Jumping a car

colino

lee that is exactly the reason why you connect directly to a fresh donor battery and via an alternate earth on the casualty vehicle. With a flat battery it is acting as a giant resistor consuming power unnecessarily as you try to start with a jump. Keep on plugging away though and believing that the hydrogen produced charging a car battery will be fooled by moving the jump lead a foot away.

chrisw

Lee is correct. Moving the leads closer to or further away from the flat battery will make no difference at all as to how it acts in the circuit. It's just like using a longer jump lead. The hydrogen isn't 'fooled' it's still there but out of range of a potential spark as it's lighter than air.

EdGasket

BeenThroughItAll wrote: »

For every example of a damaged ECU I raise you hundreds of cars successfully jump started every day.

Sure but do you want to play Russian roulette with your cars when you can quite safely charge the battery? The OP's girlfriend has waited a couple of weeks already so whats a few hours more matter to charge the battery to be 100% sure of no damage?

gilbert_and_sullivan

If you decide to jump start it, and the chances of blowing something is truly minute*, then make sure the doors of the car with the flat battery are open or you have the keys in your hand when you connect up, some models, notably Fords, are well known for triggering the central locking shut when power is connected.

*i've worked out of hundreds of car compounds, those blokes haven't got time to faff about, the Transit 'Yardy' mini bus will have perma fit very heavy jump leads, one yardy holds 'em against the flat batt terminals another yardy start the engine, this process is repeated dozens of times every day of the year.

Strider590

The ECU damage is not caused by jump starting, it's caused by connecting the leads wrongly.

12v in parallel with 12v is still 12v, it simply doubles the capacity.
However the current draw on starting could damage the alternator of the "donor", which is going to be rated at around 90Amp, where a starter may draw over 100Amp.
Whether it draws that from the battery or the alternator will depend on the internal resistance of each.

RMS2

Strider590 wrote: »

.
However the current draw on starting could damage the alternator of the "donor", which is going to be rated at around 90Amp, where a starter may draw over 100Amp.

The 'donor' will be fine, especially if you remember to start it before turning over the other car.

Strider590

RMS2 wrote: »

The 'donor' will be fine, especially if you remember to start it before turning over the other car.

It won't be fine if the other car has an underlying serious electrical fault, something people seem to forget when they just assume it needs the "magic jumpstart".

EdGasket

Strider590 wrote: »

The ECU damage is not caused by jump starting, it's caused by connecting the leads wrongly.

12v in parallel with 12v is still 12v, it simply doubles the capacity.
However the current draw on starting could damage the alternator of the "donor", which is going to be rated at around 90Amp, where a starter may draw over 100Amp.
Whether it draws that from the battery or the alternator will depend on the internal resistance of each.

That is totally wrong; you have no idea Strider; sorry. The damage to ecu and other modules is caused by voltage spikes (can briefly exceed 1Kv) which occur when you connect up the leads or use start motor on jumped car due to the inductance in the circuit, particularly but not limited to, the alternator windings. See here:

http://electronics.stackexchange.com/questions/24830/why-is-there-risk-of-overvoltage-when-jump-starting-a-vehicle

Strider590

EdGasket wrote: »

That is totally wrong; you have no idea Strider; sorry. The damage to ecu and other modules is caused by voltage spikes (can briefly exceed 1Kv) which occur when you connect up the leads or use start motor on jumped car due to the inductance in the circuit, particularly but not limited to, the alternator windings. See here:

http://electronics.stackexchange.com/questions/24830/why-is-there-risk-of-overvoltage-when-jump-starting-a-vehicle

Right...... So how is that different to not having another vehicle connected?

All your doing is putting two power supplies in parallel, the danger comes from the current passing through those leads, from battery to battery, particularly if the dead battery is actually damaged and reading half or less of the donors battery voltage, under which circumstances I can see what that guy is talking about, but under those circumstance you shouldn't be trying to jump start it.

This is why I totally disagree with amateurs jump starting vehicles, it's incredibly dangerous, people only do it because they've seen the recovery services use it as a "get you home" and think it looks simple and therefore perfectly safe.

I won't even do it myself, because i've been on the receiving end of a car battery explosion. I don't muck about with these things, the energy potential is just too high, one wrong move and it'll get very messy very quickly, particularly if something shorts across the terminals, it'll probably weld itself in place and then you'd better hope you can run.

Murielson

Anyone remember the initial question from the OP??

Some thoughts:

Is jumping the car your only option? Does she have homestart or similar where she can call someone out, get vehicle checked and confirmation of whether battery is okay (ie, just needs charging) or unserviceable?

If there is time can she get family, neighbour or someone to remove and put on charge for a while? As someone has mentioned above a bump start is another option where you get vehicle moving, in gear with clutch depressed and then life clutch to get engine turning over and started without need for the battery. Vehicle could then be run for a while to allow the alternator to charge the battery (long journey would be ideal).

Need to check for underlying problems as to why battery too low to turn engine. May need replacing or electrolyte topping up, alternator may not be charging correctly, terminals may need cleaning or connection to chassis checking.

As original post was 4 days ago maybe you have even resolved the issue by now?