Like you I had a problem with a slipping transmission, now 8 or 9 years back. I am curious about your approach to the problem. Why did you change the torque converter? I.e., what made you believe it was the source of the problem? Weres there any symptoms or diagnosis once you removed the torque converter that indicated a problem?

On the second attempt, did you change all the seals in the trans when you had it out? I had mine out at 93k miles and the seals were generally brittle and nasty. Something else maybe faulty if you didn't refresh them all. Did you clean out the valve body at the time?

Finally. did you check that solenoid A is functioning properly? IIRC it can accessed with trans in the car.

Although there are a lot of stories of bad batteries causing issues, your fault code is quite specific. Like you, I am inclined to think that once the car is running it should provide enough juice to run the systems, but a new battery wouldn't hurt.

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.