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Hyundai coupe 2006.

I tried to recharge a deeply dead battery by attaching jump wires from donor car directly to terminals of isolated battery(not in car).

I kept the donor car constant at 4000rpm for about 8 minutes, the latter few minutes in which I could smell something burning.

Went over to the hood and saw coolant leaking out of radiator cap and spilling out of overflow tank aswell and falling onto wires. Didn’t look at temp gauge yet.

Switched everything off and then on again after 5 minutes and noticed temp gauge was well past middle but not in red. However it could very well have been in red from earlier.

What happened exactly? Was it that keeping it on 4000 rpm for 5 minutes caused overheating and this had nothing to do with the connected battery, or did battery have something to do with it?

How did the coolant get past the radiator cap, can overheating damage to the cap and make it leak?

I suppose I just need to replace coolant, coolant cap and possibly burnt wires but other than that do you think any engine damage occurred?

I also saw steam coming up from where the coolant was dropping(onto wires). Would this mean the wires were burning? I didn’t want to go to close at the time.

On the bright side battery went from 2.5 volts to 11.66 volts In the 8 minutes. Does that mean it has been recharging or is capable of being recharged or is it still dead as was before?

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    1800 to 2000 rpm would have been sufficient. – Solar Mike Sep 25 at 19:14
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    Were you charging the battery or stress testing the engine's cooling system? – RaphaelP Sep 25 at 19:24
  • Battery or charging has nothing to do with overheating. – Moab Sep 25 at 19:26
  • @RaphaelP charging battery. I was trying to charge quicker by revving higher. – James Wilson Sep 25 at 19:51
  • You just found out that doing it cheap is often more expensive than doing it right the first time. With a bit of luck the engine/wires/electronics isn't damaged. An ordinary charger would have saved you this hassle. Should you get a charger, get a good one and not one of those 19.99$ battery-cookers. Bonus points: It could "revive", within limits, the battery and you are able to charge a battery while it is still in the car. – Martin Sep 26 at 8:46
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What happened exactly? Was it that keeping it on 4000 rpm for 5 minutes caused overheating and this had nothing to do with the connected battery, or did battery have something to do with it?

You were holding the engine at (relatively) high RPM for a sustained period of time with essentially no air flow over the radiator. That being the case it was going to struggle to get rid of the generated heat and therefore it overheated. The connected battery likely had nothing to do with it.

I also saw steam coming up from where the coolant was dropping(onto wires). Would this mean the wires were burning? I didn’t want to go to close at the time.

It's not clear exactly what the wires are you refer to - but the steam could be a result of the high temperature coolant no longer being under pressure and therefore boiling away, it's possible the wires were also hot, particularly if the wires were the ones running to the other battery and thus carrying substantial current.

On the bright side battery went from 2.5 volts to 11.66 volts In the 8 minutes. Does that mean it has been recharging or is capable of being recharged or is it still dead as was before?

Sounds like it was charging - but this is a poor way to charge near dead batteries. You aren't replicating the proper charging curve of a decent charger.

I was trying to charge quicker by revving higher.

This will have helped provide more charge (alternators don't often kick out much in the way of charging current at idle) but it's not a great way to charge and puts the charging car at substantial risk of damage. Buy a proper charger, it's a lot cheaper in the long run.

the latter few minutes in which I could smell something burning

Here's a free tip - if you smell burning don't ignore it!

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Your car has an electric fan at the radiator and the fan didn't engage when it had to.

Or it didn't engage its higher speed when it had to.

The result: your engine mildly overheated.

What you have to check:

  1. Why the fan didn't start? Loose wirings, burnt fuse, dead relay(s), whatever?
  2. Signs of oil or coolant leaks.
  3. Signs of oil in the coolant or coolant in the oil.
  4. Changes in engine performance and temperature.
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