[Lowfer] Re: MFJ-4035MV power supply as "float battery charger"

[Clint Turner]

Just a note or two when using "rig" supplies for floating battery banks:

> I don't use any reverse pol. protection.
> It doesn't seem to be needed.
> I use a system that charges initially with 14.5 volts and
> floats at 13.6 volts. The biggest problem is keeping water
> in the battery. If reverse pol is really needed, you could just
> use a diode.
>

A series diode (between the battery banks and the power supply) *IS* a good idea for
several reasons:

1 - Most power supplies use a "'723 and Pass Transistors" arrangement in their
regulator.  '723s have a way of dying if the voltage on the output exceeds the
voltage in the regulator input.  This would occur if a power failure were to occur
with a battery connected.

Many power supplies nowadays (such as Astrons) actually have a "backwards" diode
across the regulator inside so that backfeeding a voltage doesn't blow up the
regulator (the '723 seems to tolerate the 0.6 volts or so of drop OK) but this may
not always be of help if the battery is connected to a powered-down supply:  The
diode may not be able to charge the 100,000uf (more or less) of filter capacitance
before the '723 gets blown up.

2 - Good-quality power supplies include a crowbar circuit that shorts the output in
the event the output voltage goes too high (usually in the area of 16-17 volts.)
These are usually connected directly across the output terminals of the supply.  It
should go without saying that, if the crowbar fires (due to actual regulator failure
- but they will occasionally fire due to nearby lightning, when a load is connected
and a "glitch" occurs, due to RF, or just because of the phase of the moon) then you
suddenly have this little SCR (I saw an Astron RM-50A with a 10-15 amp TO-220 mount
SCR on it while my Drake PS-7 supply has a 40 amp stud-mount device) is put across
the battery bank.

Perhaps the sound of an SCR detonating will alert you that something just happened -
or not.  Even if you fuse the battery bank (which you *should* do) a wimpy SCR may
not survive the jolt - possibly leaving you with a shorted SCR (which is bad) or one
that is blown open (which is worse - if you don't realize it!)  I simply set the
voltage on the power supply so that, while floating, the battery sees 13.75 volts
(14.3-14.4 volts or so) with the equipment connected across the battery.  Anyway,
having an expensive fuse blow (if the fuse is larger than 30 amps) is a nuisance -
and if you have a series diode, all it takes is power-cycling the supply to reset the
crowbar - if the crowbar fired "randomly" due to noise or RF (i.e. no malfunction.)
You can always wire a Sonalert (on a full-wave bridge rectifier - so that it will
operate on either polarity) across the series diode and fuse to alert you that the
power supply has tripped out or the fuse has blown.

When I connect my PS-7 across a battery bank, I use a BFD (Big Friendly Diode - about
60 amps worth, on a heat sink) in series - just in case.

Maybe I'm saying something that everyone already knows - but I've seen several people
forget the Crowbar - and have it fire... so to speak...

***

As far as electrolyte loss in flooded-cell lead-acid batteries:  If adding water is
tedious (or, if you are like me, something that is easy to forget to do) then a
possible solution are "Hydro Caps."  These (and similar products) catalytically
recombine the hydrogen and oxygen back into water and reduce fluid loss
significantly.  I have some batteries with Hydro Caps - and some without - and the
ones "with" lose about 1/3 the water that the others do.

73,
Clint
KA7OEI
"CT"

> > Currently I use a series of 12V deep cycle marine batteries to provide
> > emergency power
> > to my Icom 746 or whenever I need to reduce all noise to the minimum for
> > weak signal work. I'd like to modify the system to always supply power
> using my
> > MFJ-4035MV power supply as the "float charger" to keep the batteries
> > charged, and when ac fails, I'd have instant change over to DC.
> >
> > I know that it's necessary to protect the power supply from reverse
> voltage by
> > inserting diodes, and have found plans to modify Astron supplies for this
> > type of work, but nothing specific on the MFJ supply. Just wondering if
> anyone
> > knows what kind of protection will be needed?