12 Volt Charging

Actually, the VR controls the current to the field (it actually
PWMs it) so as to keep the voltage at the "big" alternator terminal
(the one connected to the battery Plus) as close to 14.25V as it can.

I dont understand the reference to 11V, but the battery acts as
the "filter" to smooth the alternator ripple. The alternator
has a three-phase full-wave rectifier; its ripple content is
quite low. For a three-phase full-rectified wave whose RMS value is
14.25V, the min value of the ripple is 13V, the peak value is 15V.

Thats the way it works, even within the ripple cycle. In the long
term, every electron that flows through the "loads" comes from the
alternator. Only temporarily does the battery supply any of the
load current.

You need to read up on the electrochemistry of Flooded Lead-Acid
Batteries. You will find that if a FLAB is connected to fixed
13.65V voltage source (float voltage), the FLAB will initially draw
current (charging current). As the FLAB accumulates charge, the current
will asymtotically decrease, settling to a final steady-state
current which just equals the FLABs internal leakage (tendancy
to self-discharge, typically under 100ma).

At the nominal automotive VR setting of 14.25V, the charging
current initially decreases over time but settles to higher
final value. The higher steady-state current is due to
electrolysis, which causes gas evolution (H & O) at the plates.
At 14.25 the rate of gas evolution is still small; the steady state
current is ~250ma.

The 14.25V is a compromise setting; it keeps the battery nearly
fully charged, while not "using" electrolyte at a prodigous rate.
14.25V is too high to be used as a full-time float voltage, but it
is ok for a car that is only being driven 2 to 4 hours out of every
24 hours. 13.65V is used as a full-time float voltage where
the goal is to keep the FLAB from self-discharging when it sits
unused for months at a time.

MikeM

 

For what it's worth department. We used a 2.2AH 12V sealed lead acid battery
at work to power some test gear, and charged it off an HP lab power supply.
To get it to draw 250 ma charging current, I had to crank the voltage up to
about 14.3V.

In my car, the battery is under the rear seat, which implies some resistance
back to the alternator. With the engine running, both the instrument panel
VM, and a DVM plugged into the cigarette lighter show about 15.2V. The
car/battery will soon be 4 years old. So the batt is not being self
destructed.

I have an electric start riding lawn mower, which I trickle charge during
the off season. The battery is a 30AH car type lead acid. The charger is an
ATT 12VDC 300ma wall wart from a cordless telephone. The open circuit
voltage is 16V, and current at 14V is less than 10 ma. This may not be
enough to fully charge the battery, but it did crank over at full speed.
Also, I did not keep it charging continuously,

Tam

 

Greetings Group,

This is a hypothetical question concerning an automotive charging
system. I know that the voltage regulator controls battery charging by
regulating the field current in the alternator between 13.5 and 14.5
volts. The question I pose is this:

If the voltage from the alternator (unregulated) was held at a constant
11 volts , that's 11vdc to a peak of 15.55vdc, full ripple, no filter,
would this damage the lead-acid battery?

Theoretically, in this model, the battery works in conjunction with
alternator to supply the load instead of intermittent use. When the
alternator voltage drops below 13.5, the battery starts conducting its
supply to the load. When the alternator voltage exceeds battery voltage,
the battery is charged.

Where is the flaw in this line of thinking?

Randy Gross