[NCARC] Converting 24v to 12v

Ron Klein [email protected]
Tue, 29 Jan 2002 20:54:19 -0700 

Why not just find a 12 volt regulator and run it from the 24 volt 
battery? Use that to run the rig. You will lose some power in the drop 
across that regulator, but so be it. You might be able to find a 
switching type that will be relatively efficient.

Ron - W0OSK

-------------------

From:           	Bob Proulx <[email protected]>
To:             	"Michael Robinson" <[email protected]>
Copies to:      	[email protected], [email protected]
Subject:        	Re: [NCARC] Converting 24v to 12v
Date sent:      	Tue, 29 Jan 2002 19:42:18 -0700

> > A friend of a friend is a ham in Africa.  His vehicle
> > has a 24vdc system.  The battery has no taps to divide
> > it into 12vdc series.
> >
> > He needs about 20A to run his FT-100.  Can he put 2
> > 12v car batteries in series and tap from the center
> > connection to get the 12vdc he needs?
> 
> Yes, but... not good for other reasons.
> 
> >       24vdc                         Ground
> >         +                              -
> >                        | |
> >                       ||||
> >         .-------------||||-------------.
> >         |        +    ||||   -         |
> >         |      24vdc   | |             |
> >         |                              |
> >         |                              |
> >         |                              |
> >         |                              |
> >         |     12vdc           12vdc    |
> >         |   +  | |  -       +  | |  -  |    
> >          -----||||------v-----||||-----|
> >                | |      |      | |     |
> >                         |              |
> >                         |              |
> >                          --- FT-100 ---
> > 
> > 
> > It seems too easy.  So before I show him how to
> > blow up his radio, I thought I'd check with you
> > guys as a sanity check.
> 
> This will give 12v just fine.  But if you have a 12v battery why tie
> it into the 24v system at all?  Just so that it can charge?  If so
> then if possible I would charge it differently.  Even something as
> inefficient as a 24vdc to 110vac inverter and then 110vac to 12vdc
> trickle battery charger would probably be better for the health of the
> batteries.
> 
> Here is the problem as I see it.  The right 12v battery both charges
> and discharges.  Fine.  But the left 12v battery never gets
> discharged.  It only gets charged.  Which means it will cook off
> pretty quick in this arrangement.  Fine for an emergency but I would
> not use it this way long term.  The left 12v battery will always have
> maximum charge and therefore maximum voltage.  Which will mean that
> the right 12v battery will never get the full voltage across it and
> will never get to full charge.  Both batteries die but for opposite
> reasons.
> 
> You need some way to balance the charge of those batteries.  But this
> is an interesting idea and I would not give up yet.  This might be as
> simple as creating a switch or plug arrangement so that the radio can
> draw power from either the left or the right battery and manually
> selecting which battery you are using based upon the measured voltage.
> Always draw power from the one with the highest voltage seems like a
> safe mode of operation.
> 
> And remember that car batteries are good for short bursts of lots of
> current but not so good for deep cycling.  They have thin plates close
> together while deep cycle battery have thick plates with more spacing.
> But car batteries are cheap and available so why not?
> 
> Just my thoughts...  Maybe it will spark other ideas.
> 
> Bob
> kf0uw
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Ron Klein
----------------------
[email protected]