I don't have a multi-meter, but with a second opinion, it seems the battery sent was a dud (this model is 8 years old; it was probably sitting for a long time), and ifixit graciously sent a replacement that does charge. It reads up to 90% max charge and drains at least a bit more slowly than the older one. It could still be in part the charger, but I never could see why that would. Also this system doesn't charge by USB. I've also considered if a clean OS install would do anything, but this will do for now.