TLDR; no success yet with third-party batteries, still going strong with old Apple battery
iFixit acknowledged they had issues with some of their batteries, therefore there was no problem to get the batteries replaced for free. But after three batteries with the same issue I can only assume I was either really unlucky, or the problem was a bit more widespread than they led to believe, or there are other compatibility issues (Can Apple update the firmware to block third-party batteries, the same some printer companies block third-party cartridges? I did update to macOS Catalina). iFixit seemed confident they could supply a working battery eventually, but since they required at least 15 cycles to let the battery settle in before it could be returned, it would take about two weeks of problematic battery woes before they would ship another replacement. Therefore I decided to try another brand for now. (BTW, the iFixit iPhone battery I ordered at the same time is working excellent).
The first Mayerinc replacement battery did extraordinary well, good built quality and charge capacity around 8500+mAh, slightly over designed capacity of the original battery, giving 9+ hours of light use on a single charge. I was really happy with that, but then the battery failed suddenly ten cycles in (day after I made the post), no clue what happened. I keep a charge log using `pmset -g rawlog` that is written to disk by a LaunchDaemon script on boot. The last entry was when I woke the Macbook by opening the lid before the power cut out. Battery reports it is fully charged, just not supplying any charge. So I switched back to the old Apple battery and there are no issues aside from it being on old battery with about half the original charge capacity. Anyhow, got a replacement for the Mayerinc battery last week, but it arrived fully drained. Not a good sign. Thanks to the charge logs I can see that despite the reported capacity of 8500+mAh full charge capacity, it only charges to a capacity of about 4400mAh, about the equivalent of the 4 hours battery life it provides when idle, doing 7 "cycles" per discharge cycle! Also, the soldering on the connectors of the battery logic board to the battery and the battery connector on this second Mayerinc battery is mediocre at best, so I requested a return. Not sure if I want to try another one of the brand. The first one might have been a fluke, the second is likely just bad inventory management (production date of the flat battery was back in March 2019).
So, I'm back to my old Apple battery again, currently going at 1116 cycles, hovering around 55 to 60% battery health, and at most 5 hours battery life
I don't know if Apple or any of their authorised service centers will replace a battery on a machine of which the battery has already been removed. But I don't think they would deny your request to have it replaced at 85%. The battery in my Macbook remained around 80-85% for quite some time before dropping rapidly. I only have a partial log because I did not keep track after Januari this year because it was hovering around 80-85% all the time, but it dropped to 68% around May:
Sample date Cycle count Condition Full charge capacity
2018-01-05 818 83% 7018
2018-01-21 824 84% 7118
2018-01-30 830 84% 7123
2018-02-13 837 82% 6930
2018-02-25 846 85% 7199
2018-02-28 847 84% 7037
2018-04-24 865 83% 6981
2018-10-06 961 81% 6806
2019-01-04 1012 82% 6888
...
2019-05-15 1067 68% 5727
2019–07-27 1111 66% 5572
Currently, I am tempted to buy one of the "original" / "genuine" / "OEM" Apple batteries since the original battery works so reliably. I just don't think they are truly genuine, since Apple does not sell their batteries through regular reseller. I've heard of rumours that some of the "genuine" Apple parts sold online are goods "diverted" from the original supply chain. What is interesting is that there are new condition Apple batteries for sale on eBay that show pictures almost identical to a genuine Apple battery, including a serial number on the battery connector, but I'm going to assume that the picture is not a true reflection of the actual product you will receive. I did find an online store in my country where they supposedly sell an original apple battery, so I am tempted to try that one despite the increased cost (about €130 compared to €80 for Mayerinc/iFixit/Yanec batteries). If it is not a genuine battery, or if it fails, I'm covered by my countries return policy and consumer protection laws, without having to send the battery back to China at almost half the cost of a new replacement battery.
I haven't tried the cheap batteries on Amazon. They are often the same cheap batteries sold on eBay, usually sold through third-party resellers, sometimes even shipping from China or Hong Kong (2-3 weeks delivery), but they might be easier to return to Amazon if there are any issues. Be sure to check the charge capacity of the battery though. There are plenty of cheap batteries sold with a lower charge capacity than the original battery. High capacity Li-ion battery are generally expensive, so there has to be a reason these are so much cheaper.
Let me know if you have any success with third-party batteries, although considering the issues reported here and elsewhere online, I would encourage you to take the hit to the wallet and have it replaced by Apple. It only costs €209 at the Genius Bar in an Apple Store, but Apple authorised service centers may charge extra (shipping, handling, labour fees), hence the quote for around €300. But it is almost guaranteed good quality. Wish I had done that, although at the time it seemed ridiculous to spend so much on a device with a net worth of maybe max €800 resale value, especially since I was expecting to use it at most one or two more years before upgrading. Now I am seriously considering buying the 16" and let someone else worry about finding a suitable replacement battery.