All over Facebook and this forum. In the "old days" it was not an issue. But with modern vehicles (NOT just Crossfires), there are new concerns. We now are loaded with sensitive microprocessor-loaded modules all over the car. The power input filters on these modules do all they can to protect from voltage spikes but they can only do so much.

Voltage spikes? Yes. Starter motors in particular present inductive loads that cause voltage spikes. Your battery makes a very good capacitor to filter these spikes out. In the case of a 'dead' battery, it's capacitive reactance is diminished to some (or a great) extent. By jumping the car, you have less filtering, as the cables to the other car may not make good contact, meaning the other car's battery is not effectively filtering for you, either. Additionally, if the other car is running and your car does start, the two alternators may fight one another a bit - and again, your diminished battery is not up to filtering well until it is charged - and in the case of a dying battery, it's not going to filter well anyway - time for a new battery. THIS is why I preach replacing batteries at 48 months.

So, to sum up, charging the battery separately is safest. Aside from that, using a "booster battery" is the next best option. The third best option is jumping from another car with that cars' engine not running.

I would additionally point out, that failed SKREEMS, ABS and Seatbelt modules may, on some occasions, be caused by such voltage spikes. Again, this is why I replace batteries at 48 months. Batteries are cheaper than modules, diagnostics and being stranded.

That is a logical fallacy. It SOUNDS reasonable, until YOU munch a seatbelt module or SKREEM. I used a booster battery on my car once - and I don't mind telling you, I was a bit worried.

Read thru old posts on Facebook and here on the forum. From time to time, you will read how someone explains; "Car was fine, but I left my lights on. We got a guy to jump us, and car would not run. Turned out, we need a SKREEM* now." That is NOT a coincidence, the SKREEM was destroyed by the mechanism I described above.

*Its not always the SKREEM, in my case, I munched a SeatBelt module in this way.

It wasn't, in 1970.

Part of the big problem with cars these days, is they have been turned into rolling electronic assemblies of interacting modules and processors. What has NOT happened is the owner/driver keeping up with what has changed. Yesterdays' rules no longer apply. When they first started putting microprocessor-laden two-way radios in cars back in the 1980s, people in my field saw it coming: The automotive electrical environment is hostile, the spikes on the B+ line in your car can rise to 60 volts and more when the starter drops out, if the solenoid is sluggish or resistive. That's like 600 volts spiking thru your home wiring (five times what is designed for, that is). How do you think your home electronics would like that? Anyway, we saw a lot of failures in our products until engineering saw the light.

Automotive electronics is hardened, insofar as input power filtering - but it can only do so much.

Automotive aftermarket electronics has been a large part of my career since 1982. THAT is why I am so protective and careful with my car, truck and motorcycle when it comes to electrical issues.
This is how I earned a living over the years - and I think it's why I have had fewer electrical issues with my vehicles than others, I know what to look for cause I was the guy people came to when things went to hell. (I still am, but in a different aspect, as Im' not doing auto electronics anymore, as of July of this year.)