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... it is generally recommended not to leave the engine of the donor car running before 'briding' the two cars.
I don't see the reason for this and I don't do it like that in practice. I feel safer when the donor car's engine is running so that I don't have to shut down two cars.
Is there a technical reason why the engine of the donor car should not run?
Thank you very much in advance.
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No, quite the opposite. When you try to start a second car from the donor's battery it will draw maximum current. And you can drain the battery within just a few seconds if not careful. So always start the engine of the donor car first, so the alternator helps to keep the battery protected. You really do not want two cars with flat batteries at the same time.
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Richard MacCutchan wrote: And you can drain the battery within just a few seconds if not careful.
Eeeeh ... My battery has a capacity of 65 Ah. To drain it in one hour, you would have to draw 65A. To drain it in 3 seconds, 1/1200 hour, you would have to draw 78,000A. At 12V, that is 936 kW - almost a megawatt.
Of course that wouldn't be possible. Long ago, I worked with a company making minicomputers. They bought 5A/150A power supplies from a subcontractor claiming that they were shortcut safe. So they tested if it was true: Held a large screwdriver across the poles. The power supply survived, but not the screwdriver: It melted, and they had to cut it away from the poles with an angle grinder. I can't imagine what 78,000A would do, and I wouldn't want to stand close when it is tested. (Actually, the thought of 78,000A scares me a lot more than the thought of 78,000 V!)
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I am not an electrical engineer, but my comments are based on experience rather than theory.
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Just a thought...perhaps just problem with historical problems and exactly what problems might occur?
Certainly at one point in time with a gas engine in a car when attempting to jump it from another gas engine car if you did it incorrectly you could at a minimum damage at least the electrical system of one car.
That did include destroying a battery which of course would seem like it had 'drained' in seconds. So perhaps that is what was seen?
I would suppose that it is possible that, at least with older EV batteries, that it might have been possible to destroy them also.
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If I may add to what Richard was saying:
Quote: So always start the engine of the donor car first,
... but after connecting the two cars as to avoid unnecessary sparking. Before starting the engine, donor car has about 12.5-13 V while after starting the engine the voltage is 14-14.5 V. Also safe order of connecting is connect positive first, disconnect positive last.
The step by step procedure is:
- connect positive lead on donor
- connect positive lead on receiver
- connect negative lead on receiver
- connect negative lead on donor
- start donor
- rev donor up to 1200-1500
- try to start receiver
- disconnect receiver negative
- disconnect donor negative
- disconnect donor positive
- disconnect receiver positive
If things don't go well, flag a cab 
Mircea
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But was it the reason not to start donor in advance?
[Edit]
I'm aware about the sequence connect plus first and ground later. That is only a sequence to avoid unintentional short circuits, not really a technical issue
modified 30-Apr-23 14:46pm.
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Yes, when you connect the second lead (the negative), the donor battery has 12-12.5 V and the receiver battery has much less (typical 9-11V). You get 2 to 3 V difference which makes for a bit of sparking but not that much. If the donor engine would be running, the alternator would bring the voltage up to 14-14.5 V so you would have a 4-5 V spark. All this at currents in excess of 20-30A. There is simply no need for that kind of stress on the jumping clamps and battery posts.
The order I described also takes into account all kind of safety factors, like the remote chance of touching the car body with the positive lead and stuff like that.
Many eons ago I had a summer job in car shop and had to learn this kind of things to avoid getting smacked over the head 
Mircea
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Interesting. I've always been told (and according to the ever accurate internet):
1) Connect positive cable on dead car.
2) Connect positive cable on donor car.
3) Connect negative cable on donor car.
4) Connect cable to grounded (unpainted) metal on dead car.
The final connection is where it is most likely to spark and a dead battery can emit flammable fumes so you don't want sparks there. That's why you don't make the final connection directly to the dead battery.
Disconnect in opposite order.
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In my recipe, the final connection (and the spark) is the negative on donor. Presumably a good battery is not emitting that many fumes and all should be good. Personally I don't like looking for unpainted metal: you never know how good the contact is. I prefer battery posts or jump-starting posts for cars that have the battery in the trunk.
Mircea
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I had a truck once whose battery was nearly inaccessible. It had separate jump posts. Fortunately I only needed them once, but it sure was nice they existed then.
I understand why the donor is turned off until ready to jump start but personally I've never done it that way. I've always just left the donor running. And, I've never understood why all the advice is to do it in the order I gave. The net and manuals just give that order with no explanation. I've just blindly followed the expert's advise. Perhaps I'd better stay away from those TikTok challenges.
I have run into cases where the unpainted connection wasn't well enough grounded to work - quite frustrating. In all actuality, when I've had to jump a dead car it was usually outside, -20F, and the wind was howling like a banshee at Mach 7. Any fumes from that dead battery are a half mile away from where I needed to worry them. Under those conditions expediency, safe or not, meant directly connecting to the battery anyway.
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with relative new cars don't need to search for them, there should be a bold / stud in the near of the battery that is to be used as ground.
M.D.V. 
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
Rating helpful answers is nice, but saying thanks can be even nicer.
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Connect the negative first to get both vehicles to the same ground level. Disconnect negative last for the same reason.
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obermd wrote: Connect the negative first to get both vehicles to the same ground level.
I used to think the same but all the manual instructions of chargers and cables I have seen say to connect first the "+"
M.D.V.
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
Rating helpful answers is nice, but saying thanks can be even nicer.
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Unfortunately I'm not very good explaining things but I'll give it a try. I apologize before hand if I fail.
Here is my list of steps with explanations why it has to be done in that order.
1. connect positive lead on donor.
The only danger is if the other end of the jumper wire touches the body of the donor car. To avoid that, keep both ends of the positive (red) jumper in your hands.
2. connect positive lead on receiver
Nothing can go wrong. The receiver car body is not connected to the donor so there is no chance of a spark. You are by the receiver car now so you can proceed to step 3.
3. connect negative lead on receiver
Still nothing can go wrong. The other end of the negative (black) jumper is not connected.
4. connect negative lead on donor
Some sparking may/will occur. However you are at the donor car and the good battery will not produce much fumes.
5. start donor
You don't want to discharge the donor battery. If the engine is running the donor alternator will charge both batteries.
6. rev donor up to 1200-1500
Again you want as much help as you can get from the alternator. Revving up the donor engine will give you more juice. If receiver is very low, you can wait a few minutes at this stage to minimally charge the receiver battery. In cold whether, the charging process will also warm a bit the receiver battery and increase it's capacity.
7. try to start receiver
Don't do long tries. Pause 30 sec - 1 min between tries. That gives times to donor alternator to charge the receiver battery.
Disconnect in reverse order for the same reasons.
Mircea
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If you have a new car, read your owners manual first.
And if asked by dealership “did you use jumper cables” either as donor or receiver, act like you are hard of hearing so you do not void your warranty by admitting it!
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I have 2 cars and 1 truck. I keep rechargeable lithium jump boxes in each. They keep enough charge to work for quite awhile (I have not had one fail for lack of keeping a charge, in fact I have the same one work twice without a recharge). They each have small jumper cables which are very safe to attach. Once attached, power it on and then start your car. Has worked for all 3 vehicles in the 3+ years I have had them. If they don't then you can look for a donor car. Not exactly cheap, but worth it to not need a donor car. We have hot summers but does not an effect on them). Don't know about cold weather. I do know you want to charge them in ambient conditions. They charge with USB cable adapter in AC outlet. They could probably be charged from inside the car with USB port to 12VDC adapter.
NEXPOW Car Jump Starter, 1500A Peak
"A little time, a little trouble, your better day"
Badfinger
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100% agree with the jump pack. I have a RAV4 Hybrid with some jinky software that drained the 12V when off in the early days (OTA updates fixed it eventually). The jump pack keeps me from having to find a donor when it randomly decides to stop. I've even jumped a full size pickup from it.
Hogan
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last year I had 2 batteries (1 for car and 1 for truck) go bad the same day. what are the odds. They would not hold a charge. Jump packs came to rescue. I bought them both at the same time and still had some refund credits. (3 year warranty). Turns out they came from same batch of batteries from manufacturer. Some sort of factory defect no doubt.
"A little time, a little trouble, your better day"
Badfinger
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No one wants to hold the end of the cable while it is reeled up by the engine cooler fan of the donor car.
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There is no perfectly 'right' answer, and it depends on the state and type of the two car batteries, and the quality/capability of the alternator on the 'better' vehicle.
Historically, the main issue was that the alternator diodes would 'blow' during the jump-start if the donor car was running. The surge current into the dead car's starter was too high, especially with good jump-lead connections.
You also need the donor car and it's battery to have enough capacity when the two vehicles are initially connected (both vehicles off, mediocre donor trying to feed large dead battery could result in two nearly dead batteries, sigh).
Then there a lot of 'safety' choices as to which leads to connect securely first, and what happens when a lead slips, hence it's normal to connect the two chassis last (little chance of failure and short circuit at this stage, compared to slipping of the power terminal).
That said, on larger modern vehicles the alternator diodes now have good capacity (see articles/ videos [e.g. merlin's garage] on using them as welders!) so blowing the diodes on those beasts is less likely.
Finally, just because there's a lovely big terminal next to the battery's negative, doesn't mean it is actually a secondary chassis connection. On my car it's the common point for the positive connections - Oops.
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I miss manual transmissions:
1) Get in the car, turn the key to On.
2) Push in the clutch, put the car in gear.
2) Have friends push your car.
3) Let clutch out to engage transmission, engine turns over and starts.
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That only works if the car DOES HAVE a clutch... nowadays is not granted anymore
M.D.V. 
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
Rating helpful answers is nice, but saying thanks can be even nicer.
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On manual transmission cars?
Well, I can easily see that you could replace the mechanical speed stick with, say, a pushbutton panel giving input to some microprocessor managed clutch and gearbox movements. I just haven't seen it yet.
I wouldn't want it, though! I've been driving so much on (very) slippery ice that I demand a mechanism that sets the wheels free rolling so I can regain control over the car when it has lost the grip. (If your automatically controlled shift has an option for that, then you do have a clutch, perfectly usable for the purpose discussed here. It just has a user interface different from a pedal.)
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