Re: Car More Dead from Boosting a Dead Car Battery
- From: "Homer.Simpson" <Homer.Simpson@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: 21 Dec 2006 17:59:30 GMT
D from BC said
Taken from
http://www.canadiandriver.com/winter/050126.htm
{
Some car manufacturers do not recommend boosting a vehicle, They
state that damage to electronic controls can occur if voltage
surges flow through the electrical system during boosting.
Instead of boosting, the auto manufacturers prefer that a fully
charged battery be installed in place of the dead one or that
the dead battery be charged. This is often impractical because
of where the vehicle is sitting so most people with a dead
battery boost their vehicle - but several important steps must
be followed to prevent damage.
}
Come 'on... is it impossible to throw in some protection
electronics?
And what about this surge?...Wimpy car electronics?
Perhaps HV spikes from the booster wire inductance can zap the
computer and other loads..
Would you buy a car that has a big sticker: "Contains sensitive
electronics..do not boost"
Transients on the vehicle battery line can be a real pain for the
designer.
Load dump, as has been discussed, is normally the result of the battery
being disconnected while the alternator is at full charge. The
alternator generates a huge electrical transient as a result of the
"increased" load impedance. This transient is seen all over the
vehicle battery harness.
Most modern alternators are "centrally suppressed", in that they
incorporate a local zener to eat the overvoltage. Hence, the maximum
load dump is clamped at the source.
Some alternators are not centrally suppressed. Then the transient is
eaten by all the boxes on the harness. (or maybe the one with the
least series inductance, from the alternator). In any event, every box
usually has to be designed to eat the entire energy slug. Number and
location of such boxes vary from vehicle to vehicle.
These transients are not for sissies. A 12V system might be 70V
through 4 ohms. A 24V system might be 150V through 0.75 ohms.
Further, the spec might require a repetitive pulse for an extended
period of time.
The designer is forced to follow one of two paths:
1) Insure all circuits can handle the overvoltage.
2) Clamp it with a TVS
Solution 1 is the simplest. However, this means all MOSFETS might be
100V (or greater) - with increased RDSon. Hence they dissipate more
power under nominal conditions. Further, this decreased efficiency is
a real bummer with the controller in a 125`C environment. This means
larger packages, more substantial thermal designs, larger footprint,
etc. Any DC/DC converters must be able to maintain its V(o) of ~5V
under this transient. That's a Vin range of 4.5V (crank) to ~100V(load
dump).
Solution 2 is sometimes better. However, the TVS clamp cannot commence
until just above jump start voltage (typically 2x nominal). Further,
the worst case tolerance on the TVS might mean one can't start clamping
until ~40V for a 12V system. So, all MOSFETS are 50V - with their
reduced efficiency. The issue worsens at 24V when the jump start is
48V (or higher) and the currents are much more substantial.
Back to OP's comment..... I s'ppose the real risk is to the jumper's
car rather than the jumpee's car. His car is seeing the most
substantial impedance transient. The jumpee benefits from the load of
the dead battery. This is speculation on my part.
Cars are no longer dominated by mechanical systems - electronics is a
very major part.
.
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