Dead Battery

[Deleted User]

The 2014 Ford Focus owners handbook gives instructions on how to jump start it, the only warning relating to transients is to leave the headlamps off. Presumably they think the rest of the electronics are well enough protected by their regulators.

TrickyWicky

Bedsit_Bob wrote: »

Once the engine is running, there should be little or no current flowing through the jump leads, as the current from the alternator would pretty much balance the current in the battery.

Er not exactly.

It's the voltage that is the deciding factor here. The alternator will generate about 14 volts which means it will push current down whatever wire will take it. Voltage is a force and when the battery has a lower voltage than the voltage of the alternator, there is a potential difference. This means that the voltage from the alternator being stronger will force current down to the battery. In the case of jump leads being attached, the same thing still applies from the donor to the flat battery car.

The reason why we don't recommend leaving jumpleads for a few minutes like some folks say you should is because once the engine is running the alternator is generating power itself to keep the engine going and recharge the battery.

Voltage and current are two different things. You don't get one without the other. Current does not flow without voltage to push it. What affects current flow is resistance.

Uniscots97

just a quick update, I got a new battery. Seems a certain car stuff franchise sold me the wrong battery in the first place (it needed a higher powered battery) they replaced it free of charge. Car now working perfectly. Thanks for everyone's help.

gilbert_and_sullivan

Been interesting re-reading some of these posts.

If you ever visit a busy car storage compound you'll find that many operate a mini bus for the yard personell with permanently connected jump leads, they start dozens of cars every day, some have been flat for months, they simply hold the leads against the relative terminal and a colleague fires the flat car up.

They simply don't have time to go through such niceties as letting it charge up for 10/30 minutes before firing up...they seldom get any bother.

However like most, i'm extremely wary of jump starting from or to any of my own cars, and keep a proper pro quality jump pack charged up for such events.



It appears we have some well qualified chaps here so i'd appreciate any comments about the following scenario.

Some cars when jump started from flat will be left ticking over for an hour or so to charge them up, now with some cars they will charge fine, others require a load to be switched on to trigger the alternator or they can be left runnign all day long and won't charge.

I know this to be true as i've done enough compound work myself over many years to have seen every combination going, though some who haven't actually worked out of compounds believe this not to be the case.

Whats actually happening here technically please?