Don't you hate it when people never come back with the outcome to a problem? I do, so here it is, and there is some good stuff in here.
Firstly, contrary to the discussion above, I did have a double fault - a dead battery AND a dead charger. But I managed to recover the battery by taking it apart.
1. The SM-BCR2 Charger - Shimano have dropped the ball on this because the give no indication lights on the charger when it is plugged in and ready to supply power to the bike. I could feel it was warm so I knew the wall USB charger was delivering juice to the BCR2, but nothing else was happening. As it turns out, a perfectly functioning or dead charger both look identical if you have a 'dead' battery on the bike per the instructions and info online, although a misbehaving battery can also make the red battery indicator flash, if the charger is working.
2. The SM-BTR2 Battery - I carried out a bunch of experiments on this battery. It is easy to open it up and find 2x 14430 cells inside, along with a control board. The control board is important because it is this that determines if the battery is usable or not, not the state of the batteries. I desoldered the batteries and put them individually in a smart charger, one was slightly down and the other was very low. The smart charger recovered them both and both tested as healthy batteries, but when put back into the pack the board would not pass on the full volts to the bike. When I put a multi meter on the battery pack coax, it was still showing a fraction of a volt, and the bike was dead. My assumption is there is circuitry that kills the pack once an imbalance is detected between the cells. So I ordered a new battery. When I plugged in the new battery it lit up the stem controller shifting worked, but showed it needed a charge. This was when I discovered I had a dead charger because nothing was happening now with the good battery. I borrowed another charger and charged the new battery, but when it was done I thought I would see what it did with the old battery....well it showed a flashing red "malfunctioning battery", and still the old battery was not powering up the bike. However, I discovered that a good SM-BCR2 charger seems to remember it's last status because even even unplugged from the bike it continued to flash red. So I tried unplugging from the USB and then a few different sequences of plugging in the bike first, then the wall, wall first then bike and voila the old battery no longer registered a fault, and charged perfectly.
3. There are tons of variables in this story so make of it what you will. I do believe that without desoldering the 14430 cells from the pack and renovating them on the charger that the pack would never have recovered even with a good SM-BCR2 charger, but I will never know. I did discover you can effectively remove the cells (worth about 3.50 eur each, check out youtube "What is inside of a Shimano di2 Internal Battery" Sorry I can't post the link) and charge them with a suitable Lithium Ion smart charger, but I believe the battery pack controller circuitry is designed to kill a pack and force a replacement. Maybe I managed to fool that circuitry into reseting by trying different sequences with the SM-BCR2 charger. Anyway, now I have 2 well functioning battery packs. It says to me that you can replace the cells in a Di2 battery pack when it's dead, and the soldering isn't complicated. Photos below....oh no they are not because I haven't posted enough yet.
Last edited by Girona_Kiwi; 01-14-22 at 10:07 AM.