Ipad airs in general that wont charge

[crea2k]

Hi Jessa, im not sure if you have seen these, or if it is a signature problem or not, but I have seen a few in the last year or so where they will charge just fine when they are flat, but when they turn on they will stop charging at 1%. If you turn the ipad off it will charge fine to 100%. You change tristar and it still makes no difference.. This isnt a specific ipad I am talking about, I just wondered if you had come across this issue before and if it was the same thing at all. As I know if I have searched gsmforum for this in the past they all say its the pmic that is damaged, but just wondered on your opinion, as it seems to be a pretty common fault with the same work around (turn the ipad off to charge). I have never really investigated it further than just changing tristar so wasnt sure if some line or something to the pmic died or the pmic itself had some kind of internal issue.
I know im generalising here a bit, but it just seems to the the same pattern of things, e.g charges to 1%, wont charge unless off etc.

[Jessa_the_Professa]

Yes we've seen this many times, so let's use our brain here. Study the charging section of the PMIC in the air and tell me what are some possible explanations other than "bad PMIC"

[crea2k]

I was thinking possibly bad battery sense line or something like that , im not sure if that comes into play or not when its turned off charging ?

[crea2k]

whoops didnt mean to post this, so edited (wrong thread)

[Jessa_the_Professa]

I was thinking possibly bad battery sense line or something like that , im not sure if that comes into play or not when its turned off charging ?

bad battery sense line. sounds reasonable, no? Explain how bad battery sense line could explain this behavior.

[crea2k]

Well it could and it couldnt, im thinking when its turned on it says 'are you full ?' and get no answer so stops charging, but surely this must also happen when its turned off otherwise it would over charge the battery.

[Jessa_the_Professa]

Ok, here's a different question. Would you expect there to be a difference in overall current in the device if the device (and screen) is off versus if it is on?

and how might that apply to a line named "battery sense". None of us know, but we can all make guesses and see how they feel. What is your guess here?

[crea2k]

Charge current no, its generally pretty similar when the device is on or off, but in this case its vastly different when on and off. Im not sure how the sense circuit works, is it a voltage divider or something like that, im not overly sure, my brain hurts haha.

[Jessa_the_Professa]

Not regarding charging current, i'm asking Would you expect there to be a difference in OVERALL CURRENT CONSUMPTION in the device if the device (and screen) is off versus if it is on?

[Jessa_the_Professa]

Also--keep in mind that none of us have a manual for any of this. Your guess about how things might work is as good as mine. My job is to just ask questions that probe you to think a little differently because your current thinking has you stuck. I don't have answers---I only have questions. Many times my questions are leading you on a path, but don't get discouraged if your answer is "I've thought about it, and honestly I have no idea" that's okay. This forum is about practicing the art of logical reasoning as it applies to mobile devices.

[crea2k]

Yes I'd expect a ipad turned on to use more current than one that wasn't turned on. I'm not sue how that relates to charging though?. The problem is I don't know how exactly the sns line works so it's hard to come up with a theory on the problem.

[Jessa_the_Professa]

Yes I'd expect a ipad turned on to use more current than one that wasn't turned on. I'm not sue how that relates to charging though?. The problem is I don't know how exactly the sns line works so it's hard to come up with a theory on the problem.

None of us know, it is all just hunches and guessing. One of the things that you can draw on as you play detective is compare and contrast to changes in the same function in different devices. Isn't it true that iPad Airs seem uniquely affected by this "state dependent charging disease"? It is rare otherwise. Let's assume that for the charging function to kick in that the PMIC has to get a thumbs up from both the usb side (via tristar) AND the battery. Tristar vouches for usb side---hey bro, charger is good, bona fide apple, i see 5volts, we are good to go here. But the battery side is equally important, no? We can guess that the information might be "I'm not on fire, I am not full, and I'm available for charging right now, we are good to go here" From other faults in other devices---what do we know about how battery communicates with the board? What is the BATT_SWI line? What is the BATT_NTC line? Do these go to PMIC? Is there any other device that has a battery data line called SNS? That appears to be the only line that goes to PMIC from battery. SNS. Sense. Sensing. Sensing what? Can we hone a guess based on the label on the pad at PMIC---IBATT (that's from memory, but I think this is true). Elsewhere is VBATT. What could the I in IBATT mean?

[crea2k]

I'm guessing it's battery information then ?.
So this is similar to the iPhone faults then where someone knocks a filter off near the battery connector ?.

[Jessa_the_Professa]

"I'm guessing it's battery information then ?.
So this is similar to the iPhone faults then where someone knocks a filter off near the battery connector ?"

battery information yes--but not quite the same, something new. What line is it on iPhone 5s that get the filter knocked off--is that the same line here, or does that line also exist in the Air and this is a new piece of information from battery--if so, what might that information be? This is going to require guessing!

[crea2k]

Well I'm guessing as you mentioned current earlier that's there is possibly a current measuring line too ?. I know there is temperature and as charge level . I'd imagine there is probably cell balance too but I'd imagine that's managed by the BMs board on the battery itself.

[Jessa_the_Professa]

Ibatt—my guess is the i stands for current. SNS—my guess is sense. So current sensing. And this line in unique to iPad Air and not in phones. And it plugs into the charging section of the PMIC. And your device will charge just fine until there is current draw from the battery. I would focus there.

[crea2k]

So iv checked R8173 and R8172 and both are the resistance stated in the schematic. Id test the resistive measurements of the lines but dont have a working board now to compare with. By the way if I hold power and home to restart it, it does actually charge when turned on too, until you unplug the cable for a while, then it quits detecting it again.