I finally found an a rear battery, on a local almost-wreck that has a sordid history but able to pick it up for a fair price.
They had no charger and had run the frame battery flat, it was just over 5V when I measured it, which helped me be disappointed and value it down.
The rear battery had 36V no load so fortunately hadn't been used up without a charger.
The guy tried telling me that the manual said to fully drain often, yet he had no manuals or charger, no key, lock was broken/stolen by neighbours 4 times (state housing townhous complex in neglected/abused condition), original owner deceased estate mother inlaw who died of lung cancer (so that puts me as a rider in wimpy category as someone with terminal respiratory disease). Someone drilled through the frame to self tapper screw mount a water bottle holder, and lots of weather exposure corrossion to everything, be interesting to see inside the frame if water followed the coarse threads.
Anyway nice to have it in my hands, well, not that nice, the grips have a sticky feeling and are worn in a way mine with 4 months commuter use aren't worn, maybe sun weathering but gut feeling isn't a positive one. Charged the frame battery up to 41.6V same as the rear battery did, and both have dropped to 41.3V in the same time. I haven't load tested the frame battery but am not expecting anything from it, but if it does survive, will give it a hard time on keep cool morning commute tests.
With pedalling taller gearing the stock A2B Metro setup has got me to work in record time faster than any pedal bike and only a few minutes slower than 72V 2kW due to several stop/starts in commute. Practically there isn't any commuter gain for me deviate until something stock fails, but at least this black one has a lot more neglect and likeliness of something failing =P
It's time to upgrade to a rear shock now, the spring is bad/dangerous and I haven't ridden any other bike through a speed turn with bump that has handled it as badly as stepping the tail out. Lots of things I like about the A2B Metro, and it holds the record for able to dig out 5mm glass shards in tyre tread without incident or getting close to the tube. Only a handful of dry punctures through the year didn't phase me on the others, but it's a nice dependable quality for winter when glass cuts dominate, and I rode first half of winter on the white A2B Metro this year.
History: Front hub SS 200W, China E-mtb 200W, 3x A2B Metros, Commuter Booster beta, 2.4kW d/s w/DD kit, MAC 1.5kW road/flatbar 1250Wh EM3EV frame batt, BBS02 750W d/s MTB 1250Wh frame batt. All sold (except A2B Metros pending TLC from storage).