[VAC] Re: transformer/converter question

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Joy,
 The UniVolt is what is called a constant voltage charger, it operates
at a bit over 14 volts in most cases. It is what Livingston describes as
a single output floater, no seperate charging and power circuits.

Now, here's why the UniVolt, et. al. are crap:

To charge a 'low' battery in a decently short time, you have to have the
charging source set at about 14.4 volts or maybe a bit more. That will
cause a good charge rate and it will taper off as the battery approaches
full charge. However, that's too high to float the battery at, you'll
cook out the water. ('cook out' isn't technically correct, but you get
the idea). In an attempt to prevent cooking the battery during long idle
periods where the unit is connected to shore power, they might design
the converter so the voltage is right at 14 or maybe a bit less. The
problems however, are still there, the battery will now charge very
slowly, taking maybe 24 hours or more to be fully charged if it's down
from a couple of 'no power' days, yet that's still a bit too high for
long-term floating.

A proper 3-stage charger such as the StatPower or Inteli-Power sets
itself to 14.4 volts during the "bulk charge" part of the charging
cycle. Once the battery is 80%+ charged (just a few hours), it switches
to 13.8 volts to finish the job. When the battery is fully charged and
there's no current draw, it goes to 13.2 volts for "storage mode". The
Inteli-Power automatically switches to 14.4 volts for 15 minutes, every
21 hours, to keep the battery cells equalized; the StatPower has a
manual control for equalization.

You can easily check to see what your UniVolt is doing by connecting
your DVM ammeter in series with the battery. Set it to "10 Amps" and be
very careful how much load you turn on. You'll find that the charge rate
will be very low to non-existant when you have a couple of lights on,
yet will be 1 or 2 amps when the system is idle and the battery fully
charged. Pushing even an amp into the battery when it's already charged
will cause the water to cook out.

See:
http://www.progressivedyn.com/_frames/frames_rv_power_converters.html

and:

http://www.amplepower.com/primer/full/index.html

for more information.

Obviously, you can get by with a UniVolt (or equivilant) converter. If
you are nearly always connected to shore power, only boondock
occassionally and have plenty of time to recharge after, watch the water
level in your batteries, and don't mind a bit shorter life from them,
then the UniVolt will usually work OK. If you want to treat your
batteries with kindness, need to be able to recharge quickly from shore
power (using a small genset during extended boondocking, for instance)
and want to get away from that God-awful hum, then a new unit should be
on your shopping list.

                                            <<Jim>>