The battery in my wife's XPS 15 9550 was only lasting 40 min, so effectively dead. Thus, the need for a new one. I took a chance and purchased a replacement off of eBay for $AUD80. It arrived promptly and I noticed that when I unwrapped it, it was in fact a Dell battery. For some reason the seller put a bit of black tape over the Dell logo. Anyway, I took the bottom cover off the laptop (very easy) and did the replacement.
When I put it all back together I noticed that the touchpad would no longer click on the bottom corners. The act of screwing the new battery in place caused it to press against the underside of the touchpad so it would not work. In order to fix the problem I placed a small washer on the screw hole on both sides of the touchpad. The battery requires a number of screws to hold it in place and there are two particular screws one on each side of the touch pad. Effectively it was, one washer on each side, the new battery in place and then re-insert all the screws.
The amazing thing was the incredibly tight tolerances of the fitting. The washers I used were only 0.5mm in thickness but just the addition of that spacing caused the trackpad to work perfectly, clicking on both sides. No washers and it didn't work, washers and it's perfect.
It took me about a week to sort this out. Hope it helps other people with the same problem.
What you likely purchased is a "Rebuilt" battery - the fact that it didn't fit and the black tape over the logo likely means someone dismantled the battery and installed new cells into it.
the battery in my XPS 15 went so bad and 'swollen' that my trackpad stopped working. But I didn't have any problems with replacing a new one. Keep an eye on that battery...if it's not a genuine one.