[NCARC] Converting 24v to 12v

Ron Klein [email protected]
Wed, 30 Jan 2002 06:01:51 -0700 

Since the discussion here is usually about "shack on the belt" 
topics, I assumed the guy was running a low power vhf/uhf radio...

For higher powers a 24 volt input inverter, dc power supply 
combination is the better choice.

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]