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.