MAILBAG
Dear TJ,
The December issue (page 13) has the 7805 regulator backwards.
— John
Response: Yes it does! For those readers trying to duplicate the “Motor
Speed Controller” circuit, reverse the IN and OUT pins. I have also had requests
for the source of the IPS031 Smart FET. It is available from Digi-Key, Allied, and
Newark. — TJ
Dear TJ,
The Dec. 2005 column has a question about a telephone ringer. The easiest
way to get one is to order it from Jameco Electronics. It is the “Black Magic”
ringer, LNU-BMR12V, Jameco stock number 145816. It requires 12 volts input
and outputs 86 VAC at 20 Hz.
— Bill Stiles
2) Battery manufacturers tell us that
the maximum charging voltage
should not be exceeded 2.43 volts per
cell. What happens inside the battery
if this limit is exceeded?
— M.J.
ATo begin with, there isn’t
really a number of
charge/recharge cycles
assigned to a battery. It can
range between 200 to 1,000 —
depending on how the battery is used.
First, it’s important to understand the
chemistry of the lead-acid cell. To make
a long story short, suffice it to say
that the lead-acid cell is a secondary
battery. That is, it doesn’t generate
electricity — like an alkaline primary
cell — but stores a charge using
reversal chemistry. In the case of the
lead-acid cell, the chemistry is ”lead
rust” — lead oxide to be exact.
In laymen’s terms, you move the
lead rust from one lead plate to the
other using sulfuric acid as the carrier.
Between charge and discharge, this
“rust” goes back and forth. Ever see
what happens to a piece of iron or steel
as it goes through rust cycles? Each
time you “brush” it to get down to the
bare metal, you lose material. Same
with the plates in the battery. It’s called
shedding. Each time the cycle is reversed, some rust is shed. The deeper
the discharge, the more rust involved.
So as to how long a battery will last, it
depends on how deeply it’s discharged
and how thick the lead plates are to begin with. The more lead you start with,
the more there is to lose before there
is no longer a plate.
As to your second question,
exceeding the charging voltage per
cell affects the sulfuric acid electrolyte more than the plates. As the
charging current continues over time,
it raises the temperature of the aqueous solution to the point where it
actually boils. Now you know what
happens to boiling water — it evaporates. Since the battery needs the
electrolyte to operate ... well, figure it
out for yourself. In rare cases, high
overcharging temperatures may warp
the plates. When this happens, the
plates can puncture the separator
that goes between the positive and
negative plates (they are interlaced)
and short out — rendering the battery
useless.
BATTERY BASICS
QYou had a question about a
6V, 9. 5 Ah gel-cell battery in
the Jan. 2005 issue and
explained an interesting
method to “blow” the sulfate off the
plates of the battery by using high current pulses. My question: If the battery
is dead and cannot accept charging current from the regular chargers, how can
this method be used? Another question
please, can this method be used to
bring life back into a dead car battery?
— Zacky
AFirst, read the answer to the
question above, “For Whom
The Rust Tolls.” What I didn’t say in that answer is that
between charges and disuse, the
plates of the battery absorb sulfate
ions from the sulfuric acid electrolyte.
Basically, the lead plates get sulfated
in three stages. The first stage is surface sulfates that are quickly dissolved
16
February 2006
