Damage Jump Starting

Well. Heres a sad story of what idiot people do in idiot cars.

Was jump starting a classmates BMW x5 with my tacoma. Well. The idiot decided that he was going to go try and start it while the cables are connected, which is fine. But what he did next was scary. He floored his x5 once he got it started. I mean. He redlined his engine. Is there any chance he damaged anything vital electronics wise in my truck? Would any damage or overload appear immediately?

 

i dont think so? he would just charge your battery i believe. not too sure though

 

this

 

isnt that what your supposed to do?

 

Yo Dude you should be good as long as you connected the cables correctly, you didn't do neg to neg pos to pos. next time tell him screw call AAA

 

Voltage is not proportional to rpm, and the alternator is regulated.

Nothing to worry about here.

 

X2.... + you worry too much.

 

no i did positive to positve, negative to grounding point on his engine block.

I know nothing is amiss..yet... although it did start weird this morning...

and voltage is regulated, thank god...thats good to hear......ive got him all worried. i told him my truck is dead and that i called the cops...kid is crapping his pants right now...

i just always thought that current can backflow? or is this only when connecting pos to pos and neg to neg?

and if current ever does backflow, just for future reference, wouldnt a blown fuse be the first indication? or any problems with electricals popping up immediately? sorry just new to these trucks..old car had a surge protecting battery, this one has a standard one...

 

Lucky for you in this case, he's only hurting his engine by revving it up when it first starts. Next time shut off your motor during charge or don't do it at all.

 

So just to be clear,and to learn... an alternators output is NOT proportional to the RPM of the engine? i thought that is why you give the GOOD car in a jump a little gas? to increase alternator output?

 

Why did you do that ^^^ ?

Your truck should be fine. Usually when you jump start a car you do it with the cables still connected.

 

cause im not a huge fan of this kid.. so i figured id scare him a bit...the only reason i jumped him is cause i figured someone who doesnt like me would do the same for me.. IE.. karma.....

hopefully no electrical gremlins will pop up now.. if i wanted to live with those i would have bought a jeep grand cherokee....

 

It's a little complicated, but at low RPM an alternator's output can 'droop' even with maximum field current. The field current is how the regulator or computer controls the alternator output.

So you bump the RPM up a bit above idle, and that lets the alternator put out what the regulator/computer is requesting .

The regulator/computer is mostly just looking to keep the system voltage at the optimum level. It varies a little with temperature (higher voltage in cold weather). Usually you don't see normal operating voltages outside of 13.5 to 14.5 volts.

So, the other car would have tried to boost its alternator output until it saw 13.5-14.0 volts, which it would have probably seen just from your jumper cables being connected anyway.

tl;dr = Your truck is fine.

 

I second this gentleman's post.

 

Ah JKD good post. Now i understand.

Next time I'll just hand him the phone number for AAA and tell him to screw off. Then proceed to create tire smoke.

 

X1

 

Tell him he should FLOOR IT for 5 minutes.



Steve