven't thought about?

Yep, two alternators.  This is a really reliable way to go since you
have to fully redundant charging systems.  It does cost twice as 
much though and you'll have to decide whether you really want to pay
this much to improve a much simmpler and and already quite
reliable system.

regards,
gerard
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