[VAC] Re: Batteries and Converters

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Jim,
 Use your Digital VoltMeter (DVM) across the battery terminals after you
think it's fully charged, say when it's been plugged in to shore power
for a few days and no use of the trailer.

Here's the tricky part: The voltage must go to 14 or more to charge the
battery at any decent rate. However, if the voltage remains there after
the battery is fully charged, it'll "cook" the water out of it over a
period of time. ("Cooking" isn't really the proper term, but serves the
purpose)

A good 3-stage charger will put out about 14.6 volts until the battery
is 80% charged, then drop to 13.7 or thereabouts to bring it to 100%
charge. The first stage will require a few hours, depending on the size
of the battery bank and the (amp) capacity of the charger. The second
stage will require quite a long time, maybe 10 hours or more. When the
battery is fully charged, the output voltage will drop to 13.2, keeping
it at full charge but having nearly no water loss. Periodically, the
charger will kick into "high gear" for a short period to make sure the
battery is fully charged and cause the electrolyte to circulate or
de-stratify. This prevents sulfation, the biggest killer of lead-acid
batteries.

A regular UniVolt or equivilant holds the voltage at 14 -> 14.5 volts
more or less, charging the battery slowly, but overcharging it when it's
sitting idle.

One other point about lead-acid batteries: they need to be exercised a
bit. They should have a load applied, running them down to a small
degree, then recharged. Brand-new batteries won't reach their full
potential until a few dozen discharge/charge cycles. However, unlike
NiCad batteries, they do NOT EVER want to be fully discharged, that's
just about the worst thing you can do to them. Longest life will be
achieved when you never let them go below about 50% charge level. 

Here's a tidbit: a fully-charged lead-acid battery will have 12.6 volts
across the terminals. When the voltage drops to 12.1 volts, it's 50%
discharged; full discharge is at 10.5 volts. This means that you've gone
way too far when you notice the lights getting dim. Note that the above
readings must be taken on a battery that's been at rest (no charge or
discharge) for at least several hours, preferably overnight. The
precision required is why the "voltmeter" or "battery guage" in the
typical RV is totally useless. If you have nothing else, you should have
a good digital voltmeter builtin and at a location where you can readily
observe it. It really needs to be able to read 2 places to the right of
the decimal point.

                                                <<Jim>>