Get data back from old iPhone

iPhones in general require a good battery to turn on. It doesn’t need to work well (even a piece of junk will work) as long as it holds a passing charge. These phones are becoming collectible, so I’d encourage you to get a high quality battery and keep it around.

If you want to charge the battery, it can be done with a high amps charger - AS LONG as you watch the phone for signs of battery failure. 12 year old batteries that have not charged in years can be downright dangerous if you aren’t careful. I’ve done it before, but only under that condition to avoid problems.

Since you just need the data, find a descent quality battery for the phone and work with that to get the phone to turn on. You almost certainly need the sense line and that will only be found on a real battery. That said, if you want to try to simulate the battery, what you can do is leave the wires for the old battery soldered to the board and use a bench pperr supply to make the phone think there is a fully charged battery present (3.7V/1400 mAh or 1.4A). It may even be possible to charge it through a bench supply.