Theoretical mods that could fix premature range loss from weak cells WITHOUT replacing batteries [Active-balancer during driving]

[Infinion]

Time snip 1

Time snip 2

prof John Kelly goes over the battery construction and cell measurement/balancing in the Chevy Volt (similar architecture to Spark EV).

​

What does the BMS do?​

The BECM (Battery Energy Control Module) monitors the state of the 96s2p (96 series and 2 parallel) cells in the 2015/2016 Spark EV. The 2014 has a slightly different cell configuration, 112S3P (112 Series 3 Parallel).

How does it balance cells and how much power can it handle?​

The only issue with this BECM design is its cell balancing strategy. When the Spark EV fully charges to 100%, the BECM will top-balance all the cells by discharging them across up to 96 groups of resistors until the lowest cells reach the average cell voltage. While charging off L1/L2 EVSEs, or DCFC, the power tapers to something below the ballpark of 2-3kW. Dividing power by 96 or 112, each group must dissipate 3000 W / 96 groups = 31W of charging power.

Using the electrical power formula of Ohm's law [P=IV] where P is power, I is current in amps, and V is cell voltage and solving for current, I = P/V -> I = 31W/4.1V -> I = 7.6A of current for a 96 group balancer.

So we can infer that the BECM's balancing circuit is at least rated for around 8 amps of DC current across 96 / 112 groups of cells.

What's the problem with weak cells?​

Say you have a Spark EV that originally had 18.2 kWh of capacity with ~90% depth of discharge. After 10 years your capacity has dropped to around 14 kWh usable, or 22% degradation, with 0.146 kWh / 42 Amp-hours per cell group. However, when you drive to 90% depth of discharge, the car quickly shuts down with "propulsion power reduced". It turns out only two of your cells have degraded to 146 Wh and the rest are actually much healthier at, say, 167 Wh. However, the BECM detected the weak cells dropping below 3V and reduced your power for the entire battery. But it's not enough and the car shuts down after a only a few miles as the weak cells cross the 2.5V cutoff.

In other words, your EV is going into turtle mode early, and shutting down early because your weakest cells need to be protected, and restrict your depth of discharge.

Whenever your Spark EV top balances, The weak cells will throw away 31W / 8A of charging power so they don't become overcharged. Since the difference in capacity between our theoretical Spark EV is 167 Wh - 146 Wh = 21Wh, the car spends 40 minutes balancing on L2 and around 1h40m on L1.

And So?​

And so the BECM protects the weak cells from being overcharged, but your battery pack behaves like it has 96 weak cells with the smaller capacity, when you really have more energy stored that is unusable.

What's the solution?​

The solution is to add an active bottom balancer that charges the weak cells siphon energy from the strong ones.

How could it be done?​

It would use the same balancing lines the BECM's top balancer uses during charging. In the video above, there are service connectors for battery testing that have pins that connect to each of the 16 cell terminals on top of each battery module.



In the service manual, the connectors I am referring to are X5 - X7, X9, X10, and X13. These are male connectors, and the matching female that mates with it is https://www.aptiv.com/en/solutions/connection-systems/catalog/item?language=en&id=15345477_en
Once the minimum cell imbalance is detected in a weak cell, a 16-cell active balancer would operate, transferring energy from stronger cells in the group at <30W per cell group while the Spark EV is in operation to compensate for the difference in capacity.

There would need to be 6 balancers per battery module. This would effectively maximize the usable capacity and depth of discharge of the pack, and extend the service life of the vehicle. A top balancer to protect cell group overcharging, and a bottom balancer to protect overdischarge that the HPCM2 previously achieved by reducing power and shutting the vehicle down early.

Here's one readymade solution for exactly this purpose
https://100balancestore.com/collect...tive-balancer-module-for-lithium-battery-pack

 

[Randy960]

So, just to clarify a few points. The BMS in the Spark only balances the cells when it top charges? Or only while charging in general? It doesn't balance the cells while driving?
The service plugs you're referring to are connected to the same lines as the BMS, but they're separate plugs not normally used?
I'm loving this idea, and it would be something not too difficult to add while rebuilding a battery pack.

 

[Randy960]

A few further questions I thought of:
Would these add-on BMS modules be active all the time, and would this cause a drain if the car sits?
Would the add-on BMS and factory BMS sharing the same circuits end up causing them to fight? They should both be doing the same thing a lot of the time, but I don't know much about the factory BMS.

 

[Infinion]

So, just to clarify a few points. The BMS in the Spark only balances the cells when it top charges? Or only while charging in general? It doesn't balance the cells while driving?
The service plugs you're referring to are connected to the same lines as the BMS, but they're separate plugs not normally used?
I'm loving this idea, and it would be something not too difficult to add while rebuilding a battery pack.

Correct, the BMS presumably uses passive balancing involving a switched shunt resistor for each of the 96 / 112 groups (simple and cheap), and not between cells, so the only option to balance is to do so while charging to 100%. I would hazard to guess the reason EVs don't balance with their passive BMS while driving is because it would throw away precious energy.

The service plugs you're referring to are connected to the same lines as the BMS, but they're separate plugs not normally used?

Wait a minute... Ahhh, you're right, there should be a connector indicated in the circuit diagram if it were separate. After reviewing the battery footage, it looks like the Spark is definitely using this connection as part of the balancing harness.
It's not a dealbreaker, but it adds more complexity ... the need for a breakout PCB board or an inline Y-splitter harness to be designed with the same female and male OEM connectors, and examining the shape it needs to be a proper low-profile fit.

Would these add-on BMS modules be active all the time, and would this cause a drain if the car sits?

From what I've gleaned the modules communicate via bluetooth and can be programmed to turn on and off at custom voltage ranges. The 100balance / Daly units are on all the time and have a power consumption of 1mA while active and 100uA sleep. So with a 0.000002C discharge rate, it is slower than the intrinsic self-discharge rate of li-ion.

Would the add-on BMS and factory BMS sharing the same circuits end up causing them to fight?

Definitely possible, but since the factory bms' top-balancer only runs at 100% SOC while charging, it wouldn't overlap with the active balancer as long as the user doesn't intentionally set the active balancing starting voltage to be in the 4-4.15V region, or set the cell difference too low. There are some YT videos online showing the balancer in action on LFP cells.

 

[Randy960]

Still liking the idea, but a potential alternate option might be a more advanced 96/112s BMS wired as a piggyback to the factory BMS. If an adapter harness is needed anyway, one big one might be easier than 6 small ones. This would also work for both maximizing the factory battery, and building one with newer higher capacity cells.

 

[Infinion]

Still liking the idea, but a potential alternate option might be a more advanced 96/112s BMS wired as a piggyback to the factory BMS. If an adapter harness is needed anyway, one big one might be easier than 6 small ones. This would also work for both maximizing the factory battery, and building one with newer higher capacity cells.

I'll keep my eyes open for something like that, but I have a feeling rather than a highly integrated single centralized unit, the more typical solution will be single independent units, or a stack of 16-cell or greater units that communicate via serial data communication. https://www.thunderstruck-ev.com/bms-controller.html

There are more advantages to piggyback at the module rather than at the BMS, namely, you reduce your voltage drop across cable length and achieve better resolution. Your system also won't potentially carry the current of two balancers across the same wires. So I'd choose to get the balancer as close as possible to the module itself to save on wire and complexity.

quick voltage drop calc If the wire harness from a middle module to the BECM was 3 feet (x2 for return wire), and the wire was 18awg, a balancing pair of module cells would have a 153mV drop at 2A, 535mV drop at 7A. If 22awg, 386mV / 1350mV. Since the cells need to be balanced within 20-40mV, that voltage drop needs to be accounted for through some software presets, or with a non-current-carrying Vsense wire for each current-carrying wire to measure the Vdiff across cells.

edit: It just occured to me that you could use the same wires to check voltage during PWM operation, while no current is flowing.

good articles here
https://www.digikey.com/en/articles...n-evs---part-i-passive-balancing-technologies
https://www.digikey.com/en/articles...n-evs---part-ii-active-balancing-technologies

https://www.monolithicpower.com/en/...d-power-systems/ev-battery-management-systems

 

[Randy960]

I wasn't factoring in that the original BMS might be adjusted to compensate for the resistance of the leads, and it's a good point that the amperage my be too high with two BMSs working together. Also, the only 96s BMS units I was really finding are larger boxes. A simple Y-harness with that small 16s BMS would likely fit in the gaps between the modules.

 

[FalconFour]

Unfortunately I think there are some fundamental misunderstandings here, betrayed by this line in particular:

Whenever your Spark EV top balances, The weak cells will throw away 31W / 8A of charging power so they don't become overcharged.

"That's not how this works. That's not how any of this works!"

Balancing is *not* a critical function of every charge cycle. Balancing is, in fact, very rarely needed on a battery pack - and at a very slow rate.

I get it, lithium batteries are a new tech, and complicated. But they adhere to the laws of physics, particularly "energy is neither created nor destroyed". Cells are discharged and recharged in perfect unison when in series - that is, exactly the same current (amps) are drawn from (or charged into) each one at all times. Balancing is the act of taking one cell and discharging it (or, in active balancing, charging it) in isolation.

Maybe this notion of "balancing needs 31 watts / 8 amps" comes from believing that the cells must somehow "fall out of balance" when they are discharged, which is what makes the voltages look all disoriented and wonky on a dead battery - e.g. a low cell is wildly far out of "balance" of the others. But that is not "balancing" or "being out of balance". That is just a factor that the different cells now have different capacities over time. When the same current (amps) is applied to all of them at the same time, the cells with a slightly lower capacity will reveal themselves to be dead (low voltage) first.

If you take that discharged pack with all its wildly varied voltages, and merely charge it - without any BMS doing any balancing whatsoever - the pack will (if it's been top-balanced recently) return to a near-perfect uniform balance at the top when it's charged. In fact, it'll appear mostly balanced shortly after you start charging it, as all the low cells come up from the "death cliff" (see the far left of the chart, below):


Balance is just needed to make up for minor variance in internal resistance of the cells. Following the "energy is neither created nor destroyed" idea, consider that with higher internal resistance on some cells versus others, that means some cells will heat more than others when they are discharged or charged (e.g. when fast charging or with perky acceleration). That additional heat (energy) produced (wasted) by the weaker cells will result in more energy (watts) being needed to recharge the cell, thus it falls slightly out of balance with the others. That minor deviation is all that needs to be compensated by the balancing of the BMS.

Now, as for using active balancing to compensate for a weak cell (and give it more effective capacity by discharging other cells into it while driving)? Tricky, tricky. In theory it could be possible, but in practice... has anyone ever implemented such a thing? It would be causing "imbalance" all throughout the drive cycle, and would have to almost constantly be re-balancing - especially when recharging (and could certainly cause problems when DC charging). Personally, I'd much rather hear about a replacement BMS that properly reacts to the low cell of a pack, and adjusts the usable range accordingly, instead of... completely bricking Sparks when they get too low.

 

[Infinion]

FalconFour, first welcome to the forums. I'm going to have a spat with you. Thank you for recently posting your insights and your thoughts regarding the Spark EVs BMS. I very much agree that the handling of weak cells is very poorly implemented, although it might truly be the fault of the Hybrid Powertrain Control Module 2.

Unfortunately I think there are some fundamental misunderstandings here, betrayed by this line in particular:

Whenever your Spark EV top balances, The weak cells will throw away 31W / 8A of charging power so they don't become overcharged.

"That's not how this works. That's not how any of this works!"

Balancing is *not* a critical function of every charge cycle. Balancing is, in fact, very rarely needed on a battery pack - and at a very slow rate.

Lol, well feel free to personally verify this by observing the OBD II PIDs.

I admire your certainty on the matter, but I'm not sure what you're arguing here. Do you have a problem with the individual cell balancing power I calculated? And forget for a moment whether it balances every charge cycle (it does). When it top-balances, what do you think the balancing current is and what will you base those numbers on?

I get it, lithium batteries are a new tech, and complicated. But they adhere to the laws of physics, particularly "energy is neither created nor destroyed". Cells are discharged and recharged in perfect unison when in series - that is, exactly the same current (amps) are drawn from (or charged into) each one at all times. Balancing is the act of taking one cell and discharging it (or, in active balancing, charging it) in isolation.

Assuming I don't understand battery tech and am breaking energy conservation laws is a profoundly presumptuous thing to say. What violation is this based on?

Curiously, in the same paragraph, you seem to describe battery charging in ideal terms where cell capacities and internal resistances are perfectly matched. This is most certainly a gross oversimplification. You'd be describing idealized batteries. So how now do certain physical realities cease to exist? Exactly which laws can be broken and by whom?

Maybe this notion of "balancing needs 31 watts / 8 amps" comes from believing that the cells must somehow "fall out of balance" when they are discharged,which is what makes the voltages look all disoriented and wonky on a dead battery - e.g. a low cell is wildly far out of "balance" of the others

It was actually quite clearly outlined in my first post if you read it. It was never about how much power balancing needed. Rather, how much power the system must be designed to handle per cell, given a specific input power and observing conservation of energy laws that you presumed I did not understand, along with battery tech. The subheading is below, you should read it.

How does it balance cells and how much power can it handle?​

It would be funny if the entire premise of your post calling me out as violating the 1st law of thermodynamics was because of some personal incredulity that prevented you from considering such an efficient battery could need balancing when it fully charges, at all, any time, ever.

But that is not "balancing" or "being out of balance". That is just a factor that the different cells now have different capacities over time. When the same current (amps) is applied to all of them at the same time, the cells with a slightly lower capacity will reveal themselves to be dead (low voltage) first.

Ok.
Actually, it's quite the contrary. Cells do fall out of balance during discharge and charge cycles. Your position that it doesn't confounds me. The reactions in Li-ion cells are not perfect, and we're even discussing degraded cells here.

Balancing is the act of matching cell voltages through deliberate discharge of individual cells, so that charging and discharging can occur symmetrically across the pack and where the act of balancing does extend to compensating for weak cells. Balancing can be done to the 96 series cells simultaneously, and it ensures the usable capacity, power, and efficiency will all be maximized. If not for the efficiency and power in a top-balancer, it is to protect the weak cells from going overvoltage. That's one of the reasons GM top-balances, and why this thread also considers an added active bottom-balancer.

Being out of balance describes cells deviating in voltage. Besides capacity, real non-ideal batteries have dynamic resistance over their discharge curve. Here's an example from DOI:10.3390/en6105538



If your weak cell enters the high resistance zones first, then by definition it will operate less efficiently for the same current, and continued discharging and charging will further exacerbate its departure from the rest of the pack's state of charge. Cells that have different capacities will reach undervoltage and overvoltage cutoffs at different times for the same current. I believe we agree on this point. During this time we get a larger variation between minimum and maximum cell voltage, which is where the term "out of balance" comes from. If this wasn't a concern and li-ion stayed in perfect unison there wouldn't be a market for Li-ion BMSs. Everything would be perfectly matched always and forever.

If you take that discharged pack with all its wildly varied voltages, and merely charge it - without any BMS doing any balancing whatsoever - the pack will (if it's been top-balanced recently) return to a near-perfect uniform balance at the top when it's charged. In fact, it'll appear mostly balanced shortly after you start charging it, as all the low cells come up from the "death cliff" (see the far left of the chart, below):

Yes I agree, except voltage only roughly correlates SOC and you wouldn't look at this curve to estimate those widely varied voltages. You would want a table showing open circuit voltage if the cell chemistries had settled in equilibrium. Like this one below.

Balance is just needed to make up for minor variance in internal resistance of the cells. Following the "energy is neither created nor destroyed" idea, consider that with higher internal resistance on some cells versus others, that means some cells will heat more than others when they are discharged or charged (e.g. when fast charging or with perky acceleration). That additional heat (energy) produced (wasted) by the weaker cells will result in more energy (watts) being needed to recharge the cell, thus it falls slightly out of balance with the others. That minor deviation is all that needs to be compensated by the balancing of the BMS.

Yes I agree with most of that, and not only will degraded cells have a higher internal resistance, the dynamic resistance will amplify this difference if the degraded cell is not in the same SOC as other series cells.

I'd like to hear how minor of a deviation you believe this causes, how quickly it balances and how long. Care to guess?

Now, as for using active balancing to compensate for a weak cell (and give it more effective capacity by discharging other cells into it while driving)? Tricky, tricky. In theory it could be possible, but in practice... has anyone ever implemented such a thing? It would be causing "imbalance" all throughout the drive cycle, and would have to almost constantly be re-balancing - especially when recharging (and could certainly cause problems when DC charging). Personally, I'd much rather hear about a replacement BMS that properly reacts to the low cell of a pack, and adjusts the usable range accordingly, instead of... completely bricking Sparks when they get too low.

I'm absolutely certain there is no replacement to the OEM BMS. Anything coming close would be entirely custom and must communicate through GMLAN. Too many unknowns and too much reverse engineering.

 

[FalconFour]

Well, respectfully, from one reasonably well-educated battery guru to another, let us spar then.

I admire your certainty on the matter, but I'm not sure what you're arguing here. Do you have a problem with the individual cell balancing power I calculated?

Absolutely. There's no way I would imagine they'd build 8 amps of discharge current and 30-odd watts of heat dissipation *per cell*, times *96 cells*, into the BMS board. That would be so magnificently inefficient, it would require liquid cooling for the BMS and would show as significant energy loss in the charging. I think those calculations suffer "abstracted math gone too long in a vacuum" - numbers obtained from some source that were put through too many seemingly-logical transformations to come to the conclusion. Instead, I suspect a balance current of 1 amp or so, periodically performed to maintain a top balance as needed.

How "as needed"?

What violation is this based on?

The violation I see is that balancing is needed only where the cell "leaks" energy outside the (logically) sealed system of the battery during its usage. That would come in the form of imbalanced cell resistance, but that's about it. Any other time, the cells will exhibit an "imbalance" at low state of charge, but that isn't a true imbalance - it's just the result of the cells having differing capacity. When recharged, as they walk their SOC% range to reach full charge together, they naturally come back into balance - no external balancing needed, except to recover from the minor imbalance that differing resistance carries.

That is to say, it doesn't make any sense to have 8 amps of balance capability, when it only needs to apply a balance while it's resting, after charging. It can take its merry time to work off whatever small imbalance may turn up.

However, the BECM detected the weak cells dropping below 3V and reduced your power for the entire battery. But it's not enough and the car shuts down after a only a few miles as the weak cells cross the 2.5V cutoff.

This part in particular seems untested and strange to me. The fault level is in the service manual as 1.75v - that's the only "cutoff" the car seems to know/react to. And in my car today, testing it with the heater, I found the "propulsion power reduced" appears at 2.5v, which is pretty damn crazy (because 2.5v is far into the death curve - that cell is totally dead and the car *should* shut off, but it keeps going). From here, nerd to nerd, I admit I took a bit less credibility from the rest of the writing - it's so easy to see the behavior, it reads like an ideal but untested claim.

Besides capacity, real non-ideal batteries have dynamic resistance over their discharge curve. Here's an example from DOI:10.3390/en6105538

This is actually new to me. Cells have a higher resistance at both ends of SOC%? I knew at the low end - as to me, it seems like C-rate has a SOC% component to it (that is to say: it's more stressful to put a high load on a low-charged cell), but at the high end, I always thought the cell was at its absolute peak performance (taking high load would be "good" for it). I'll have to chew on this a bit...

Your position that it doesn't confounds me.

Never was a position that they don't fall out of balance - merely that they don't do it _significantly_. In fact, I've had several battery banks at home charging/discharging almost fully, every day, slowly (about 0.1 C-rate), with only protective BMS watching over them. The cells never drift - after 300+ cycles, the drift is so minor it only takes a few minutes for my iCharger X12 (with about 2-amp balance capability) to regain a perfect top balance - in 64Ah banks. That's my experience - cells don't drift in balance (in any significant way at least) unless they're abused. To that point, I also have a Storm2 Liquid battery bank (90Wh - made of 8x 18650 NMC cells, 2p/4s) that's been through about 700 cycles at ~>1C rate, with pretty high heat and stress... its cells end up drifting out of balance regularly and somewhat chronically, yet they didn't equip the BMS board with a balance circuit, so I have to manually balance it. I have experience at both ends.

Lol, well feel free to personally verify this by observing the OBD II PIDs.

Point me to how to obtain the data, and I'll gladly scoop it up. Currently I've been using the Car Scanner app to get data, and though it provides critically useful data, I imagine there's a lot it doesn't show.

I'm absolutely certain there is no replacement to the OEM BMS. Anything coming close would be entirely custom and must communicate through GMLAN. Too many unknowns and too much reverse engineering.

totally with ya there... but then again, just glance over at what's happening with Nissan Leaf replacement packs. It doesn't seem completely out of the question, especially if the Chevy Volt and Chevy Spark can trade notes.

 

[Randy960]

Your nerd fight (meant respectfully, I enjoy a discussion between two people who know more about a subject than me) has me thinking about the idea of a piggyback BMS. There's really two things we want improved on the factory BMS, balancing and a safety cutoff that protects the battery without bricking the car. The balancing part is easy if we're already adding a second BMS. Cloning all the the CAN messages and functions from the factory BMS into an aftermarket one probably isn't feasible unless GM really wants to help, but getting a programmable BMS to spit out the "turtle mode" signal or something along those lines should be doable. Even if that doesn't work, triggering a relay that disconnects something like a temp sensor might cause a non-critical error that lets you know to back off before the battery actually bricks.