While we're on this subject - batteries too flat to start the car - I've been very nervous, for years, about jump starting any vehicle with electronic systems so I have tended just not to do it, preferring to either charge up the car's existing battery or fit another know good battery.
However, over the last couple of years I've rather changed my mind. The "Family Fleet" is now becoming quite geriatric - 2007 Astra, 2008 Jazz, 2010 Panda and 2012 Punto (The ibiza and Rio are both younger and I haven't yet had to jump them). Although older all these cars have electronic control mechanisms so I've tended not to jump them but they've all experienced undercharged battery problems within the last couple of years. So I've been getting "brave" and started jumping them when needed. The Jazz in particular has had to be jumped several times due to grandchildren twice leaving the interior light on overnight and then, of late, the battery failing to hold charge after several days inaction. The Astra too must have been jumped three? times before we gave up and bought a new battery for it. The Punto was jumped about 3 times too but over a period of just a few days because it started playing up at the beginning of a long holiday weekend and it took several days for Kenny (Harrison Motors) to get a new one for us, so I was jumping it to keep my boy mobile - it would start when hot and maybe a couple of hours after stopping, but wouldn't start "cold" after an overnight. It just would go over comp slowly a few revs and then die. Our own Panda often sits for days without running and sometimes would just struggle into life, sometimes not. It really needs a new battery, and I'll have to definitely buy one before this winter starts, but in the meantime I've done a "recond" charge with the CTEK and it seems to be "living" now the weather is warmer (I also stick the CTEK on it a couple of times a week.)
In addition to the above, in the last year or so, I've jumped my pal's Jag "S" type, two of the four Jazz's which "live" in the street and the HRV Honda which lives at the end of the road. I've not had a single problem with any of them and these are all cars with who's owners I have almost daily contact so I'm sure I'd know about it if they'd had problems.
So, how am I "getting away" with this seemingly heinous crime? Well, for starters, I've done a lot of reading up on it and chatting to people I know in garages. The main thing which seems to be the villain is voltage spikes which exceed the designed max voltage an electronic component is designed to work with. So, I reasoned, if you never connect anything which can induce a higher than acceptable voltage then you're probably going to be "safe". The other thing you have to watch out for is whether there is a Battery Condition Monitor/sensor present. This is often a wee "gismo", typically mounted on the negative terminal clamp of the battery but can be mounted elsewhere on the earth lead. As a general rule, if it's got stop/start then it's probably got a BCM. If it has one of these then don't connect the neg jump lead clamp directly to the neg battery terminal, go for the point where the earth lead is connected to the vehicle body/chassis - that way the sensor stays included in circuit as intended and you're unlikely to "blow" anything. It's good practice not to connect the neg jump lead direct to the battery terminal anyway for reasons of safety (avoiding sparks near the battery). I believe a very few (probably Luxury brands) have these sensors on the pos terminal clamp too so be careful where you put that pos jump lead if it has one - I've not so far came across one on a pos terminal clamp though.
Now, how to avoid - or at least minimize - the possibility of those system wrecking voltage spikes (and their attendant high currents which is actually what burns stuff out). First off I would avoid jumping car to car if at all possible. I'll talk a little more about that in a minute. A jump pack or slave battery, intelligently used, is your best bet. The vehicle is fitted with a 12 volt battery and it's charging system will raise system voltage to around 14, maybe 15 volts during charging. I believe some modern overrun (smart charge) systems can peak out at 18 volts? So the electronic components which are receiving battery voltage (there are many systems, like some sensors, which receive regulated supply, typically 5 volts) are "hardened" to work with voltages, and their resultant currents, of that order of magnitude. So, the simple act of connecting up a 12 volt battery is very unlikely to blow anything. I have a nice big (77ah) battery which came out of my old Cordoba tdi and what I like to do is connect it up with the jump leads and then let it sit for a few minutes just to put a bit of surface charge on the car's discharged battery plates thus somewhat equalizing the difference before turning the ignition key. When/if the engine starts then DO NOT immediately disconnect the jump leads. With the jump battery connected the alternator regulator (ECU) will be "seeing" a goodly source (battery) voltage - at least 11 volts probably - so it will start to deliver a "normal" rate of charge to the battery leads. - Alternators deliver a very good rate of charge at tickover but you might want to just raise the revs to 1500/2000 rpm to maximize charging, no need to rev the "nuts" off it though - Leave the jump/slave battery connected for a few minutes while this initial charging gets done - I tend to go for about 5 minutes - The slave battery coupled with the alternator output will put a very good surface charge on the "flat" battery's plates. Now when you disconnect the slave battery the alternator ECU (regulator pack, or whatever you want to call it) will continue to "see" this charge state of the car's own battery and continue to deliver a "normal" rate of charge to the car's battery. This "surface charge" is caused by chemical reaction on the very surface of the plates and it will take time before it goes "deeper" into the plates which is why you'll need to either drive around for 20 minutes or so or put a charger on the battery (preferred anyway) when you get home so that the charge can "saturate" the plates. Fail to do this, ie. turn the engine off after just a few minutes running, and you'll find there's not yet enough charge in the battery to do a restart because the surface charge on the plates will rapidly disperse.
Using a jump pack, as far as the car's electronics is concerned, is very similar to me using that big battery, in that when the engine starts the alternator ecu/regulator "sees" the jump pack voltage so it doesn't do anything "stupid" as it delivers charge to it's flat battery. I would probably leave the jump connected for a few minutes after starting to allow surface charge to build up on the flat battery's plates thus minimizing voltage drop when the jump pack is disconnected.
One of the worst things you can do is connect up the batteries - remember neg lead last - (typically directly terminal to terminal because many people don't know about sparks being a problem or have any idea they probably have a BCM) and immediately go for a start. As soon as the engine fires up they then disconnect the jump leads, often while someone else keeps the engine reving "just in case it dies". This can be the point at which an electronic component somewhere shouts "uncle" (60's language) and shuffles off to a higher existence (dies). Why? You've got a very discharged battery in the car to which you've connected your slave source. Then you start the engine which spins up the alternator so it's potential to produce charge is entirely at the, instantaneous, "mercy" of the regulator (ECU controlling the alternator field). OK, because you've got the slave battery connected the starter turns, there's enough electricity for the ignition and fueling systems to work and the "beast" awakens! Great. So, within moments, you - or another helpful person, whips the jump leads off thus killing the voltage from the slave which the alternator ECU has been happily using as a reference. Because you didn't allow time for a surface charge to build up on the car's battery plates, the only point of reference the alternator's ECU now has is the very low voltage of the car's flat battery so one minute it's "seeing" the slave voltage (say 12 volts) and then, virtually instantaneously as the jump lead is disconnected, it "sees" whatever the car's flat battery is managing to retain (maybe 5 volts? who knows. The lower this voltage the greater the problem) So it immediately raises the field excitation to max which whacks the alternator output through the roof, helped no end by the revving of the engine, and, if you're unlucky, exceeds the max designed voltage limits for some of the other electronics (Spike) whereupon they will, virtually instantaneously, "clock out"!
Jumping vehicle to vehicle? I really just don't do it but if you must then disconnect the vehicle leads on the "good" battery so it's isolated from the car and use it just like a stand alone slave. Another safer way to do it is to connect up the jump leads between the two cars - paying attention to safety and BCM considerations - remove the ignition key from the car with the flat battery then start up the car with the "good" battery and let it run for maybe 10 minutes. This will put a "useable" charge onto the plates of the flat battery - unless the "flat" battery is a hopeless case. Now disconnect the jump leads and try starting the "flat" car. Unless the battery is totally "done" it'll probably start and because the slave car is not now part of the system there's really no risk of damage to either vehicle.
What I would absolutely not do on a modern vehicle, is connect up the jump leads car to car, run the "good" car's engine and start up the "poorly" car then disconnect the jump leads with both cars still running. In fact I wouldn't do anything which involved both cars having their ignition keys in the run position both at the same time. I'm not all that enthusiastic about just using the good car to put a charge into the poorly car - as I've mentioned above. I just don't like to have two car's electronics having access to each other!