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Slow supercharging

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Anyone experienced only 30kw supercharging at South Mimms recently? For the last three months about half the times I visit it doesn't get above 30kw even though I am in a 2018 X 100D with about 120 miles in the battery. One time it started out at 30kw and after 10 minutes went up to 70kw or more but it seems increasingly rare there to get 120kw. I know it slows down for the last 15% or so but this is getting annoying and turns a 40 minute session into 90 minutes. I saw a lot of cars arriving and leaving more quickly last might and guess they may have had the same issue. I get free supercharging. Any thoughts?
 
One time it started out at 30kw and after 10 minutes went up to 70kw or more but it seems increasingly rare there to get 120kw.

Was there another car on the paired stall? This sounds like normal behaviour for that case: you arrive and are the second car sharing the supercharger, so get only 30kW, then either (a) the car that was there first departs and you now have "first dibs" on the supercharger, but by this time your car has reached a level of charge that it can't take the full 120kW, or (b) the car that was there first is getting closer to full so is using less than 70kW and another 30kW "chunk" can be released for you to use.
 
Was there another car on the paired stall? This sounds like normal behaviour for that case: you arrive and are the second car sharing the supercharger, so get only 30kW, then either (a) the car that was there first departs and you now have "first dibs" on the supercharger, but by this time your car has reached a level of charge that it can't take the full 120kW, or (b) the car that was there first is getting closer to full so is using less than 70kW and another 30kW "chunk" can be released for you to use.

Thanks. Not sure what a paired stall is? There are about 12 separate stalls there and when I arrived about 6 or 7 were in use. About 4 cars then left over the next 15 -20 mins but the charge on my stall stayed at 30kw. By then I was up to about 140 miles with the max in cold weather usually 247.
 
Thanks. Not sure what a paired stall is?

They all have numbers on them - 1a to 6b.

Each pair (1a/1b, 2a/2b etc) shares a supercharger cabinet with a total of 145kW of capacity. The first car to arrive gets as much as it wants. When a second car arrives, it gets only whatever capacity the first car isn't using, except that a minimum of 30kW is given to the second car, and the capacity can only be shared in large chunks.

So, when arriving at a supercharger you ideally pick a stall that isn't already shared with someone else - though if half the stalls are in use then you may not have the option. If you are forced to share and can guess who is likely to leave next, sharing with them is obviously best - obviously you can't easily guess, but if you followed someone else into there, sharing with them is a bad idea.

1A vs 1B doesn't matter in itself - it's whoever got there first out of A and B that gets the lion's share.

At most sites, the stalls run 1A,1B,2A,2B along the side, so people tend to pick the best stalls just avoiding parking next to someone else, but this isn't the case everywhere (a few sites have all the 'A' on one side and all the 'B' on another), so it's best to check the numbers if you care.

Usually the impact of picking a shared stall isn't that bad - you have to be unlucky to pick someone who arrived not long before you, was completely empty, and intends to stay there until fairly full.
 
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Thanks so much and this makes perfect sense now you have kindly explained. This rather undermines the supercharging offering doesn't it if you have to play tactical games instead of getting 120kw from every stall automatically. Is this just the UK or worldwide?
 
Is this just the UK or worldwide?

Worldwide, the equipment is the same everywhere.

The only exception is the relatively small number of new "Urban Supercharger" installations (I think only in USA/China so far), where the sharing is fixed at 50:50 (so 72kW each) rather than first-come-first-served. These are typically at city-centre shopping malls, with significant numbers of local owners without home charging who want to go for their "weekly fill up". For that use it's apparently considered more useful to have a long but consistent charge time rather than the typically-shorter-but-sometimes-you-are-unlucky system used on en-route superchargers.

Some locations are slightly worse (very old locations in USA, or European locations with lower grid voltage), where the amount of power to be shared between the two stalls is lower, but the mode of operation is the same.

Whether this "undermines" supercharging depends how you like to look at it. Obviously in an ideal world there'd be enough capacity for everybody to charge at maximum rate all the time. However, that's infeasible to provide and at peak times there will be more cars than capacity. Would you prefer there to be 7 full-speed stalls and a higher risk that you have to queue, or 12 stalls as at present where there's a risk that your charge might take a little longer?
 
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Pablodog, is it possible that you are arriving with a cold battery? Is your regen limited when you arrive? Are you driving for a long enough period before supercharging?

I suggest reading these two pages:
Supercharging
Supercharger | Tesla

Tummy, thanks and I had been driving for 1 hour. Not sure what you mean by regen limited - I haven't reduced the regenerative braking if that's what you mean? I think is is as arg has said and I need to be more savvy when selecting a stall.
 
Tummy, thanks and I had been driving for 1 hour. Not sure what you mean by regen limited - I haven't reduced the regenerative braking if that's what you mean? I think is is as arg has said and I need to be more savvy when selecting a stall.
By regen limited, do you have the yellow dotted lines around the circular energy display in the instrument cluster? That would indicate that your battery is cold.

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