Levenkay
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 4:27 pm

IssacZachary wrote:Let's say all your cells are at 60AH, except one, which is at 30AH. When that cell is depleted all the other cells will still have half their AH's. But at that point you won't able to go any farther. The whole pack will be "empty" but only because one single cell is depleted. The other 95 cells still have another 30AH's to go. It would be technically possible to use a small amount of charge from each of the good cells in order to charge the bad cell up and keep going. It's not unicorn breeding, it's science.
You're imagining some way of transferring energy among arbitrary cells or groups of cells??? I'd be fascinated to learn how you think any such scheme would be implemented. Independent switchmode converters associated with each cell of the battery, each driving a separate winding on some massive transformer? Or maybe stepping up each cell's voltage to pump current into a common (and isolated from the cell) high voltage bus? Or using enough switching semiconductors to allow individual cells to be disconnected from, or spliced into, the battery's series string at will? Do you know of some clever way of doing that that doesn't involve the entire pack's current flowing, in series, through as many extra switches as there are cells in the battery?

It may be simple science, but that doesn't mean it's practical.

LeftieBiker
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 6:11 pm

To put it another way, a BMS can do two things: connect cells to the main bus, and disconnect them. I think Isaac is thinking this can be done to dynamically balance cells while driving, but unless the drivetrain shuts down for a few seconds - or minutes - to allow this energy transfer within the pack, I don't see how it could be done either. Maybe if it were configured to do it only while the car was On but in Park it would be theoretically possible, but as I noted earlier, Leaf cells don't degrade one or a few at a time, they ALL degrade more or less evenly. A cell at half the capacity of the rest would be defective, and would qualify for warranty replacement.
2013 "Brilliant Silver" SV with Premium Package and no QC, and 2009 Vectrix VX-1 with 18 Leaf cells.

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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 6:23 pm

LeftieBiker wrote:The scenario you describe would be a manufacturing defect, because the cells generally degrade more or less uniformly. Any pack with cells at half the capacity of the rest should be eligible for warranty replacement.


Very true! I don't know how uniformily they degrade. However, I do know that it is not completely uniform. And as far as I understand, I am no longer qualified to get a pack replacement. So would it be worth it? As I said before, it would likely be much easier to get a wrecked leaf, or buy a few modules off eBay if you can determine the degredation of them, and replace individual modules.

Levenkay wrote:You're imagining some way of transferring energy among arbitrary cells or groups of cells??? I'd be fascinated to learn how you think any such scheme would be implemented. Independent switchmode converters associated with each cell of the battery, each driving a separate winding on some massive transformer? Or maybe stepping up each cell's voltage to pump current into a common (and isolated from the cell) high voltage bus? Or using enough switching semiconductors to allow individual cells to be disconnected from, or spliced into, the battery's series string at will? Do you know of some clever way of doing that that doesn't involve the entire pack's current flowing, in series, through as many extra switches as there are cells in the battery?

It may be simple science, but that doesn't mean it's practical.


I didn't say it was practical. But it can be done.

For an example, with 95 5PDT relays and 2 3PDT relays, a single isolated step up transformer and a circuit board to measure each cell's voltage, calculate each cell's capacity and charge and to switch the relays and control the transformer the theoretical device could simply find the highest capacity cell and step that up into the lowest capacity cell, then find the next highest and step that into the next lowest, and keep doing that until all the cells are averaged out during the hour or so of driving.
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LeftieBiker
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 6:30 pm

We've seen what happens to range and performance when a single bad cell gets shut off by the BMS during driving. Now imagine two cells getting taken offline while one charges the other. The car would be in Limp Home Mode and would be barely driveable.
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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 6:37 pm

LeftieBiker wrote:We've seen what happens to range and performance when a single bad cell gets shut off by the BMS during driving. Now imagine two cells getting taken offline while one charges the other. The car would be in Limp Home Mode and would be barely driveable.

There's no need to take any cells offline. Simply take a little more current from a better cell and feed it into a bad cell through an isolated transformer. All the cells would still be in series.
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LeftieBiker
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 10:54 pm

There's no need to take any cells offline. Simply take a little more current from a better cell and feed it into a bad cell through an isolated transformer. All the cells would still be in series.


As long as the cells are being drained, they can't be charged. They are being drained at all times except during regeneration. So, as occurred to me just a few minutes ago, it may be possible to dynamically balance the cells by directing regen current only to the lowest cells in the pack; the BMS could take the higher cells out of the circuit for the duration of the regen event, thus charging the weaker cells. This would be much trickier than it sounds, but it might be possible. That isn't what is being claimed for this device, though, AFAIK.
2013 "Brilliant Silver" SV with Premium Package and no QC, and 2009 Vectrix VX-1 with 18 Leaf cells.

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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 11:31 pm

LeftieBiker wrote: As long as the cells are being drained, they can't be charged. They are being drained at all times except during regeneration. So, as occurred to me just a few minutes ago, it may be possible to dynamically balance the cells by directing regen current only to the lowest cells in the pack; the BMS could take the higher cells out of the circuit for the duration of the regen event, thus charging the weaker cells. This would be much trickier than it sounds, but it might be possible. That isn't what is being claimed for this device, though, AFAIK.

Mmm... I like your idea of only recharging the lower cells.

Still, how do you know you can't charge or discharge a cell as it's being used by the rest of the pack? Imagine if you put a lithium-ion cell charger on a single cell as you drive along with the cell still connected in series with the whole pack. Why wouldn't that work? Am I missing something here? Sure, if the charger charges at 10 amps and you're using 50 amps to cruise down the highway then your cell would still be discharging at 40 amps plus the 10 amps from the charger. But that would still be a lower discharge rate than the other cells.
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LeftieBiker
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Re: New device boosts road time for Leaf by 50%

Mon Nov 27, 2017 11:55 pm

Still, how do you know you can't charge or discharge a cell as it's being used by the rest of the pack? Imagine if you put a lithium-ion cell charger on a single cell as you drive along with the cell still connected in series with the whole pack. Why wouldn't that work? Am I missing something here? Sure, if the charger charges at 10 amps and you're using 50 amps to cruise down the highway then your cell would still be discharging at 40 amps plus the 10 amps from the charger. But that would still be a lower discharge rate than the other cells.


You can certainly discharge a cell in use - that's what is happening. Charging a cell means reversing the direction of current flow, and that can only happen by taking it out of the main bus circuit and charging it with no drain. What you describe above is reducing the drain from a cell, not charging it. I suspect that what you propose would result in little or no net gain of range, because you would be adding extra losses to the system by adding that "internal charger" and also by making the other cells work harder.

I think that using regeneration current to briefly charge only the weakest cells (first, until balance was reestablished) would be the most workable method for dynamically balancing cells, but I really don't know that much about this subject. If I didn't have an electric bicycle and electric motorcycle (using Leaf cells) I'd still know virtually nothing. A recently as 2012 I struggled with the concept of AC motors. ;-)
Last edited by LeftieBiker on Tue Nov 28, 2017 10:29 am, edited 1 time in total.
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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 7:45 am

The system could charge a cell in use if your "cell charger" is putting out more than what the pack is draining. For an example, if you're draining the pack at 50 amps and you're feeding 100 amps into a cell, it would be charging at 50 amps, the other 50 amps being used to drive the pack. But simply discharging it at a lower rate would be plenty fine to help extend range if that cell has degraded more than the other cells.

Now yes, it would be putting a greater load on the other cells, but that's kind of the whole point if you have cells that are degraded more than other cells. Or like you said, do it when regenerating. But regardless, whether it would be helpful or not would all depend on the efficiency. If losses were less than 10% it could potentially help range. After all, say half your cells are 60AH and the other half are 50AH then you only have about 5AH to transfer between them during driving. At a 90% efficiency that would be 4.5AH transferred with about 0.5AH lost. So your total AH would be about 54.5AH instead of only 50AH. Almost a 10% gain in range.

The problem with regenerating only the lower capacity cells is that if you only put the lower capacity cells in series your voltage will be much lower. A lower voltage means you would have a lot less "motor braking." You could potentially lose valuable regen energy and actually lower your whole range. Plus I don't see any practical way of taking cells out of series and putting them in their own series while driving.
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LeftieBiker
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 10:31 am

The weaker cells would remain connected to the main bus. Voltage would be lower, but this balancing regen would hopefully only last a couple of seconds, and not occur every time you braked. You could perhaps even select it as a 'balance mode'.
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