To the best of my knowledge that's exactly right chris. The big thing is that the battery sensor that's on our negative leads needs to remain in the circuit so it can "learn" what's going on with the state of charge of the battery.
I've been very interested in this. I have 2 "smart" chargers, a cheapie from Lidl and an all singing all dancing expensive one from CTEK. I decided to contact CTEK for their advice and got a very informative reply back which I wrote up here:
https://www.fiatforum.com/leisure-l...-stop-start-vehicles.html?483890=#post4564452
The long and the short of it seems to be that the manufacturers are worried you might cause damage to electronic components by connecting a charger to the vehicle whilst the battery is still in circuit with the vehicle so they virtually all recommend disconnecting the battery negative lead - as do FIAT (which is what chris is saying above.) When you dig more deeply into this it seems to be a problem with cheaper, especially transformer type, battery chargers. What does the damage is that chargers of this type can't iron out surge spikes from the mains and if one of these occurs during charging it can "spike" the car and damage electronic components. Newer electronic chargers, which are typically smaller and lighter than the older type because they don't have a big transformer inside, employ much more sophisticated control circuitry and so virtually eliminate this problem.
CTEK's recommendation is that charging with both battery leads still connected is absolutely OK (in fact part of the "kit" that comes with the charger includes a plug which can be permanently wired into the vehicle so it can be charged by simply plugging the charger in when you park up) However it's important that the negative charger lead is NOT connected directly to the battery post itself. If you connect as recommended by chris you will achieve this. Or, perhaps a simpler way of achieving it is to connect the charger's positive lead directly to the battery positive post/clamp and the charger's negative lead to a good chassis earth point. This way that "pesky" battery sensor will be bound to be in circuit.
On my Ibiza - which is stop/start and has a sensor on it's negative terminal - it has a body earth bolt on the inner wing next to the battery where the negative lead earths to the body and it's actually shown in the manual as being the point to connect to for charging and jump starting (jump starting for the very brave!). On my boy's 2012 stop/start Punto there is not such a convenient earth point so I simply connect the negative charger lead to a clean bit of engine metal. - The engine is, of course, earthed to the battery with a dirty great earth lead so by connecting in this way, pretty much no matter what the car or whether it has stop/start or not, you will never "by pass" the battery sensor.
Because it's so easy to remember I always connect in this way. Positive charger lead to positive battery terminal/post and negative charger lead to engine block/cylinder head (I'd probably avoid connecting directly in the region of the alternator "just in case" although actually the alternator casing itself is earthed so would work). Being an "Old Codger" and not in any way an electronics expert and despite the reassurances from CTEK, I was absolutely "bricking it" the first time I tried it. That was on my boy's Punto. But it worked just fine. In fact, because we didn't realize the battery was duff we actually charged it in this way a few times before buying a new battery. Next up was my 2016 Ibiza, which is a real basket of electronic gizmos! It's not getting much use what with all this lockdown nonsense so I put the CTEK on it once a month just to keep the battery up to scratch. I've also charged the Honda Jazz belonging to my younger boy's wife by connecting to the battery pos and engine block and it's been absolutely fine. I mention this because Hondas are notorious for not liking you "fiddling" with their electrics when the battery is still connected.
So I think that you can very happily charge most cars with the battery still in circuit as long as you use a modern smart charger and don't connect that negative charger lead directly to the negative battery post. I have read that a very small number of vehicles have a sensor on the positive battery terminal and an even smaller number have one on both. I've no idea what to do if you have one of these? However they must be very few and far between, I've certainly never seen one.
I probably now have to say that although I believe all the above to be true and it's certainly worked for me on several makes of car, it's not to say your's might just go "POP" when you try it! So it's got to be completely up to you whether you want to try this and if it all goes wrong I'll be the first to commiserate with you but that's as far as I'll go!
