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Works with Magsafe, shuts down on battery (96% charged)

Would love any tips on troubleshooting this situation. Macbook Pro 15" Retina Mid 2015 - A1398_820-00138, has battery A1618. It works fine with Magsafe connector plugged in. Battery shows healthy with 95% charge(checked with coconut battery too). However it powers-off the moment magsafe is removed - i.e. will not run off of battery.

I have Electronics Engineering background and am quiet comfortable with micro-electronics troubleshooting. Have repaired quiet a few appliances/gadgets/computers etc. So I decided to troubleshoot this myself so I can learn something new. Found Louis's youtube channel and it was great to inspire some confidence into me... I bought the needed tools - cheaper versions of what Louis recommends as I wanted to just do it for myself(not work as pro) for learning. I got a USB microscope, tweezers, bought the schematics and board-view. I had multilmeters, old soldering iron, but may upgrade it to a better one for SMD type work, and may get a cheap rework hot air-gun if needed. I built a DIY arduino oscilloscope as didn't want to plunge more money into this. I understood the charging circuit and all the SMBus communication stuff.

Here is what I have done so far :

1. First did the SMC reset, PRAM reset etc. with old battery.

2. Old battery - cell voltages with no power applied. cell-pair#1: 12v, cell-pair#2: 8v, cell-pair#3: 0v. At Battery Connector J5 Terminal 1-4, the voltage is about 2V.

3. With this old battery, when the magsafe power is plugged in, I can see the charger putting out ~12v on PPVBAT_G3H_CONN at Battery Connector J5 Terminal 1-4. The Battery indicator light is green. Battery shows ~96% charge, and shows as "Normal" in the Energy Saver Panel (same in coconut battery app)

When I unplug the magsafe, PPVBAT_G3H_CONN at Battery Connector J5 Terminal 1-4 drops down to about 2v

4. I got a new OEM battery from eBay(reputed seller) which incidentally had very similar cell voltages as my old battery -i.e. cell-pair#1: 12v, cell-pair#2: 8v, cell-pair#3: 0v. Also at Battery Connector J5 Terminal 1-4, the voltage is about 2V.

5. When this new battery was plugged in, the charger is putting out ~12v on PPVBAT_G3H_CONN at Battery Connector J5 Terminal 1-4. The Battery indicator light however is amber. Battery shows as 46% charged and displays warning - "Replace battery soon" in the Energy Saver Panel(same in coconut battery app)

6. I have checked voltages on R7150 and R7120, but found they were fine. Note that there is no board view for A1398_820-00138, so had to use the next one close to it which is A1398_820-3787. It was bit of learning, but could identify most of the test points.

7. Again did SMC reset, PRAM reset etc. with new battery - no change in status reported.

8. It is quiet possible that the new battery was faulty - I was able to return for a full refund.

9. I researched battery repair and understood the SMC world a bit more (http://be2works.com/hardware/) . The Battery SMC Chip is - TI BQ20Z451. I was about to plunge into getting the adapter (costs about 30) to do SMC chip reset, but held back as it was not clear if I had to unsolder this chip or it could be done via the battery connector. Plus I am constrained by time I have on hand.

However I did understand how the battery and SMC is supposed to work - i.e. battery health is manipulated by SMC chip and that is what Macbook will see.

Any pointers to where I can look next ?

1. Is it the battery or the board ? Anything could be wrong at board level that I can check ?

2. The battery A1618 for this model is not cheap - runs about 80-130$. I am trying to get a working battery A1618 locally(one guys on CL has it, but will not get it till after next week) so I can test it and buy only if it works. There is another battery A1517 for the A1398 models of 2012-2014 which has same power specs and sells for ~40-60$. However its battery connector is located at about 15 mm offset from center compared to A1618 battery connector I have - possible to do soldering hack, but not sure if its worth it.

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Update to my original question:

I got a new battery from another source. That resolved the problem.

To check further, I compared the battery cell voltages for the battery that is working fine and the one which is not switching over. Both the batteries show similar cell voltages. This indicates the problem may be with the battery controller (on the battery that is not switching over).

I checked on what can be done - there seems to be a way to reprogram that battery controller, but it requires additional time and $ investment. Its sad that I will have to trash it even though the cells are fine...

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