From: garnett@news.Colorado.EDU (Santiago de la Paz)
Subject: Basic Electricity (was Re: Jump start with car? DONT DO IT !!!!!!)
Organization: University of Colorado, Boulder
Date: Wed, 15 Apr 1992 00:31:43 GMT

mel@derringer.EBay.Sun.COM (Melvin Cagle) writes:
|> The difference is your Ford is expecting 70-90 amps. and your poor bike is not.......
|>      DoD 0497                                                                                                                                                                                                                            |>                                                                                                                                          
|>                                                                                                                                                                                                                    |>                                                                                                                                                                      

This argument comes around fairly frequently, and I'm still surprised that
it gets so much arguing time; I suppose that it really is confusing.  The
concepts are simple, though, so perhaps a straightforward explanation of
the processes involved would help to (finally) resolve this round.

The deal is this: for the most part, you never need to worry about jump-
starting a motorcycle from a car, for the same reasons that for the most
part you never have to worry about jump-starting a car from a car.  The
size of the charging battery (the car) has *nothing* to do with the
rate of charge or the potential for damage!

Imagine for a moment a near-infinite current source, say a battery in the
car the size of the sun, or Chuck Roger's FJ baggage (:^), ie near-
infinite amperage at 12 volts.  Now, consider your motorcycle battery, 
sitting there not fully discharged, but unable to run the starter.  Open-
circuited, it will still present nearly 12 volts: equipotential.  No current
flows.  When you turn the crank, it is the *car* battery that is going to
provide the inrush current.  No doubt you may think this is the basic 
problem, but read on, for answers follow.

Consider the opposite example: motorcycle battery fully discharged (0 volts)
but able to accept a charge (if it's not, then buy a new battery, don't
jumpstart).  If we jump, a current flows which is limited by the impedance
of the connecting cables and the internal impedance of the battery.  This
internal impedance is simple to measure: place a load on the battery and 
measure the output voltage at the terminals, change the load and take the
new voltage, then compute the impedance from the delta-voltage divided by
the delta current.  This is normally higher than you might think, and is
what limits the voltage when you short the battery's terminals -- batteries
are not those nice theoretical "zero resistance" EMF sources that you 
learned about in Circuits For Software Designers.  The short circuit
current is not high because of this, only on the order of 300 amps.

This means that inrush current, upon jumping from a car is at most 
short-circuit current, less because of cable impedance.  But inrush 
current is instantaneous: under the heavycurrent, counter-emf due to 
electrode material and gas buildup quickly reduces the actual charging 
current, an effect you'll see under *any* charging conditions, whether 
caused by jump-starting from a car or charging from a normal battery
charger.  Note that batteries consistently are designed to prevent
gas buildup which insulates the internal electrodes, but these are
chemical mechanisms (like the additive of something, perhaps calcium)
which are quickly overwhelmed when the high current appears.  When the
counter-emf builds near to 12 volts, the charging voltage drops to 
the difference.

After the initial inrush, the charging voltage is just the impressed
voltage minus counter-emf, and the charging current is that divided 
by the (already found) high inherent resistance of the system: a low
value, something like 20-50 amps.

SO, the SIZE of the charger makes no difference, just so long as it
is a 12-volt charger and your motorcycle battery is a 12-volt
battery.  The only case where inrush current could make a difference
is one where the battery is so far gone that you needed to replace
it *anyway*: when the connections internal to the battery are corroded
to the extent that they are broken like fuses when the inrush flows.
In a healthy battery, this won't make a difference, but in one abused
by many charging/discharging cycles in addition to overflows and
weathering, the internal connects will be eaten away and blow apart
when the inrush current flows, merely because of the lowered cross-
section.  When this happens after a jumpstart from a car, no doubt
the "big battery supplying too much current" would be blamed, but
chances are that you'd end up losing the battery soon afterwards,
anyway.

The idea is this: if you've taken care of your bike, don't sweat making the
occasional jumpstart from a car.  It's not going to hurt anything.  On
the other hand, if you've never even seen your battery, don't do a jump
start, just buy a new battery and install it right there.  AND DON'T LET
IT DISCHARGE AGAIN!

~james

James Garnett                                   garnett@boulder.Colorado.EDU
UnixOps/Distributed Computing                       ...!ncar!boulder!garnett
University of Colorado at Boulder                            +1 303 492 0264