The Natural Philosopher wrote:
On 11/04/2020 09:53, Mike Clarke wrote:
On 10/04/2020 22:35, The Natural Philosopher wrote:
On 10/04/2020 20:28, Bert Coules wrote:

Do I need to disconnect the car's own connecting leads
first?

no

Some manufacturers recommend that you do disconnect the battery. Some
cheapo chargers might have a very spiky voltage pattern that could
upset the onboard electronics.

You need a hell of a lot of current to spike a battery over 14v

Car electrical systems need to have a wide voltage tolerance.
Thus occurs during "load dumps".

The voltage seen on a car electrical bus *can* spike, but
the electronics are also specified to survive this.

This is one reason you should not connect relatively "naive"
electronics to the raw electrical system of the car.
(A "car stereo" is not the same as a "home stereo" from
a "width of supply rating" point of view. The "car stereo"
is just a bit armored by comparison.)

You might ask yourself, if the car battery impedance is as low
as we think it is, how do *70V spikes* appear on the car
electrical bus ? As a Natural Philosopher, you can go find
an answer for us. I haven't seen any oscilloscope pictures
of car electrical systems which prove these spikes are present.

As an illustration of this, the specsmanship, we can use a
TDA2003, an audio amplifier intended for cheap car radios.

UTC TDA2003 LINEAR INTEGRATED CIRCUIT

ABSOLUTE MAXIMUM RATINGS(Ta=25°C)

PARAMETER SYMBOL VALUE UNIT
Peak Supply Voltage Vs 40 V
DC Supply Voltage Vs 28 V
Operating Supply Voltage Vs 18 V

Nominal

Supply Voltage Vs 8 to 18 V

Now, that's an integrated circuit, where the nominal battery
voltage might be 12.6V, or maybe 14.4V fully charged or under
charge or whatever.

The 18V value is what results, if you jam the field winding
to battery, as an alternator test. The alternator can run
the battery up to around 18V, by forcing a lot of current
into the battery. Your mechanic at the shop might try this.
That's why this stereo expects on occasion, to see 18V, and
the car radio will continue to play.

Well, what do the other voltages mean ?

The 40V value is a measure of load dump. If there is a
transient on the car electrical bus, the 40V value can be
achieved. The TDA2003 is designed to disconnect itself,
at the 40V level, in an attempt to protect the IC innards.
The behavior of some part of the IC changes, when the supply
goes over 18V.

BUILT-IN PROTECTION SYSTEMS

LOAD DUMP VOLTAGE SURGE

The UTC TDA2003 has a circuit which enables it to withstand a
voltage pulse train, on pin 5, of the type shown in Fig. 23.

If the supply voltage peaks to more than 40V, then an LC filter
must be inserted between the supply and pin 5, in
order to assure that the pulses at pin 5 will be held
within the limits shown in Fig.22.

A suggested LC network is shown in Fig.23. With this network,
a train of pulses with amplitude up to 120V and width of
2ms can be applied at point A. This type of protection is ON
when the supply voltage (pulsed or DC) exceeds 18V. For
this reason the maximum "operating" supply voltage is 18V.

There can be spikes on battery, but you would hope that things
like CANBUS or whatever, the spikes would not be seen there.

What is the battery doing when those spikes are present ?
You got me there. Looks like the battery impedance at
frequency, is insufficient to deal with the spike in any
useful way. It suggests an impedance versus frequency curve,
of weird shape or behavior. Even bench DC power supplies,
have impedance versus frequency curves.

A cheap battery charger, is a half wave or full wave rectified device
running off a transformer. A pulsating DC waveform is being applied
to battery, but staying at relatively low voltage levels. A good reason
for not making sparks around the battery area, is igniting any
evolved hydrogen (more of a problem around the battery types
where the battery caps are removable, as those have vents). It's
for that reason, you could connect the charger while it's off.
The electrical spike issue, the car design has taken some
of that into account, with load dump specs and the like.

If you're jump starting a car, that's usually done with "hot" cabling.
The cabling is connected in a particular sequence, to keep "sparks"
away from the battery. You can connect the "hot" to the "hot" first,
without sparks. The "ground" alligator jaw, gets fastened to a screw
on the metal framework, rather than to the battery terminal, and this keeps
the spark away from the very top of the battery. As it's when the
second lead is connected (or disconnected), when a small spark flies.
When jump starting a car, those "sparks" near the frame, also
hint about the state of the two electrical systems on the cars,
such as if the second car is a dead short or something. You might
see very large sparks or "welding behavior", if the second car
had a dead short of some sort. It's better to be making sparks like
that, near the frame, than near the top of the battery.

Jump starting a car, is a very good time to be testing the
limits of load dump specs.

Paul