If you were to straighten the spring assembly out…how long a piece of wire do you think it would be…that last battery thru that long run…very thin gauge + long run = high voltage loss
Low voltage of battery put in very high current draw to feed the boost circuit to make 5vdc from say 3.7 or lower vdc, high current flow thru thin wire = heat
I recommend on all parallel connections thru these thin steel springs is to spring bypass with a piece of 22ga copper wire, this way the current flows thru the wire and not that low current flow type spring steel
In fact check out a test one of our awesome member here did showing the voltage loss thru springs (spring #4 looks about the same gauge)…now multiply the loss thru springs x 5-10 times due to the length from the - side to the circuitry at the board (even a full charge battery would probably surge current if it was the first battery placed in that very long high resistance run)
That kind of setup is a very very poor conductor of power…all current is parallel thru battery all concentrated in that one single run