Cell Balancing in Leaf

[Ingineer]

The pack should be self balancing all the time

If you mean it will stay in balance by itself, (passively) then yes. But there is no way it can actively balance with the hardware provided except during a top-balance.

 

[Ingineer]

From an article in AutoBlog Green interviewing Bob Stempel:

Battery makers are paying more attention to cell-to-cell variation: "Once you get those cells where each one is virtually identical to the next one, the pack process becomes a lot less complicated. And people are working on the controls. When we started with li-ion, you literally had to measure the temperature and the voltage drop in every cell, so the complexity of the controls was horrendous. Now it's getting much simpler."

This is the key to making a pack last a long time and the cost (complexity) drops fast when you don't have to have cell-level monitoring/balancing. I suspect the only reason the Leaf even has active balancing is because they want to make absolutely sure it lasts with these first generation cells and there are no problems that could damage the program. Expect future large-scale lithium implementations to eliminate active balancing.

 

[bowthom]

Maybe the dealer performs a "serious pack balancing" during the scheduled maintenance service.
Maybe pack balance anomolies are why fully charged range has been showing some variance between owners.

 

[Ingineer]

Maybe the dealer performs a "serious pack balancing" during the scheduled maintenace service.
Maybe pack balance anomolies are why fully charged range has been showing some variance between owners.

As I wrote above, if there is a "serious" pack imbalance, that likely means defective cells. I imagine your car gets a new pack at that point.

 

[garygid]

What you think is "likely", and what you "imagine" does not necessarily apply to the LEAF battery pack, to what Nissan is or will do with the pack, or even apply to any other currently-known cells.

But, indeed it would be easier if all the cells, and all the conditions inside and around them were perfectly identical, so no cells ever needed to be replaced.

However, in my experience battery cells generally do not enjoy such "perfection", and (almost always) gradually drift out of balance if not "equalized" in some way. There are almost always differences in internal leakage (self-discharge) that create the imbalance, given enough time.

 

[Ingineer]

What you think is "likely", and what you "imagine" does not necessarily apply to the LEAF battery pack, to what Nissan is or will do with the pack, or even apply to any other currently-known cells.

But, indeed it would be easier if all the cells, and all the conditions inside and around them were perfectly identical, so no cells ever needed to be replaced.

However, in my experience battery cells generally do not enjoy such "perfection", and (almost always) gradually drift out of balance if not "equalized" in some way. There are almost always differences in internal leakage (self-discharge) that create the imbalance, given enough time.

Well Gary, I don't have to "imagine" all the experience you "likely" have with these new high tolerance, large-format Lithium cells... Forget what you know about the ones you've experienced, as these are different animals!

Come back in a few years when the real-world results are in, then you can point fingers. Until then we are all speculating to some degree, even Nissan!

I am simply trying to share my knowledge, If you choose to believe something else you are welcome to it.

Since we've got a few years to kill, Here's a good video I recommend you watch. (I promise it's not sarcastic! =)

 

[garygid]

A cell goes bad, then they all go bad at the same time?

Or, no cell ever goes bad?

If a cell is replaced in an older pack, the new cell amoung older "brothers" will be different.

True, new chemistries might be developed such that different "equalizing" methods (like over-charging) can be used.

However, I thought we were discussing what this LEAF battery pack will do, not a pack 10 or 20 years in the future.

You "imagine" Nissan will replace the Pack if one cell goes bad. I expect them to replace the bad cell, probably using the procedures described in their LEAF Service Manuals.

So, apparently our expectations differ, and you might indeed have information that I do not.

 

[charlie1300]

Well, this is all very interesting speculation, but I've noticed the OP has dropped out of the discussion. What ultimately happened to the Leaf in question? Did he take it to the dealer to have the battery checked? Did a full recharge make it better? Is he just being extra conservative with his trips? Kelangst, are you still with us?

 

[Herm]

Temperature variations will also age cells at different rates.. perhaps Nissan will mix&match modules from different packs (with a similar history profile) when they repair a defective pack... if they keep track of the history of each module. I would love to get hold of a bunch of those surplus used cells

All these cells, even new ones, will need occasional balancing.. still I hope Nissan is only using 80% of a 30kwh pack.

 

[mwalsh]

still I hope Nissan is only using 80% of a 30kwh pack.

+1. That would be nice.

 

[Randy]

I'm by no means a battery expert, but if Nissan were using 80% of a 30kwh pack, then why would they have gone to the trouble of offering an 80% battery charge timer for longer battery life?

Wouldn't that be 80% of 80% or about 64% charging? That doesn't make sense....

 

[AndyH]

Right, I added the same kind of balancer to the 21 cells on my E-motorcycle.

The trick is in making sure that the charging current stays low enough for long enough, and that the heat is dissipated properly. I have the shunts OUTSIDE the battery pack, so that excessive battery heating is not an issue.

If the imbalance is (for example) 10% on a 60 amp-hour battery, a half-amp shunt has to work for 12 hours to equalize. That would be sixteen 45-minute "balance" periods for just a 10% imbalance.

The main trick is getting all the cells well-balanced the first time.

Then, one can usually just balance occasionally to maintain the balance.

10% is extremely bad! I doubt seriously the cells in the Leaf are that poorly manufactured! Careful production controls can yield near-identical cells which if treated identically will not need ANY balancing. This is why as battery technology improves we will not need to have balancers. The EV-Energy cells (Panasonic/Toyota) used in the PHV Prius are such an example. If you want a good understanding of Lithium-Ion battery technology from an expert, [here] is a good video from Jay Wittacre @ CMU.

We've had that video since mid-December.
http://www.mynissanleaf.com/viewtopic.php?f=9&t=2002

Careful production controls (and post-production QC/screening/culling) can allow a manufacturer to build packs from very well matched cells (or parallel stronger with weaker if that gives the desired performance). However - and this is significant! - LiCo have the longest production track record in the lithium ion 'world' and they still can't produce perfect cells. LiFePO4 has about 10 years of production time and they're nowhere near 'near-identical'. The PHEV Prius is using NCA cells (lithium nickel cobalt aluminum) and like the LiMn in the Leaf, they're still somewhat young.

As for management and balancing - even if the cells come off the line with absolutely identical properties, the cells change as they age. So even perfect cells today will become less perfect as time goes on.

Management is needed in a new pack and will be more necessary as the pack's used.

IF Nissan's using some type of resistive balancing, I doubt it's as low as 20mA. The lowest capability Chinese management boards have 100 to 250mA bleed capability. Gary, I think your BMS uses 500mA shunts. The board I'm using with my bike now (and the one on my test bench) is capable of 1000mA continuous and 1500mA intermittent. Smaller shunts can be used, but they significantly slow the balancing process as the charge current must stay below the shunt capability - and should be even lower to keep the BMSs cooler. A larger shunt with a lower duty cycle doesn't create any heat when the pack's balanced, but allows quick balancing when necessary.

Andy

 

[AndyH]

I'm by no means a battery expert, but if Nissan were using 80% of a 30kwh pack, then why would they have gone to the trouble of offering an 80% battery charge timer for longer battery life?

Wouldn't that be 80% of 80% or about 64% charging? That doesn't make sense....

Sure it does!

LiFePO4 carries the bulk of its energy in the middle so it makes sense to use the middle 80% as you've suggested. Lithium cobalt and similar cells carry more of their energy earlier in discharge, so it's more common to use 80% capacity, but 'slide it up' toward the top.

Instead of lopping the top and bottom 10% off to leave 80% in the middle, it's very likely that the Leaf's pack is using the 80% between 95% and 15%.

This was roughly the proportion used by NREL when they modeled the Leaf for the climate control study. http://www.nrel.gov/docs/fy11osti/49252.pdf

Motor Power (kW) 80
Battery Capacity (kWh) 24
Battery Delta State of Charge (SOC) 84%
Battery Maximum SOC 95%
Thermal Management No active cooling
Accessory Load 300W

 

[TomT]

I thought we had pretty thoroughly debunked the 30Kw rumor... It's 24Kw, period.

I'm by no means a battery expert, but if Nissan were using 80% of a 30kwh pack, then why would they have gone to the trouble of offering an 80% battery charge timer for longer battery life?

 

[Ingineer]

After closely looking at the cut-away pack they had on display at the Leaf test-drive event, the battery controller did not look like it had any significant thermal dissipation capability. I would doubt seriously if it exceeds 100ma dissipation per cell. I'm not just guessing, allow me to explain my reasoning:

Here's an image from the service manual:

The service manual claims that the bypass function is built into an ASIC, and that the "bypass switch" is triggered by cell voltage. This means only during charge will balancing occur. Carefully note this diagram:

If this is the case, it's unlikely they can implement much of a balancing charge, as thermal dissipation in an ASIC with at least 4 other channels (see diagram) is going to be limited by it's package. Since ASIC dissipation is limited, the charger must be set to a low constant current while this phase is operating. It cannot balance the cells at any point except by top balancing during charge, as this the only time the voltage in the cell would hit the bypass threshold.

They are probably using an ASIC similar to Linear's LTC6802. It supports up to 12 channels per device, and has an onboard bypass FET with typical on resistance of 10ohms. Do the math, if they drove the on-board FETs fully, (in a perfect world) that would be ~350mA maximum possible discharge. However, at that point the FET would have to dissipate at least 1.25W of power, and that's only one channel! Note this diagram from the LTC6802 datasheet:

It pretty much speaks for itself! Even if they are only using 4 channels per ASIC, that limits your discharge current do about ~75ma max before thermal runaway. Even if they are using a much better part, I'll still bet anyone here that they aren't much over 100ma per cell. At 100ma that would put total balancing energy dissipation at somewhere close to 50 watts of heat near the end of balancing. This heat would need to be disposed of in the battery controller somehow, which is substantial but feasible.

 

[AndyH]

Assuming all the work is being done on the ASIC or completely within the controller box, I'd agree. Since I (we?) don't know, I'm not betting!

Note this from the datasheet you referenced:

On-Chip Passive Cell Balancing Switches Provision for Off-Chip Passive Balancing

There are other ways to balance as well. Maybe the ASIC is doing charge transfer and maybe the balancing method only works with the single high cell at a time.

Realistically, the cells should be fairly close at least initially, and since we have two in parallel, that gives AESC/Nissan a bit more wiggle room to create a uniform 'cell' from what they have. In addition, since we'll never charge to 100%, that 5-10% 'on top' that we'll never use allows more wiggle room. It'll be interesting to see how these cells perform as they age, as they won't age in lock-step.

Nissan's biggest challenge is making enough cells to fill cars, while also keeping battery cost down. Choosing two of three - fast and cheap - suggests that better management would be more cost effective than 100% perfect cell matching - especially since management is more necessary as the pack ages anyway.

It'll be fun to see which of us is more 'right' as our info and understanding unfolds.

 

[AndyH]

I thought we had pretty thoroughly debunked the 30Kw rumor...

I'm by no means a battery expert, but if Nissan were using 80% of a 30kwh pack, then why would they have gone to the trouble of offering an 80% battery charge timer for longer battery life?

What makes you think it's either rumor or debunked?

What changes do you recommend?

(total capacity) - 20%*(total capacity) = 24kWh

30 - .2*30 = 24
30-6=24

 

[LEAFfan]

I thought we had pretty thoroughly debunked the 30Kw rumor...

I'm by no means a battery expert, but if Nissan were using 80% of a 30kwh pack, then why would they have gone to the trouble of offering an 80% battery charge timer for longer battery life?

What makes you think it's either rumor or debunked?

What changes do you recommend?

(total capacity) - 20%*(total capacity) = 24kWh

30 - .2*30 = 24
30-6=24

+1!

 

[AndyH]

Active cell balancing 24/7 using charge transfer - no generated heat and no need to wait for "100% charge" to balance the pack.

http://www.atmel.com/dyn/resources/prod_documents/doc9184.pdf

This method has the advantage of high balancing currents (>100 mA, up to 1A) plus the fact that
balancing is independent of the cell voltages.

With the ATA6870, one discharge control signal DISCHn is available for each battery cell. The
discharge pins DISCHn are switched on and off via the SPI interface.

The concept for transferring the energy from a battery cell to its lower neighbor cells is shown in
Figure 3-2 on page 3. In principle, this is a modular concept that can be used for all cells in a
stack. A module generally consists of one switching transistor (PMOS), one diode and one
inductor. For a battery stack of n cells, n-1 modules are needed.

 

[evnow]

mods, perhaps you can move the battery charging discussion to its own thread? Thanks

Done.