LeftieBiker wrote: 12.56 volts is pretty typical for a Leaf, and does not indicated a failed 12 volt battery.
but, apparently the replacement TCU 3G unit requires a higher power requirement than the unit it replaced, so what was acceptable, 12.5 volt, now is a problem. Myself and others have replaced the 12 battery, my new battery sits at 12.95 volts as reported by Leaf Spy and our TCU problems have gone away without a painful visit to a Nissan Service Dept. They are in the dark about the cause, much less the fix for this problem.
Also the new TCU units have been out in use since the last week in 2016, and right after that the country went into a deep freeze. That's never a good thing for a weak car battery. Mine suffered, and my new TCU stopped being reliable.
I had already purchased a replacement, but due to recovering from a surgery, I simply blew it off. Once installed, TCU worked!