Optimized charging for longer trips?

Fredrik Lidén

Fredrik Lidén

Member
I have been trying to figure out what tactics to use when driving for longer trips with the Mini Cooper SE. Apparently the charging curve drops quite abruptly at about 75 to 80% charge, so it is perhaps a waste of time to keep charging past 80%. The charging also takes longer if the battery is below about 20%. So in this case the optimal driving would be to stop and charge when the battery is at about 20% and stop charging at 80%. This would get you from one place to another in the shortest time!? Comments are very welcome
 

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Are you charging at a DC fast charger? The worst case time from 0% to 80% is typically around 35 minutes. Not too bad. But, yes, above 80% it definitely slows down. On my road trip last week I was getting a charge from 30'ish% to 80% in 21 minutes (two separate times).
 
The picture I enclosed was from FASTNED, DC fast charging. The Mini appears to be able to receive between 40 and 50kW for most of the charge curve according to their measurements. It will be different with other types of charging of course. I presume the curve will be straighter for lower capacity charging (but I have not seen any data).
 
BMWi3_022020.webp
This is data for a BMW i3 and I believe that the Mini Cooper SE has the 33 kWh battery (blue curve above). In this case it looks good all the way down to below 10% charge, and charging above 80% takes much longer time.
 
The i3 uses a Korean Samsung battery and the SE uses a Chinese CATL battery (which was touted as having better cold-weather performance than the Samsung). I would guess the two batteries display different charging profiles. My experience parallels that of @GvilleGuy.
 
Fredrik Lidén said:
The charging also takes longer if the battery is below about 20%.
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That seems strange to me, the slowdown at the top end is to protect the battery from overcharging. Is it a battery chemistry thing, or something to do with EVSE ramp-up or handshaking? It'd be an interesting exercise to measure CCS charging at different starting points, like 10%, 20%, 30%, etc.

My real-world experience is that the SE does a great job charging at near maximum until 80%, and then tapers slowly until somewhere between 85%-90%, then slows way down. I basically stop charging around 85%, since the time to get the last 15% SoC isn't worthwhile. Keep in mind that's only adding about 20 miles of range.
 
This is my strategy for trips:
  • Find hotels with charging by using Plugshare's lodging filter, or AirBNB's new EV charging filter
  • Plan your longer stops around mealtime or points of interest (the curve doesn't really hit a wall until around 95%).
  • Get a TeslaTap (countries where the Tesla connector is used) so you can take advantage of Tesla's vast AC charing infrastructure (not the Superchargers... yet!)
  • Plan your trips with A Better Route Planner. Install and register with ALL charge station apps BEFORE going on the trip
  • Don't drive too fast. Aerodynamic drag is exponential. Headwinds, rain, and cold weather all eat info efficiency. Pre-condition the cabin while on grid power.
  • Pack lots of water and things to do. I like to take photos at charge stops and go on short walks. Before you know it the car is waiting on you :)
  • When in town, keep an eye out for charging stations. They are often in prime parking locations. Of course, do not park without charging!
 
So i looked at the MINI app and have the following results. I have a 50kW charger (free) with a 1/2 hour limit. The limit is only new, so I do have some longer sessions from a few months ago. From real data:

9-80 in 33 minutes
18-85 in 31 minutes
25-90 in 32 minutes
31-95 in 36 minutes
20-100 in 1h 7 minutes

subtracting differences between similar start percentages gives the extra times, so:

85-93 in 9 minutes
95-100 in 24 minutes

This leads me to think that for a long journey, going to 90 adds less than 8 minutes (from 80). From 90-95 somewhere around 8-10 minutes, but from 95-100 takes almost as long as from to 25-90%. Not worth it unless you need every km (mile) of range to your next stop. For my money, the sweet spot is 90% - going to 100 adds almost 1/2 hour for 25km (15 miles) range.
 
I haven't done many fast charging sessions, but vader's excellent data reflects my own experience. I was pleasantly surprised at the relatively low time penalty for charging from 80% to 95%.

The Fastned charging curve for the SE can be found in this Inside EVs article.

As I understand it, the recommended strategy for road-tripping EVs is to arrive at the fast charger with the lowest state of charge you're comfortable with and charge no more than necessary to make it to the next fast charger. That should keep you in the "meat" of the charging curve and minimize the amount of time spent at each charger. Of course, this assumes you don't need to charge past the curve drop-off to make it to the next fast charger. ;)
 
I've now read this from several unverified sources (forum and YouTube comments) - that a 0-to-100% charge on the Mini is around a 4-to-92% charge with the built-in buffer. What does our forum brain think of this figure? Would it be in the ball park of correct?
 
The gross capacity of the battery is 32.6kWh and the available capacity is 28.9kWh, which means we have 11.34% buffer total. I don't know how that buffer is distributed between the "top" and "bottom" of the battery, but the range you give adds up to 88%, so should be in the ballpark.
 
GvilleGuy said:
I've now read this from several unverified sources (forum and YouTube comments) - that a 0-to-100% charge on the Mini is around a 4-to-92% charge with the built-in buffer. What does our forum brain think of this figure? Would it be in the ball park of correct?
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Normally the bottom buffer is larger since running a battery down to 0 volts can lead to permanent damage. If a car is left for over a year at 0% user SoC it should be able to survive due to the bottom buffer.

The i3/SE have no charge limit controls and the manuals say to fully charge the car as often as possible. I found it interesting that they warn the battery must be charged to at least 80% when stored for 4+ weeks.

Based on that, the 4-92% distribution makes sense. I also have an Audi e-tron which splits the battery 8-96%. Later models reduced the top buffer to 8-98.75%.
Puppethead said:
Which is why we can ABC – Always Be Charging. The MINI battery is designed to protect itself from under- and over-charging, unlike those poor Tesla owners who have to carefully monitor their charge state to avoid battery damage ;).
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I don't really see that as a downside. It's great to have that extra battery capability for trips. VW, Audi, Porsche, and Tesla all have daily limits in the 80-90% range. It will be interesting to see how the Ioniq 5 and EV6 systems work.
 
Puppethead said:
It was meant lightheartedly. I actually equate Tesla owners (and apparently VW, Audio, Porsche) as power users while MINI owners have a keep-it-simple EV.
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I have a Porsche Taycan in garage (SE on the way -- fingers crossed). It's not overly complicated. I have my General charging profile set to 85%. Like the Ronco Showtime Rotisserie ... I just ....((studio audience)) set it and forget it.
 
Novadar said:
I have a Porsche Taycan in garage (SE on the way -- fingers crossed). It's not overly complicated. I have my General charging profile set to 85%. Like the Ronco Showtime Rotisserie ... I just ....((studio audience)) set it and forget it.
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What a perfect pair!
 
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