Is it save to solder the cell only iPhones battery to the BMS board?

Hi Aaron,

Spot welding is preferable, mainly for the reason that heat will quickly destroy a Li-Ion battery. Using a spot welder means there's a momentary intense flash of heat on a very small spot on the metal, while soldering means a much longer exposure time and wider area and thus more chance for the heat to transfer down the connections to the lithium itself.

That's not to say you can't do it with soldering, but you'll want to avoid getting the heat to the battery as much as possible. That means be very brief with the soldering iron, and solder as far away from the main part of the battery as possible. If you have some hemostats, you can clamp them between the solder point and the battery and they'll act as heat sinks to keep it from traveling down the metal strip.

Before I got my spot welder I did quite a bit of soldering directly on Ni-Cad batteries and so far I've had good luck with being quick and following the guidelines above.

@aaron75 I did realize which battery you were talking about, and I'm aware that the terminal strips should be made of zinc on the replacement cell. 5-7 seconds sounds long to me; I would use flux and quickly tin both leads first, then use the barest touch of the iron possible to get the BMS connected. Don't worry about it looking good or being smooth; you want a solid electrical connection in the very shortest amount of time possible.