Even with a bad battery the iPad should still recognize being plugged in by flashing the battery symbol. First thing I would try is the chargeport, which is a solder job on the ipad mini. Hopefully that solve your problem. Unfortunately, it may not. Thats because the Mini has a known weakness that sometimes it's power management chip gives up the ghost out of nowhere and for no apparent reason. If this is the case, the ipad will not boot, charge, flash the battery symbol, or otherwise show any signs of life whatsoever. This can fixed but it is a very advanced motherboard level repair