By ‘4’ I assume absorbtion stage? If so, it’s going to mess with the current tapering by opening doors/boot so it probably just jumps into the Analyse & Float stage for safety. It’s pretty much good to go once in absortion state anyway…
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Interesting.
YOu could be right there.
It usually goes to 3 straight off the bat, within seconds on mine, and then to 4 pretty quickly.
Good point about that safety thing. However, it don’t always to it (respond quickly to a door open), so who knows,
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Trust me, CTEK probably tested this for all scenarios, including unexpected extra current loads during any stage of the charge. I had a crappy old so called “smart charger” that refused to start charging my Vel Satis with the battery in situ in the car because the known parasitic drain meant the computer said no. Once I disconnected the car’s cables from the battery the charger worked perfectly. But that’s Renault and crappy smart chargers, another story!
Why would it jump to ‘fully charged’ (green light) though, if it wasn’t. You’d think it’d either pause it, or turn off or something, rather than going to fully charged? As i mentioned too, it don’t do this all the time, and when it does, its not the same time duration to move from ‘4’ after opening/closing a door. Sometimes a few mins, sometimes a lot more.
Down to cost I guess, they could do that if they wanted to design and build a “super smart” charger, but why bother for an edge case? 99% of users wouldn’t start opening doors/boots etc in the absorbtion phase.
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Yes maybe.
Saying that, I’ve done it for about 3 years with no ill effects!
You’re probably the 1% 
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Can you use the cigarette lighter charger adaptor for the ctek mx5s on an NC?
No (add five characters here).
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I did it 'your way ’ earlier, not opening any doors etc. All fine. I was excited and all pumped up to charge it this morning. Shows what an exciting life I lead.
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I finally got to charge my ND3 yesterday. Same as you experienced it rapidly wacks through stage 1 and 2 for desulphation and soft start and is straight into stage 3 ‘BULK’ in a matter of seconds. The voltage then starts to ramp up from about 13 Volts to around 14.45 Volts at about +0.01 volts increase every 5 seconds or so. It sat at 14.45V for the last 5-10 minutes of the BULK for a total of 25 minutes before it switched on to stage 4 ‘Absorbtion’. About 5.5 hours in ‘4’ at around 14.44V before finishing in ‘6’ FLOAT and settled at 13.56V. Total charge time ~6 hours. I assume it was probably around 75% SOC before CTEK charging as it only took 25 minutes in BULK.
Disconnected charger, closed bonnet, unlocked, sat in and started her up. No issues at all. No confusion, no unexpected warnings/alerts. i-Stop lit up briefly in amber as expected and worked perfectly once engine warmed up in normal driving. Just to confirm I didn’t disconnect the battery clamps, remove the wheels, remove the seats, phone a special number, use an app to reset anything or perform an indian rain dance! 
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Great write up,
Do you think it would give the same result (as in no issues) using the black croc on a nut, rather than the negative battery port?
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Not a blind bit of difference, no.
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Your selfless work and ‘experiment’ has answered a 5+ year question, full of rambling theories, threads, posts, guesses, ‘might bes’, ‘think it is’, on this subject of using an MXS (whatever version) on an ND2/3, in normal mode, ‘as a charger’ in the way one would do with an ND1, and directly then seeing if there were any ramifications in doing so. Thank you 
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No big deal. Needed charging and checking anyway after leaving parked too long up to last weekend. I was never expecting any issues. Apart from me not driving it enough, car’s been faultless so far and a joy to own.
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So I asked CTEK about this and all they would say, not very helpfully IMHO, is:
"A MXS 5.0 will charge your battery to 100% SOC if you connect the clamps correctly.
Please connect the to the positions described in the instruction manual for the car so
the BMS can record the charging. If you changed the battery please reset the BMS
with Mazda or universal service tool."
So you should connect the CTEK to +ve and chassis (not -ve battery terminal) so the ‘BMS can record the charging’. It’s all very inconclusive as to the correct/best way to do this (battery connected to car or not, and where to connect the charger if it is) as the manual is silent on this and you can get the issue I an others have had with the BMS thinking the SOC is low until you drive a lot of miles or disconnect and reconnect the battery.
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I actually emailed them myself y’day, regarding opening/closing doors when in stage 4 (absorption mode), to see if it effects it at all, as in the hunch mark67^ suggested earlier in the thread. Be interesting if it does. I haven’t heard back from them yet.
" and you can get the issue I an others have had with the BMS thinking the SOC is low until you drive a lot of miles or disconnect and reconnect the battery." : This is after charging directly on the battery posts or via the chassis negative earth with the charger’s aligator clip? As I posted previously, I charged directly on the battery posts with no issues. Apart from CTEK’s general vague instructions to charge directly on the battery posts with supporting diagrams showing this while also stating with contradiction to use the chassis negative earth connection, especially if car has BSM with current sensor, where else outside of CTEK’s publications (ideally within Mazda’s own publications) does it state to connect to the chassis or directly on the battery for the ND?
I was connecting to chassis (so BMS is ‘aware’) as per email from CTEK above. Mazda manual says nothing useful either way.
If you connect direct to the battery I guess there’s a risk that, whilst the BMS may not switch off stop/start, it may take a while for it to work out the battery was fully charged and potentially cause gassing for a while as it tried to further charge it itself. But who knows?!
At the end of the day connecting to chassis is supposed to let the BMS know what the charger has been doing hence why CTEK recommend it. It’s just that in the MX-5 ND case the BMS has some kind of bug/design anomaly so it can’t work it out. (Works as expected in at least some other cars, like my 2015 Audi.).l
“it may take a while for it to work out the battery was fully charged and potentially cause gassing for a while as it tried to further charge it itself. But who knows?!” I can asure you this would never happen. The alternator regulates the output to around 14.2 Volts max so it will never cause gassing unless there’s a fault in the regulator. Even if you drove the car every day for several hours the battery would never reach 100% SOC due to the capped charge voltage of 14.2 Volts. It might charge to about 90-95% maximum SOC via the alternator. The CTEK or other good chargers will raise the charging voltage to 14.4-14.7V depending on ambient temperature and this is the only way to reach 100% SOC. You can check this yourself after charging and starting with a multimeter or using an ODB scan tool, It will never be higher than around 14.2 Volts with engine running. Gassing the battery is actualy a much higher risk using an external charger(especially a poorly bad designed one!) than via a modern car’s alternator.
Regards the BMS, The CTEK might confuse it as it also uses ‘PULSE’ type charging during maintenance mode. Pulse charging in nothing like the normal steady current charging the battery expects during bulk, absortion charge stages or during normal driving. But we also don’t know if the current sensor can sense actual direction of current flow, so it could also be confusing the CTEK charge as a major parasitic drain. Unless we can get detailed specs on the current sensor and the BMS algorithm we will never know for sure.