Date:         24 May 1996 09:57:25 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         "Harvey Chao" <Harvey_Chao@smtp.svl.trw.com>
Subject:      F.Y.I. - regulating chargin

                                                                      5/24/96
                                                                      9:21 AM
 F.Y.I. - regulating charging current (long)

There have been a few recent messages about excessive current when recharging
a depleated axu. battery so as not to blow the wiring or alternator.

One suggestion was adding resistors to the charging circuit.

I think that there is a better (mre elegant)solution.  Perhaps some of you out
there can confirm this for me as it has been just a "few"years since I was in
school on this topic.

If I recall, the charging rate of a battery is determined by (the difference
in voltage between the battery under charge and the applied voltage) and by
the internal resistance of the battery.  In theory, if the difference (delta
v) is high enough, and the source is an ideal unlimited source, you can pour
unlimited current into the battery until it boils and explodes and or melts
the wiring and itself.  This is because the internal resistance of a healthy
battery is very low where the only limiting factor is this internal resistance
of the battery. [Note - a bad battery, one that is "sulfated" and won't accept
a charge  - has a very high internal resistance, that's why it won't accept
any significant current flow when you put a nominal 14 or so charging voltage
across it]

Think of a battery as a very low value resistance in series with an imaginary
"perfect" source of unlimited  current .  As such, the only thing that limits
the amount of discharge amps in a direct short is the value of this internal
resistance.  Since we know that a direct short can produce 100s of amps at a
nominal 11-12 volts the internal resistance is very very low (amps =
volts/resistance - so if amps is really big, and volts is around 12, then
resistance is on the order of  0.1 ohms or less.)

Let's say that the alternator output is regulated to 14 volts up to the rated
output of the alternator (the voltage will start to drop off after that max
output current), and that the open circuit voltage of the discharged battery
to be recharged is 10 volts.  OHm's law says current = volts/ohms =>
(14-10)/0.1 = 40, using a very conservative 0.1 ohm value of internal
resistance, it could easily be 0.05  or less which would double the current to
80 amps or more.

We can't do anything about the internal resistance of the battery under
charge, but we can limit the applied voltage and thus reduce the delta v.

Automotive chargint circuits apply a constant voltage to the battery and allow
the current to vary with the state of charge of the battery - really high at
first (max delta v) and then tapering as the voltage of the battery under
charge rises with it's state of charge.   Adjustable voltage regulators  for
alternators are available - I used to have one on one of my cars.  The crude
implementation is to manually lower the alternator output voltage  by
resetting the variable regulator's adjustment setting with a screwdriver. 
Just gotta remember to put it back after the battery gets abeyond it's initial
high current surge at the start of the recharge cycle.   

 I am sure that it is possible to take a standared regulator, "gut" it so that
the physical carrier for the alternator brushes remains, and bring the leads
out to a seperate regulator circuit that is dual mode, 1) under "normal" loads
it regulates as a stock regulator, by only regulating the output voltage of
the alternator at around 14 volts.  It will have a second function 2) whereby
it senses the output level of the alternator current, and when the current
approaches a preselt limit (alternator rating) it reduces the voltage
regulation from 14 to some lessor value so that the current output does not
exceed rating.  Any "takers" on this out there???

[I know that existing standard circuts will tend to be self limiting and do
this to a degree  - as the current approaches max, the voltage drops, but the
standard regulator keeps trying to keep the voltage up and so the alternator
runs "wide open" limited only by resistance losses in it's  windings and
diodes.  The problem is that it keeps trying to supply more current than it is
rated at.  When the current goes beyound rated value, and the resistance of
the windings and diodes are fixed, there is increased heat generated  in those
items (amps X amps  x resistance) due to the  current  flow that is above
limit -  that's when diodes blow and windings melt.]