Jump starting - a new one on me

[Steve Medlock]

My elderly friend rang to say his car wouldn't start and that he had missed hospital and vaccination appointments. I told him I thought it was just a flat battery and I would come over. I took tools and a fully charged HD battery. The starter was starting to spin but not engaging so we put jump leads across and ran my engine for a few minutes but no joy. So I removed his battery and connected my fresh one but the starter still wouldn't engage so we though maybe stuck solenoid perhaps. As the starter motor is pretty much inaccessible we drove down to the little local garage and we asked them to have a look at it when they were free.

Anyway my friend rang this morning and said they managed to jump start it but then fitted a new battery for him. When I asked how they had managed to jump it, they had told him that you have to put the headlights on when jump starting as this wakes up the alternator and that hardly anyone knows to do this - a new one on me I have to say.

Cynical me thinks they tapped the starter with a hammer and then fitted a new battery?

 

[t-tony]

I'm with you on that one Steve. Sounds like BS to me.

Tony.

 

[Duncodin]

My elderly friend rang to say his car wouldn't start and that he had missed hospital and vaccination appointments. I told him I thought it was just a flat battery and I would come over. I took tools and a fully charged HD battery. The starter was starting to spin but not engaging so we put jump leads across and ran my engine for a few minutes but no joy. So I removed his battery and connected my fresh one but the starter still wouldn't engage so we though maybe stuck solenoid perhaps. As the starter motor is pretty much inaccessible we drove down to the little local garage and we asked them to have a look at it when they were free.

Anyway my friend rang this morning and said they managed to jump start it but then fitted a new battery for him. When I asked how they had managed to jump it, they had told him that you have to put the headlights on when jump starting as this wakes up the alternator and that hardly anyone knows to do this - a new one on me I have to say.

Cynical me thinks they tapped the starter with a hammer and then fitted a new battery?

Yep. Headlights on . . . And you have to stick pencils up your nose and say 'wibble'.

 

[Synclare]

My understanding is that if jump starting a car with flat battery, once started turn on a high current device such as headlights before disconnecting the slave battery. This prevents a current spike that can damage electronics.

 

[the Nefyn cat]

You need to "wake up" the alternator to start a car? I've just checked and it's still not April 1st.

 

[Duncodin]

My understanding is that if jump starting a car with flat battery, once started turn on a high current device such as headlights before disconnecting the slave battery. This prevents a current spike that can damage electronics.

I never heard of that one. Whether there's one slave battery or 10 slave batteries. Connected in parallel. The voltage is still only 12v. Whereas once the engine is running the alternator should be putting out around 14v so disconnecting 12v batteries shouldn't cause a voltage spike.

 

[ChrisD]

I never heard of that one. Whether there's one slave battery or 10 slave batteries. Connected in parallel. The voltage is still only 12v. Whereas once the engine is running the alternator should be putting out around 14v so disconnecting 12v batteries shouldn't cause a voltage spike.

The effect is commonly known as 'load dump'. You can read all about it here: https://en.wikipedia.org/wiki/Load_dump.

 

[Duncodin]

The effect is commonly known as 'load dump'. You can read all about it here: https://en.wikipedia.org/wiki/Load_dump.

I meant disconnecting a jump start battery while leaving the original battery connected to the system. The original 'flat' battery is, I would have thought, a big enough load that the alternator wouldn't mind.

 

[Synclare]

Yes but once you disconnect the 12v slave battery (or 10 of them in parallel) the flat battery that replaces them is not 12v

I never heard of that one. Whether there's one slave battery or 10 slave batteries. Connected in parallel. The voltage is still only 12v. Whereas once the engine is running the alternator should be putting out around 14v so disconnecting 12v batteries shouldn't cause a voltage spike.

 

[Duncodin]

Yes but once you disconnect the 12v slave battery (or 10 of them in parallel) the flat battery that replaces them is not 12v

Some would argue that when a 12v battery goes flat it isn't the volts that have done a runner - it's the amps.

You often hear people saying "My battery can't be flat because it shows 12v on a volt meter.

A volt meter is almost no load so, with no load, an almost flat battery will still show 12v on a volt meter (2 volts per cell). Only when you put a load onto the flat battery will the volts drop as it struggles to supply the amps being pulled by the load.

But in the case of a car that's just been jump started there's no real load on the battery but the alternator is churning out 14v and the flat battery is acting as a capacitor soaking up any spikes

 

[Duncodin]

My understanding is that if jump starting a car with flat battery, once started turn on a high current device such as headlights before disconnecting the slave battery. This prevents a current spike that can damage electronics.

Well. That's really Gasted my Flabber.

 

[the Nefyn cat]

Well. That's really Gasted my Flabber.

View attachment 323957

Surely that bit refers to after the engine has been started, whereas the original post seemed to suggest that the load should be applied before trying to jump it. But I may have mis-read it.
It's a bu**er getting older, hope it doesn't happen to me.

 

[Duncodin]

Surely that bit refers to after the engine has been started, whereas the original post seemed to suggest that the load should be applied before trying to jump it. But I may have mis-read it.
It's a bu**er getting older, hope it doesn't happen to me.

You are correct. My first response was to the statement about applying load "before" jump starting.

But then synclare suggested applying load "after" jump starting and I dared to disagree with that too. Incorrectly it seems.

 

[Richard29]

I dont think turning the lights on wakes up the starter motor. I always assumed turning the key did that.