What tools should be carried in a modern car?

:)

I think we're talking at cross purposes. My point was that I switch the donor engine off before trying to start the flat car and that it's fine to run the donor engine to charge the dead battery.

I think there's a risk with both cars of a spike as long as they are running, so if the donor isn't running there's no danger to it of a spike as the spike comes from the alternator suddenly having less load. If it's not turning in the first place then no problem.

Thanks,

Peter.

Have you ever looked at the spikes produced by a starter motor in operation? It's an inductive load switching on and off rapidly with a current of up to several hundred amps.

Which of course the vehicle's system is designed to accommodate.

Unlike jump starting...

Chris

Ah, OK. Completely pointless however, and running the risk of two non- starters...

There is no risk to the donor car at disconnection, regardless of whether it is running or not, at least according to official AA data supplied to patrols.

Chris

My BMW with boot mounted battery has lugs on the engine *precisely* for jump starting. Even tells you about them in the handbook.

The battery acts as a capacitor smoothing out the flow

&, more to the point, all the induction is in the starter, which isn't being fed by the alternator. Quite a few cars & alternators have got built in protection anyway.

Perhaps it's old enough to not have electronics that are sensitive to spikes? Or perhaps the designers took that into account?

The handbook for all recent Fords has a section on jump-starting that specifically explains the precautions that must be undertaken to avoid electronic damage.

Chris

My Prius has a jump point under the bonnet, too - although the battery is in the boot.

[...]

If the "starting" battery is flat, can you drive electrically, then bump start it?

Chris

quite a few have not

The problem we now have with the newer vehicles is the wide range of voltages used within the electronic systems .

No, if the 'starting' battery is flat, you're buggered.

Stupid bloody system, but the 'starting' battery is needed to power up the computers etc.

It's got more electronics than is the norm even today.

My point exactly. Designing any car electronics which are *likely* to be damaged by a jump start is a total nonsense.

Does this insist you use jump leads with surge protection? And I hope if it does it insists they are checked for correct operation regularly.

This is mostly lower than 12 volts so it can be regulated. The car 12 volt system can vary from approx 10-14. And the whole process of regulation will include protection from 'spikes'.

Whats the voltage of the electrical system that drives the car

Lots.

I recall something like 480v.

But that's no excuse not to use a transformer for emergency starting.

If its a DC voltage then it cant be directly transformed down it would need to be shunted through an ocilliator circuit.

A circuit that could handle that level of drawdown of current is going to be difficult to produce , probably be cheaper to have a 480 volt starter motor and a small transformer to power up the electronics

It is bloody stupid though that this problem hasnt been invisaged even

[...]

No. I'll quote verbatim from the Focus handbook:

"*To disconnect the leads*

To reduce voltage peaks on disconnection, switch on the blower and heated rear screen in the vehicle with the flat battery.

! Do not switch on the headlights in place of the heated rear screen. The peak voltage could blow the bulbs"

It's the AA that has the requirement of surge protection. It's built in to the battery box they carry, and if I remember correctly has status led's to confirm proper operation. Standard kit also includes a separate device to be used with normal jump leads, and some years ago a written instruction to patrols was given that it was a disciplinary offence to fail to use it. This was done after a number of events when damage was caused to customer's vehicles after jump starting.

Chris

In message , Chris Whelan writes

The AA used a power unit to start mine on the one occasion I needed it.