Problem Z3 Compact bootloop - battery too low


I bought a Z3 Compact at the flea market. I tried it on the spot with an external battery but it didn't turn on, only the led turned on red, stayed on for a while and then went off, showing nothing on the screen.

When I got home, I left it on the charger for about half an hour, after this time the LED stayed on red, the screen black. I pressed the power button and the phone turned on, showing the Xperia logo but then turned off. It immediately restarted and shut off. And so on. All this time, the led lights up red and then goes out when the screen is turned off and the pattern repeats.

Am I to understand that at first the battery didn't even have enough current to turn on the led, then it had enough current to display the logo but not to continue the boot procedure?

The cover of the phone can be seen to be disassembled, it was barely holding on with a little glue. I managed to disassemble it, then the battery, unplugged it and unplugged it but the behavior repeats itself.

Unfortunately, I don't know how I could charge the battery directly, the contacts are too small, I don't know which is + and -.

Any chance I can bring him back to life?