Battery (I think the 12V one!)

[Nigey]

Anyone had the same issue? I recently (don't know how) drained the 12V battery as fitted under the bonnet down to 5 volts. I believe this battery was replaced by the previous owner. I asked a Leaf Expert if there was a particular way to change it. I was told "no probs, disconnect the old and put a new un' straight in" I did so with a new Yuesa 54AH battery.
This is when the fun really started. On depressing the brake pedal and pressing the start button, all dashboard indicator lights came on and the warning KEY FAULT. I tried the spare key. Same message. I had the firm that supplied the car connect their diagnostic tool (a generic one for all EVs and not a specific Leaf one) He cancelled the 10 in number FAULT messages and the car started.
The guy who did the diagnostic check on his admission was not an expert and said he'd peruse it "further with the Experts" I await his results.
I have 2 questions:
1. Assuming the battery I took out was OK as its now, after charging, showing 12.7V, what caused it to go flat
2. What caused all the dashboard lights to come on and the KEY FAULT warning light to come up?
Thanks for reading my "epistle" and looking forward to an understandable cause and reason!
All good fun! Nigel

[VStar650CL]

Leafs (Leaves?) are bizarre with the codes they throw when the 12V battery goes flat. Those codes will all have occurred when the battery was in "never-never land" below 8V, before replacement and not after. So your guy was correct that swapping it wasn't problematic, those codes were already there before the swap. Once they get goofy, erasing the codes is the only way to get them restarted. The 12.7V on your old battery isn't totally healthy (12.8V is) and any automotive battery besides a deep cycle type can be seriously harmed by a trip into never land, losing as much as 20% of capacity and CCA with each recharge. Voltage recovery won't tell you that. On a '14, if it never had a new battery, there's a good chance it simply died of old age and gradual depletion.

[dsevier]

We just bought a used '14 Leaf S. On the 2nd day of ownership, my wife said the car would not start and would not turn off. When I got out there, every dash icon and warning light was illuminated. I also could not turn it off. I measured the 12v batt voltage and it was 8.5v. So I "jumped" it with a portable battery. After a minute or so I was able to turn it on and off. I replaced the battery and the dead one was probably the original as it was Nissan brand. After installing the new one, I had the warning light of vehicle profile with exclamation point in the center of the dash (traction sysyem?) illuminated. But a few minutes later it was off and Leaf Spy showed no DTCs. Anyway, not sure why Nissan felt it necessary to show all the dash lights when just the 12v battery icon would have been more helpful but everything seems ok now.

[VStar650CL]

At 8.5V the battery isn't delivering enough current to maintain voltage regulation for all the various devices on the CANbus. As they gradually drop off the bus, the devices still alive start throwing codes for the devices they can't "hear" anymore. The result is a temporary Christmas tree on the dash when power is restored. You were actually fortunate that it didn't go low enough to start throwing powertrain codes. Many of those won't self-clear, and the ones in the powerhead generally disable the vehicle. None of that stuff happens when you disconnect as opposed to the battery gradually dying, because when you disconnect, everything works properly up to the instant of disconnection.