[NCARC] Converting 24v to 12v

[Bob Proulx]

> 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