The figure it gives for range after a charge is based on historic values, so is very dependant on how it's been driven. To get an idea of the state of the battery in terms of usable capacity, run it down to a known percentage (check the value before you charge), then charge it up and monitor how long it charges for as well as the rate at which you are charging (set on the dash, you can also monitor it using Mercedes Me). Work out the kWh from that. It'll only be a rough idea as we'll never know for sure how much energy is used to cool the battery during charge plus efficiency of charger etc, but it's a guide. From that, you should be able to work out, again, roughly, what capacity the battery says it has. Further complicated by the fact we don't know what 100% means - it won't be the full 6.38kWh I don't think as that would be very bad for the battery. Probably likely to be about 90% of actual capacity? - could even be a bit less to try and give the battery a longer useful life. Similarly, 20% might not be real either. Again, probably 20% of allowed usable capacity, as discharging to 0% will be very bad for them too. Like I say, it'll be a rough guide. Wouldn't surprise me if we only get to use about 5kWh.
The mileage you get from the battery is very dependant on many factors. Speed, acceleration, accessories you may have running like A/C, demist, terrain. They aren't particularly efficient on pure battery power, sadly, as there is that auto box sitting in the way.
The 'guess-o-meter' on the dash is a bloody liar by the way. Hopelessly optimistic, particularly in winter.
I'm not aware that you can improve the battery condition with charging. If it has suffered due to some issue, the loss of capacity will be permanent - a fact of life with lithium-ion chemistry.
No idea on the noise though - if it's tyres or anything on the driveline other than the ICE itself, then you'd hear it when on battery power. It'll do motorway speeds on battery , so you should be able to test that theory easily enough. You can also easily force the ICE to kick in by putting it in Sport so it'll be very easy to see if its ICE or 'something else'. If it does seem like ICE related, you can always mess with the gears to see if it happens at specific engine revs. rather than a specific roadspeed IYSWIM.
If you've read much of this thread you'll no doubt(!) have seen me pissing and moaning about how unrefined the ICE sounds on this car IMO. Particularly bad when cold. That said, it doesn't sound anything other than vaguely 'drone-ey' inside the car on ours once its warmed up and driving at normal speeds/revs. Just don't sit at the lights and rev it to try and impress passers-by. An AMG V8 it ain't. The 2l Zetec in our Westfield sounds epic in comparison.
