I would like to offer a few observations concerning battery charging and
chargers:
A good battery, with a full charge, produces 12.6 volts. An alternator is
regulated to produce a steady 14.6 volts under all conditions, within it's
capacity. If the load on the alternator increases, the internal
regulator increases the field current to maintain the 14.6 volt output. With 12.6
volts from the battery and 14.6 volts from the alternator, all of the
automobile's electrical loads are being supplied by the alternator.
The battery cannot supply current as long as it is being fed from a higher
voltage source.
On a long trip, such as the one I'm taking next month of 15-16 hours duration,
the battery is constantly being fed the 14.6 volts from the alternator. No
harm is done to the battery by this constant higher voltage. The only time
the battery will ever supply current on this trip will be when I start the
car after refueling and eating -- about five times total. I don't believe
the battery would be harmed by this if I drove the car 24 hours a day, seven
days a week, for weeks at a time, without ever restarting the car.
An Ideal battery has zero internal resistance. A real battery has two types
of resistance -- resistance to charging current, and resistance to discharge
current. On a fully charged battery, the resistance to charging current is
high, and the resistance to discharge current is low. On a weak, or
discharged, battery, the opposite situation exists -- the resistance to
charging current is low, and the resistance to discharge current is high.
I have a heavy duty battery charger (60 amps charge, 240 amp boost). The only
components in the charger are a transformer with a multi-tap primary, a
rectifier, a switch, an ammeter, and a timer. The switch has three
positions-- slow charge, fast charge, and boost. The only difference in the
three positions is the tap on the primary of the transformer, which changes
the output voltage of the secondary. There is no regulation whatever.
When I connect the charger to a very flat battery, the ammeter shows an
initial charge current of up to 60 amps. As the battery charges, the charging
current slowly tapers off to a minimum value, on the order of 10 amps. On the
slow setting, the output voltage is 13.4 V; on the fast setting, it is 14.8 V;
and on the boost setting, it is 17.6 V. These measurements were made with no
load. I haven't measured the voltage when it is charging a flat battery at 60
amps, but I'm certain the voltage goes down from the heavy loading.
Given the above, it appears that the battery itself provides the regulation of
the charging current, for a given charging voltage. A constant charging
voltage, whether in storage on in operation, of 14.6 volts will maintain a
full charge without any damage to the battery. I don't know if the battery
needs to be used occasionally or not, but I don't think so. Not as long as
it maintains a full charge. I would think that a power supply, set to deliver
13.5 V or so, would do the job of maintaining a battery for long term storage.
Just for "feel good" until better information is available, you might want to
rig up the charger setup with a switch so that you could shut it off, and
connect the battery to a load for a short period of time. You would only need
to do this on occasion, just whenever you happened to think of it. (I don't
believe that is necessary, but I'm not a battery expert, so I can't be sure).
As a point of reference, at our nuclear power plants (at the Tennessee Valley Authority),
we maintain the equivalent of 13.5 volts on our batteries at all times,
with an equalizing voltage of 13.98 applied on a periodic basis. I say
"the equivalent" because our batteries are either 125 volts or 250 volts,
and consist of multiple 12 volt batteries, very similar to car batteries,
arranged in series/parallel as required to produce the needed voltage and
current capability. Because of the large number of cells involved, and the
fact that they are arranged in parallel, it is necessary to equalize the
charge on these battery setups. Because these batteries are used to perform
safety functions, they are tested on a very frequent basis, so the need for
discharging on occasion is never addressed -- they are automatically
discharged as part of the testing.
Not directly related to charging, but of interest none the less, is the
internal resistance to discharge current. A fully charged battery has a
terminal voltage of 12.6 V, whereas a very flat battery has a terminal
voltage of about 11.6 V. The difference of one volt is not enough to cause
real problems in an automobile. The problem stems from the high internal
resistance of the flat battery. When you measure the voltage, the meter has
very little current draw (compared to the typical battery load, it is
virtually zero), so no voltage is dropped over the internal resistance, and
the terminal voltage stays high. When a heavy load is place on the battery,
the high current draw causes a large voltage drop on the internal resistance,
reducing the terminal voltage to a very low value. The low voltage, combined
with the high circuit resistance, is what causes the starter to groan rather
than spin the engine. To get a true measure of a battery's condition using a
voltmeter, it is necessary to load it heavily, heavily enough to get a voltage
drop across the internal resistance.
Totally unrelated to anything we're talking about, but of interest anyway, I
believe, are the batteries used in portable radios. The next time you have a
9 volt radio battery go bad, replace it with a good 6 V battery just as an
experiment. You will find that it will work just fine, although the volume
won't be as high. What happens when a 9 volt battery goes bad is the same as
what happens when a car battery goes bad -- the internal resistance goes up.
This internal resistance causes distortion. When the radio is playing a loud
passage, the high current causes a voltage drop across the internal
resistance, dropping the output voltage of the battery, and, in effect,
turning down the volume. Conversely, on soft passages, the voltage drop is
not as pronounced, and the battery voltage goes up, cranking up the volume.
Turning down the loud sounds and turning up the low ones is not the way to
enjoy music! Replacing the 9 volt with a good 6 volt cures the problem, as
the internal resistance is still quite low. On any radio, when the volume is
turned up to the point that the battery can't keep up, you get distortion.
Operating a 9 volt radio with a 6 volt battery will lower the volume at which
this distortion occurs.
Dan Masters, Alcoa, TN DANMAS@aol.com
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