The Obstacle Course on the Path to Repurposing Used Electric Vehicle Batteries (EVB). Part IV: Impact of Cell Imbalance
This is the fourth blog post in our series, this time we’re going to discuss the cell imbalance. First, there are two types of imbalance: state-of-health (SOH) imbalance, and state-of-charge (SOC) imbalance. In an electric vehicle (EV), SOH imbalance may reduce a battery’s overall lifetime, and SOC imbalance reduces the car’s driveable range. Both are undesirable, and they are interrelated. Together, these imbalances present several challenges when it comes to testing and then repurposing the battery. Spoiler alert, severe imbalance leads to longer test times and higher repurposing costs.
In this blog, we’ll discuss:
What is cell imbalance and why does it matter?
What is its effect on state-of-health (SOH) and state-of-charge (SOC)?
How does this affect the repurposing of used batteries?
How does this affect the vehicle’s battery?
What is cell imbalance and why does it matter?
Just as a chain is only as strong as its weakest link, a battery has only as much capacity and lifetime as its weakest cell. Thus, batteries perform best when all the cells are close to the same state-of-health (SOH). See figure one for an example of what a well-balanced module looks like versus one that is imbalanced. Reminder: a module is a battery composed of multiple cells.
Figure 1 - Plot of SOH by cell for used electric bus modules with 8 cells per module. In Module A (left), the cells are relatively balanced whereas in Module B (right) the cells are imbalanced.
There are many things that can cause cell imbalance, including but not limited to, temperature variations and charging behaviors. For example, in a larger vehicle like a truck or bus that has multiple battery packs in different locations e.g. a battery pack on top of a bus is subjected to much more heat than the other packs. We won’t go into too much more detail since this is a bigger and much more complex topic.
Let’s look back at the two used bus battery modules from Figure 1, but here we include the module SOH (the red line) represents how usable that entire unit is. You can see in the module that has high imbalance, it is limited to its weakest cell!
Figure 2: charts from Figure 1 with the module level SOH% (red line)
What is its effect on state-of-health (SOH) and state-of-charge (SOC)?
Why does this happen? Remember when we said that a module is only as strong as its weakest cell? Well let’s dive into an example of what that means by using Module B - the imbalanced bus module. Cell 2 has the lowest SOH at 79% whereas cell 5 has the highest at 89%.
This means that the module will only charge up to the maximum that cell 2 can take. This means the maximum state-of-charge (SOC) of the module is limited. You may experience this phenomenon over time with your laptop and cell phone batteries where it takes longer to charge and doesn’t last as long between charges.
In addition to working best when cells are in balance. Batteries also perform best when they are not always at their upper and lower limits of the SOC. Basically you don’t want to always charge your battery up to 100% or drain it to near 0%. Hot tip: It’s also not good for the battery to remain at a high or low SOC for long periods of time, so if you’re going on vacation, don’t leave your car fully charged or near empty.
If you’ve been reading about the GM recall, you may have heard they tell the Bolt owners to only charge up the battery to 90% and not let the range dip below 70 miles. This is a similar idea to limit the amount of stress on the battery to reduce degradation. What does that mean for state-of-health (SOH)? Over time, the weak cells get weaker because it’s constantly hitting its upper and lower limits. When you scale this across hundreds to thousands of cells in an EV battery pack, you can see where this becomes a bit unruly.
How does this affect the repurposing of used batteries?
Ideally, imbalance is addressed in the battery’s first life (don’t worry we’re working on something), but if not, it needs to be addressed in its second-life. Otherwise, you would reject module B for a second-life application because the imbalanced cells in Module B will cause the capacity to fade faster than that of Module A.
One option is to rebalance the cells to make the module more usable. The challenge? The process, just like battery testing, takes specialized equipment, labor, and a lot of time. This becomes a bit of a challenge because we are adding testing and rebalancing cost and time which will inevitably add cost to a second-life battery system. However, balancing will make more batteries suitable for repurposing, which helps reduce the need to mine for and manufacture new batteries.
As you know from our prior blog on battery grading, measuring a battery’s SOH is a challenge. What you may not know is that measuring SOC is a challenge as well. In a vehicle (similarly in your laptop and cell phone), you have a prediction that gives you a rough idea. However, when you get used batteries disaggregated from a larger system (oftentimes from multiple vehicles), there is no way to know. Take a look at the battery modules we’ve received (see Figure 3), it is a black box unless you perform the same type of testing we mentioned in our battery grading blog.
Figure 3: image of BYD module, image of Valence module.
In fact, when we performed our initial tests when we first got our modules, the cells were not at a consistent SOC. The practice should be to get to below 50% SOC, but even when we get the same bus batteries from the same supplier, it is not easy to do.
These are not insurmountable challenges, but they do make it harder to qualify and repurpose as many batteries as possible. We are now in the midst of our third case study of grading used electric vehicle batteries to evaluate their potential for reuse. Follow us on social media so you’ll know when we publish our next case study.
How does this affect the vehicle’s battery?
We’ve seen some interesting chatter recently about how some experts in the EV aftermarket are having a debate about how to do this for degraded battery packs. We’re starting to see early EV adopters needing battery replacements. The question on whether to put in a fresh pack versus replacing the most aged modules is a big debate with really no consistent results. The financial impact is nothing to scoff about either.
In Car Buzz, replacing a Tesla pack can cost you over $16,000! If you had to cover this out of warranty, that usually means the vehicle is over 8 years old, so you really have to ask, is it worth it? Replacing only a bad module costs a fraction of that, but it is only a temporary fix if not done properly.
A VICE article covers a different Tesla vehicle where the cost to replace the pack was $22,500 when the value of the vehicle was estimated to be $23,000. Ouch! Again, the driver was able to find an independent repair shop to replace a module for a fraction of the cost, but the vehicle only lasted a few weeks before again experiencing imbalancing issues.
Generally though, the automakers we’ve spoken to are seeking to remanufacture and replace faulty modules. This is by far the most cost-effective and sustainable solution for all parties involved. It does require more advanced diagnostics to do it right.
Contact us at info@rejouleenergy.com if you want to leverage our technology to test your batteries in or out of the vehicle. What are your thoughts? Did we leave anything out? Please comment and share so we can all engage in conversation and learn from each other.
Cheers,
The ReJouligans
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This document was prepared as a result of work sponsored by the California Energy Commission. It does not necessarily represent the views of the Energy Commission, its employees, or the State of California. Neither the Commission, the State of California, nor the Commission’s employees, contractors, or subcontractors makes any warranty, express or implied, or assumes any legal liability for the information in this document; nor does any party represent that the use of this information will not infringe upon privately owned rights. This document has not been approved or disapproved by the Commission, nor has the Commission passed upon the accuracy of the information in this document.©2021 ReJoule Incorporated. All Rights Reserved.
