Hi and
to the forum.
I now have a few questions for you:
1) Were you able to save any changes to the controller before your initial test ride?
2) Did you apply the brakes at high speed during the test ride?
3) Has anything been altered on the electrical installation since the test ride?
My gut feeling is that the EM3EV 52V battery may have inadvertently damaged the controller while trying to protect the battery from being overcharged:
A 2 wire type BMS such as ours, costs more (it requires more mosfets) than an equivalent 3-wire type, but it means that the battery can be ran in parallel with another equivalent battery and also means it is fully protected against over-charge (impossible with a 3-wire type BMS that are generally used in this application).
I suspect that the
BMS on this battery is suddenly disconnecting the battery from the controller while it is under high current regenerative charging, and disconnecting the battery under load can result in damage to the Magic Pie's controller, due to a sudden voltage spike that has nowhere to go except back into the electronics of the controller.
Loose connections on a battery cable can also result in controller failure for the similar reasons
(sudden disconnection of the battery under load).
There is another post on this forum regarding regen problems with a EM3EV battery where regen would not work unless the battery voltage was below 55V:
Hi, new member here. Hoping for some help getting my Magic Pie 4 regen working properly with a new battery.
I purchased my MP4 2.5 years ago with a 36V golden motor battery and everything worked flawlessly, including strong regen.
I recently decided to upgrade the battery to a 50V 14.7Ah battery from em3ev. The battery is awesome, and really gets good power out of the MP. The only issue is that regen only works when the battery is under 55V or so. Full charge is up over 58V, so I need to ride for many miles before I can use regen. This is especially annoying because I live at the top of a large hill...with my old battery it was nice to be able to regen on the way down, but that is no longer possible unless I only charge to 60% or so.
I have a Cycle Analyst so I can see exactly where it cuts off...if the battery is around 55V and I hit the ebrakes, V will spike up to around 56V and I get no regen. If the battery is a little lower and the Voltage spike is like 55.8 V I will get some weak regen (-100W or less), but not much.
If the battery is much lower...like under 54V, then I get full regen (-600 or more W).
This makes me think that the overprotective
BMS on the EM3EV battery could potentially cause controller failure if it suddenly opens the battery circuit as the controller voltage rises above 56V during high speed regen braking.
Although your battery worked fine with your Bafang mid drive setup, that motor is incapable of producing regen and would therefore not produce any voltage that could cause the
BMS high voltage protection to operate.
Another forum member
(also using a EM3EV battery) has recently suffered controller failure after
accidentally activating his regen on his MP5.
Have a close look at the 5 pins on the throttle connector to make sure that none of them have been accidentally bent over, and also make sure that there is no signs of water inside the connector.
If you had another throttle to test, it should at least prove whether the original throttle was faulty, but if my suspicions are correct, I think it could be the controller itself that has failed.
Alan 
