Absolutely fantastic topic and actually the reason why I registered here.
Trying to revive a run down S8 I got handed down that is still good for a fun project and trying to make it run on alternative power sources. I have some questions ...
So 4.4v is the absolute limit? Would 4.8v cook it? 4.8 would be 4x 1.2v rechargable AAs. Also, has anyone of you tried to scope out the lowest possible voltage it is still able to run? Lets say if it was able to still function at 2.9 or even 2.8 I'd try to hook it up to some LifePo4 cell since I love working and tinkering with them (they're really good-natured and don't go up in your face if you look funny at them like LiPos for example). They typically have 3.2v but will of course dip under 3.0 at some point.
Also, is the small pcb on the battery just the BMS of the cell or is there more to it?

 

I've converted two Galaxy S7 phones in the same way, also to use as remote-controlled web cameras on the island where I have my summer house. Basically I don't want Li-Ion-batterys sitting inside the phones as the temperature will go much lower than the freezing point, and there is a potential fire hazard if the phones tries to charge them (which they shouldn't but...).

I power them via a 12V car battery (solar panels on the roof), and from the car battery I simply use 12V USB car cigarette-chargers, then the USB cable is soldered as in this thread to the disassembled battery PCB. But I had to test my way with some chargers, and use high-capacity car chargers to make the phones power up. Although the chargers nominally will give 5V there seems to be no problems. Actually when I log the voltage from the phone's battery manager, it varies between 4.1 and 4.7 volts on different days, not sure if it's the chargers which are inexact and varying (e.g. due to temperature or something), but anyway...

I put up my phones October 8, just before closing off the cottage for the winter. Due to dangerous ice conditions on the sea I cannot access the island from mid October to about Christmas (this is in northern Sweden, close to the arctic circle). My problem now is that the battery level steadily goes down, and I fear that when it reach a low enough level the phones will shut down (despite having fine voltage). I assume that the battery manager counts the discharge and somehow ignores the voltage. The phone with the lowest battery reading is at 10% now, so let's see what happens during the next week or so.

If they shut down (which I fear) I will try next winter to also power them via the USB port in the phone, so that the phone knows that it's not running on battery. However, trying to power them only via the USB (with the battery removed, and nothing soldered) didn't work at all.

 

My problem now is that the battery level steadily goes down, and I fear that when it reach a low enough level the phones will shut down (despite having fine voltage). I assume that the battery manager counts the discharge and somehow ignores the voltage. The phone with the lowest battery reading is at 10% now, so let's see what happens during the next week or so.

Ok, so here is a follow-up to my own post. It seems like the phones are not turning themselves off even though the battery goes down. At least one of the phones have now been down to 1% battery since more than a week, but it's still running (and does not go lower than 1%). But the phones behave differently, which is a bit strange.



Phone 1 (blue line) goes down with a smooth slope (disregard the peak at Oct 14, I turned off and restarted Phone 1 that day, so it jumped back to ~85%). Phone 2 (red line) has no smooth "going down pattern", it jumps up and down all the time, even if the long-term trend is also decreasing battery level.

Phone 1 should consume slightly more battery, since it has a BlueTooth connection active all the time. This is to a BlueTooth relay which I use to turn on/off a big 50 Watt LED lamp if I need to take a picture or video when it's dark. But still, why do shape of the curves behave so differently?

This is kind of strange, although of course all that matters for me is that they actually do work, and seem to ignore the low battery level. So I'm kind of hoping that they will be running at least until Christmas, when I expect to be able to take the snow-mobile and go out on the frozen sea to the cottage. The ice is slowly setting now, which can be seen in this Phone 2 early morning picture from November 8...

 

But the phones behave differently, which is a bit strange. Phone 1 (blue line) goes down with a smooth slope (disregard the peak at Oct 14, I turned off and restarted Phone 1 that day, so it jumped back to ~85%). Phone 2 (red line) has no smooth "going down pattern", it jumps up and down all the time, even if the long-term trend is also decreasing battery level.

Ok, so just another update. Phones are still running ok, but now it's very evident that something strange is going on. Phone 1 has had 1% battery for two weeks now, while Phone 2 seems to fluctuate up and down around 60% or so. As the two phones are totally identical, including all settings, I don't really understand what's going on...?

 

Thought I'd add my experience to this thread since I am also trying to power a samsung phone from the battery connector but without having a battery. I haven't figured out how to power the phone from but connector but I did figure out that I can start and power the phone using the original samsung charger and cable but with the battery removed. I have a Note 8 which has the same battery connector as the phones mentioned.

The trick I found is to press and hold down the power button and then plug in the charger cable into the charger port. If done correctly, the phone will boot up like normal. Sometimes it will not boot and show a yellow exclamation warning symbol with a temp gauge in it. In that case just take out the charger wire and try again.

When the phone is running, the battery percentage level shows really weird readings and tends to fall to 1% but the phone never shuts off.

 

I just done the mod yesterday and it will work, you need strip the PCB out from battery then use that to connect 4volts to battery terminals, if you connect direcly to motherboard you need connect resistor also and have aswell skill and proper tools to do that tiny solder. After connecting wire to battery terminal at battery PCB, the phone wont turn on, this is normal(most of people will fail at this point). To fix this issue you need to plug microUSB connector into a charger for 1second, then you can disconnect that one, after that the phone will turn on. if you disconnect power from battery terminals you need always cycle microusb first. I was managed to fit DC-DC converter into battery spot, so i can use 5-30volts without any issues, actually i have it on my car and it is mailage diary.