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This weekend, my daughter asked me to take her on an impromptu trip to Christkindlmarkt, a local German Christmas market. It was about an hour away, and it looked like I had just enough charge to make it there and back comfortably, so we headed out. However, as one might expect, this was a busy venue, and between traffic and parking it added about an hour to the trip. We arrived with 25% charge, making it look like I would definitely want to stop and fast charge for at least a few minutes on the way home.
We walked around, explored the shops and performances, then made our way back to my Macan. As I sat down in the driver’s seat and began looking up directions to the fast charger, I discovered that while we were shopping the car had charged itself - we were now at 40% and had more than enough range to get home! Could it be that my Macan came with an undocumented feature - a cosmic ray collector that could charge the car when needed, from seemingly nothing, without prompting?
Pleased, we started home and indeed made it all the way back with another 15% to spare. It was real - “found energy”. We went inside, and I checked the car about an hour later. It had risen to 22% before leveling off. In all, between 15% at the market and 7% at home, I collected an astounding 22% charge yesterday from thin air.
On its own, this sounds miraculous, but it was not the beginning of the story, and by the time it happened I actually had a pretty good idea what was going on.
Friday morning I took a trip to drop off some skis for tuning, and thought I’d stop and fast charge on the way home to collect information about how the Macan handles cold weather charging. The car had sat at a relatively low 40% all week so after the trip to the ski shop it would be ready for a deep charge test. However, when I got to the fast charger I discovered something troubling - I couldn’t charge faster than about 35kw. I tried three or four different cabinets, then drove 8 miles to another location, only to find the same result. By this time between the multiple false starts to charging, the battery was in the “sweet spot”, nice and warm. This level of performance couldn’t be explained away by a cold battery!
I charged up enough to get home and then booked a service appointment (two weeks out). In the meantime I began wondering if the battery needed some sort of recalibration, which I’ve heard of vaguely. Usually someone will just make an off-hand remark that it’s a good idea to occasionally run the battery down and then charge it fully. The idea being that the vehicle battery has many cells and if your cells aren’t evenly charged then it will severely limit both the capacity of the battery but also the rate of charge you can achieve, since it won’t be able to charge all of the cells in parallel at the same rate. I sort of think of it as “fragmentation” in old school computer jargon, leaving gaps in your usable space and a lot of capacity stranded.
Indeed I came across a few posts about instructions for a battery “rebalancing” on Taycan and Macan forums. I’ve also seen various posts over time about people having trouble with low DC charging speeds (referring to a Porsche service procedure), surprisingly low range at high SoC, and other issues that could have similar root cause.
Further investigation found that there are two general kinds of rebalancing - a “top balancing” where a pack full of cells that is mostly charged will top off low cells with energy from high cells, and “bottom balancing” where “empty” cells and those with some charge left are leveled off. This is important because the collective performance of the pack can be limited on the top end by the cells that are most full, and on the low end by cells that are most empty, as the car relies on pulling power from many cells together.
This is the case for all EVs, I think, but additionally the Macan has two “banks” of cells, a split pack as a part of its 800v design that can split into a 2 x 400v. So the battery management is quite complicated.
So Friday night I charged the Macan up to 85%, trying to trigger a “top balance” and when I woke up the next morning the vehicle was down to 75%. It seems implausible that the car just vaporized 10% (~9kwh) of energy overnight, so to me this was an indicator that some rebalancing had occurred, some energy had shuffled around and made room for more charge. Or perhaps even just a recalculation of available capacity. I charged it up to 100% first thing in the morning, let it sit a bit, and then took it for a drive to burn off some of the energy.
It was Saturday afternoon that my daughter came to me wanting to go to the market, where I believe a “bottom balancing” occurred while we shopped, and again later at home. My theory is that when I arrived at the market I had some empty cells artificially limiting the 0% charge point. While we shopped those cells got charged up by other cells, leveling things out and restoring usable range, the “phantom charge”.
I did not charge on Saturday after getting home. I wanted to give it more time to rebalance, if it could. On Sunday I headed out to pick up my skis that I had dropped off Friday, having just enough charge to make it to the shop and then to a DC fast charger, and when I arrived at the fast charger I found my fast charging speed had been restored, for the most part. Hard to say exactly where the peak would be because the station was quite busy and the cabinets share energy, but I hit 200kw so I was happy.
The moral of the story, for me, is that it is indeed good to stretch the legs of the battery occasionally and remind the battery management system of its full capacity.
I have some theories of how I got things so imbalanced.
1) My car has been somewhat of a garage queen the last few months, only driving 700 miles in two months. Generally keeping the charge between 50-80%.
2) My trips are usually fairly short. Less than 10 miles maybe three times a week. I don’t know how the car pulls energy from the battery, but if it doesn’t always pull from all cells evenly all the time I could see how short bursts of usage might create “gaps” in charge that worsen over time if you never hit a rebalance trigger.
3) I have a solar AC charger, which is great but it throttles supply based on available excess solar power. This means that if my house suddenly needs more power, or if a cloud passes overhead, the charge to the car can lower to what’s available. You can go from 9.6kw on a sunny midday down to 1.5kw on a cloudy day. I don’t know if the car charges cells in subgroups, or if it always charges all cells in parallel, or what exactly. I can imagine it might interleave charging smaller groups of cells when presented with a lower power AC charge.
Definitely something I’ll be keeping an eye on, and I’m glad to have discovered it and experimented with it in somewhat of a controlled situation, rather than randomly discovering it on a road trip. I will take note to be better about exercising the full range of the battery occasionally.
Working through it, I imagine if I had been planning a trip, I would have charged up the night before, rather than leaving at 40%, which may have triggered the top rebalance, and then it would have auto-charged the newly freed space as well overnight. So I may have contrived the situation a bit inadvertently, but clearly it needed the bottom balancing as well.
This could explain all sorts of weirdness reported on the forums though, from slow DC charging to “I lost X kWh overnight”, to “My car shows 160 miles of range at 80% charge”.
I’m keeping the service appointment and will ask if my car has the latest software or not.
We walked around, explored the shops and performances, then made our way back to my Macan. As I sat down in the driver’s seat and began looking up directions to the fast charger, I discovered that while we were shopping the car had charged itself - we were now at 40% and had more than enough range to get home! Could it be that my Macan came with an undocumented feature - a cosmic ray collector that could charge the car when needed, from seemingly nothing, without prompting?
Pleased, we started home and indeed made it all the way back with another 15% to spare. It was real - “found energy”. We went inside, and I checked the car about an hour later. It had risen to 22% before leveling off. In all, between 15% at the market and 7% at home, I collected an astounding 22% charge yesterday from thin air.
On its own, this sounds miraculous, but it was not the beginning of the story, and by the time it happened I actually had a pretty good idea what was going on.
Friday morning I took a trip to drop off some skis for tuning, and thought I’d stop and fast charge on the way home to collect information about how the Macan handles cold weather charging. The car had sat at a relatively low 40% all week so after the trip to the ski shop it would be ready for a deep charge test. However, when I got to the fast charger I discovered something troubling - I couldn’t charge faster than about 35kw. I tried three or four different cabinets, then drove 8 miles to another location, only to find the same result. By this time between the multiple false starts to charging, the battery was in the “sweet spot”, nice and warm. This level of performance couldn’t be explained away by a cold battery!
I charged up enough to get home and then booked a service appointment (two weeks out). In the meantime I began wondering if the battery needed some sort of recalibration, which I’ve heard of vaguely. Usually someone will just make an off-hand remark that it’s a good idea to occasionally run the battery down and then charge it fully. The idea being that the vehicle battery has many cells and if your cells aren’t evenly charged then it will severely limit both the capacity of the battery but also the rate of charge you can achieve, since it won’t be able to charge all of the cells in parallel at the same rate. I sort of think of it as “fragmentation” in old school computer jargon, leaving gaps in your usable space and a lot of capacity stranded.
Indeed I came across a few posts about instructions for a battery “rebalancing” on Taycan and Macan forums. I’ve also seen various posts over time about people having trouble with low DC charging speeds (referring to a Porsche service procedure), surprisingly low range at high SoC, and other issues that could have similar root cause.
Further investigation found that there are two general kinds of rebalancing - a “top balancing” where a pack full of cells that is mostly charged will top off low cells with energy from high cells, and “bottom balancing” where “empty” cells and those with some charge left are leveled off. This is important because the collective performance of the pack can be limited on the top end by the cells that are most full, and on the low end by cells that are most empty, as the car relies on pulling power from many cells together.
This is the case for all EVs, I think, but additionally the Macan has two “banks” of cells, a split pack as a part of its 800v design that can split into a 2 x 400v. So the battery management is quite complicated.
So Friday night I charged the Macan up to 85%, trying to trigger a “top balance” and when I woke up the next morning the vehicle was down to 75%. It seems implausible that the car just vaporized 10% (~9kwh) of energy overnight, so to me this was an indicator that some rebalancing had occurred, some energy had shuffled around and made room for more charge. Or perhaps even just a recalculation of available capacity. I charged it up to 100% first thing in the morning, let it sit a bit, and then took it for a drive to burn off some of the energy.
It was Saturday afternoon that my daughter came to me wanting to go to the market, where I believe a “bottom balancing” occurred while we shopped, and again later at home. My theory is that when I arrived at the market I had some empty cells artificially limiting the 0% charge point. While we shopped those cells got charged up by other cells, leveling things out and restoring usable range, the “phantom charge”.
I did not charge on Saturday after getting home. I wanted to give it more time to rebalance, if it could. On Sunday I headed out to pick up my skis that I had dropped off Friday, having just enough charge to make it to the shop and then to a DC fast charger, and when I arrived at the fast charger I found my fast charging speed had been restored, for the most part. Hard to say exactly where the peak would be because the station was quite busy and the cabinets share energy, but I hit 200kw so I was happy.
The moral of the story, for me, is that it is indeed good to stretch the legs of the battery occasionally and remind the battery management system of its full capacity.
I have some theories of how I got things so imbalanced.
1) My car has been somewhat of a garage queen the last few months, only driving 700 miles in two months. Generally keeping the charge between 50-80%.
2) My trips are usually fairly short. Less than 10 miles maybe three times a week. I don’t know how the car pulls energy from the battery, but if it doesn’t always pull from all cells evenly all the time I could see how short bursts of usage might create “gaps” in charge that worsen over time if you never hit a rebalance trigger.
3) I have a solar AC charger, which is great but it throttles supply based on available excess solar power. This means that if my house suddenly needs more power, or if a cloud passes overhead, the charge to the car can lower to what’s available. You can go from 9.6kw on a sunny midday down to 1.5kw on a cloudy day. I don’t know if the car charges cells in subgroups, or if it always charges all cells in parallel, or what exactly. I can imagine it might interleave charging smaller groups of cells when presented with a lower power AC charge.
Definitely something I’ll be keeping an eye on, and I’m glad to have discovered it and experimented with it in somewhat of a controlled situation, rather than randomly discovering it on a road trip. I will take note to be better about exercising the full range of the battery occasionally.
Working through it, I imagine if I had been planning a trip, I would have charged up the night before, rather than leaving at 40%, which may have triggered the top rebalance, and then it would have auto-charged the newly freed space as well overnight. So I may have contrived the situation a bit inadvertently, but clearly it needed the bottom balancing as well.
This could explain all sorts of weirdness reported on the forums though, from slow DC charging to “I lost X kWh overnight”, to “My car shows 160 miles of range at 80% charge”.
I’m keeping the service appointment and will ask if my car has the latest software or not.
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