Connecting Lipo Packs

Hi people.

I have 2 lipo packs salvaged from cheap faulty car jump starters.

Both charge up to 12-13 volts one is a 4 cell pack the other is a 3 cell.

Can I connect them together in series?

Or will I get a PHATOMB lol.

Oh and I have to re wire the charge input as neither my fake Imax B6 or capacity checker has the correct plug type.