I'll add my own experiences to the HV battery failure knowledge in hopes it may encourage others to heed 12v battery problems more seriously.
In the UK only 180 451EDs were sold hence a lack of familiarity in the dealer network. I bought ours used in 2016 at three years and 7,000 miles old. Finding one at all was tough. I had to travel far to see and buy a coupe model. It was on a BAP contract.
I was concerned that the dealer had had the car a little while. One of the ad. photos showed 0% SOC. Troubling but it was on sale-or-return and I was itching to drive electric. What could possibly go wrong?
As soon as the car was delivered I built the ED BMSdiag unit and it gave the following (short-form) results which looked all good to my untrained eyes.
Reading data........
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Time [hh:mm]: 16:01, ODO : 11803 km
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SOC : 99.5 %, realSOC: 96.7 %
HV : 384.4 V, -1.80 A, -0.69 kW
LV : 14.3 V
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ENTER command... (? for help)
CMD >>rpt
Reading data....................OK
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--- Battery Status Report ---
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Time [hh:mm]: 16:01, ODO : 11803 km
Battery Status : OK
Battery Production [Y/M/D]: 2013/7/11
Battery verified [Y/M/D]: 2013/7/11
realSOC : 96.7 %, SOC: 99.5 %
Charged capacity : 50.3 Ah, min. Cell# 15
BMS estimate : 50.9 Ah, value @25degC
Measurement done : 75 day(s) ago
* Watch timer for OCV state, redo test! *
Estimation factor: 0.753, poor!
*** Don't trust and redo measurement! ***
OCV timer : 6056 s, @12.6degC
OCV state : HV ON, can't meas. OCV
DC isolation : 62324 kOhm, OK
Voltage Distribution (dV= 25 mV):
*-------------------------->----[==]----*
4115 01 > [4135; 4135; 4137] < 00 4140
min [p25; median; p75] max
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Getting on fine with the car for a few months, then after maybe three or four days of not being used, the 12v battery was flat. Called Smart's warranty line, a tech arrived, left the 12v on charge for an hour, and all went back to normal.
Things continued OK until the following year when - again after a few days not not being used - the 12v was flat again. Figuring the car was now 4.5 years old, it wasn't unusual for a battery to fail so we DIY replaced it. A few more months, a few more days of no use and the 12v was flat again. Only this time, after a 12v charge, we got a 'drive sys. Engine off' error (see photo). This reset itself after a couple of key off/key on attempts and all was fine.
This error came up a few more times after a flat 12v battery, but would always clear when 12v was restored and the key turned a few times.
End of last year the car was due annual maintenance. I had tracked down the flat 12v issues to a phantom ≈4Amp drain, when everything was turned off. So I flagged this to the dealer. Apart from the warnings there'd never been a problem with the HV battery during our ownership, and it was on around 60% SOC when it went in.
Call from the dealer next day with bad news. The diagnosis scan showed both the HV and 12v batteries had failed ('broken' was the technical diagnosis) and they'd also found a 'module fault' (DC-DC converter?).
After 9 months awaiting delivery of a new HV battery, we're finally back on the road with brand new 12v/HV batteries, and the unspecified module replaced, all courtesy of the BAP.
To be fair to the dealer and MB UK they were attentive, courteous and sympathetic throughout, keeping us informed and loaning us various demo MBs and Smarts. And I guess their knowledge of the 451ED has expanded somewhat.