Replacement battery not detected

Since you cleared out the CMOS RAM, it sounds like the BIOS doesn't like the pack. The iFixit batteries are aftermarket, and Dell BIOSes are touchy about accepting them. I got one for my E7440 from iFixit and when I enter the BIOS on the system (running A25, on purpose) it will complain by saying Not a Dell battery.

This doesn’t surprise me one bit - Dell was affected by the Sony recall in 2005/6. The issue now is Sony made them hyper paranoid about 3rd party batteries :(. It works in ePSA or normal boot modes, but if I dare go into the BIOS it fails. More then likely your tired OEM works because it has the right information, but the 3rd party is different. Unless you find one where they cloned it and have good compatibility, you will often run into problems with Dell and 3rd party packs. For the internal packs, I recommend going OEM due to how particular Dell has designed the battery authentication to be, and the amount of issues the knockoffs tend to have. I can use 3rd party packs that do *enough* in my E7440, but that means I need to carry an OEM pack with me to make the machine happy in some situations…. even if it’s nearing the end of its life. You can’t do in a sealed XPS!

As much as I hate to say it, you’re better off returning the aftermarket pack and eating the restocking fee. Buy the OEM one from PartsPeople or another reputable supplier and you won’t have to deal with their BS around this. Try an exchange if you want, but it's usually due to BIOS rejection.