Function of third battery contact on old iPod Touch/iPhones? - Page 1
Forum section: electronics / repair

author: lpc32
author_rank: Frequent Contributor
timestamp: March 10, 2015, 04:19:05 pm
content: I'm trying to boot an iPod Touch 2G without its original battery. As far as I know it should be fine other than having no battery and a screen that's only good in the top 1/4th.It powers up with just the plus/minus battery connections, but as far as I can tell it doesn't really/fully boot. I'm assuming it has to do with the 3rd battery contact not being connected. If it's just a thermistor, any idea what it expects to read there?

author: lpc32
author_rank: Frequent Contributor
timestamp: March 11, 2015, 02:18:56 pm
content: Well, in the original battery the thermistor connection does act, at least superficially, like an NTC. ~10Kohm at 22C, ~8K at a few degrees higher. But the iPod still doesn't boot with 10K, nor 1K, or 100K. Maybe there's more elaborate signaling going on there?A strange thing I noticed. When I connect power through a breadboard + jumper, the iPod screen turns off after ~10 seconds. Without these two elements it stays on indefinitely. Any idea why or how that might be?

author: ConKbot
author_rank: Super Contributor
timestamp: March 12, 2015, 04:53:22 pm
content: In most every lithium pack there is a temp sensor for charging safety on good products. Though I wouldnt be surprised if apple soon replaced that with an crypto rom and temp sensor IC just to make sure only authentic batteries work

author: Zucca
author_rank: Unknown
timestamp: March 13, 2015, 08:14:41 am
content: I think you need to scope out the signal on the third terminal... Do you have a O'scope?

author: lpc32
author_rank: Frequent Contributor
timestamp: March 16, 2015, 04:38:27 pm
content: No scope. And it might not be a good idea to try charging an inflated and abused battery. While it worked when I charged it once before opening the device (popped front panel notwithstanding), it's now punctured/squashed since I had to pry it away from glue.I have an incoming iPhone battery from the same era, which I expect is logically compatible. But the problem could also be something entirely unrelated to the battery.

author: abyrvalg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 11:25:10 pm
content: Many phone batteries have some digital supervisor IC communicating with the phone over that 3rd terminal. In this case you can extract the small pcb containing that IC from a dead battery and connect your external supply right where the lithium cell was connected to the pcb.

author: lpc32
author_rank: Frequent Contributor
timestamp: March 17, 2015, 03:52:47 pm
content: What I read suggests LiPo/Li-ion aren't really toxic, but are there still particular things to avoid doing?

author: ConKbot
author_rank: Super Contributor
timestamp: March 17, 2015, 04:58:30 pm
content: Quote from: lpc32 on March 17, 2015, 03:52:47 pmOverchargingoverdischarging and then recharging the cell. (discharging a cell to below 2.5v resting is safe, but charging it afterwards is not)reverse charging. (discharging a multi-cell series pack to low levels will most likely reverse charge one of the weaker/lower capacity cells, so avoid that. )discharging too rapidlycharging too rapidlyoverheatingpuncturingcrushingbreathing the resultant fumes from doing above said activities. If you do, seek medical attention, as Ive heard of people having long term issues afterwards due to either swelling or scar tissue buildup.But other than that, totally safe

author: lpc32
author_rank: Frequent Contributor
timestamp: March 17, 2015, 05:37:21 pm
content: I mean when taking apart the battery to reclaim the PCB part. Puncture and crushing: too late. But so far I survived.

author: sacherjj
author_rank: Frequent Contributor
timestamp: March 17, 2015, 05:41:47 pm
content: Third wire is either thermistor or 1-wire IC interface. We use a 1-wire fuel gauge chip for our custom pack.

author: abyrvalg
author_rank: Frequent Contributor
timestamp: March 17, 2015, 11:37:28 pm
content: All the dreaded chemicals live inside a sealed container with 2 terminals going to PCB that is outside this container. Then there is an outer case that holds both things together. Destroying the outer case to get to the PCB should't release any chemicals (unless you've damaged the inner container occasionally).

author: lpc32
author_rank: Frequent Contributor
timestamp: March 20, 2015, 12:34:54 am
content: I initially thought there's some electronics also under the opaque battery shell, but no, it's just what's tape-wrapped externally.But running a battery thru this and on to the iPod doesn't work. The screen flahses briefly and that's it, unlike running a battery direct where it stays on. Same thing when skipping the sense/control line. In both cases the voltage readings are different, in different ways, than what I see with a battery connected directly.