In every EV battery pack, dozens or even hundreds of cells work together to power the vehicle. But if just one of them falls behind, it can drag down the entire pack’s performance, especially its range.
In this blog, we’ll break down what battery imbalance is, how it hurts your range and what fleet technicians can do to fix it before it becomes a bigger problem.
Why does cell imbalance happen?
Electric vehicle battery packs contain many cells arranged into modules in series and/or parallel. These cells are intended to charge and discharge all at the same time. However, over time, slight differences emerge thanks to tiny (and unavoidable) manufacturing defects. Some cells store or release energy faster or slower than others, and this is referred to as cellular imbalance.
Cell imbalance typically results from manufacturing inconsistencies, variations in temperature across the pack, repeated fast charging or deep cycling and uneven ageing rates.
In most EVs, the battery management system (BMS) monitors every cell and intervenes when one reaches its voltage limit before the others. To protect the cell from overcharging, the BMS activates passive balancing, using resistors to dissipate excess energy as heat. This allows charging to continue briefly, but only within safe limits. If the imbalance becomes too great (for example, if one cell consistently reaches its maximum capacity long before the others), the BMS will eventually slow down or halt charging altogether to prevent damage.
How imbalance reduces EV range and performance
When a battery is imbalanced, its full capacity cannot be used. A weak or overcharged cell will trigger BMS protections, cutting off charging or discharging prematurely. This directly impacts:
- Driving range – even a 10% state of charge (SoC) difference can reduce usable capacity by 15% to 20%, limiting how far the vehicle can go.
- Charging times – the charger may shut off early to prevent overvoltage in fully charged cells, even if others are still low.
- System efficiency – healthy cells remain underutilised, while outlier cells carry extra stress, reducing overall energy throughput.
- Thermal stability – imbalanced cells generate more heat under load or during charging, increasing the risk of thermal hotspots and placing additional strain on cooling systems.
- Battery lifespan – chronic imbalance accelerates wear on outlier cells through overcycling, resulting in uneven ageing and earlier failure or the need for reconditioning.
- Regenerative braking performance – high-voltage cells can restrict the system’s ability to recover energy, limiting regenerative braking efficiency.
- Power delivery – in high-load or fast-charging scenarios, imbalanced cells may trigger voltage or current cutoffs, reducing available torque and potentially activating fault codes.
- Reliability under stress – imbalanced packs are more vulnerable during high-demand events such as fast charging or steep hill climbs.
As the imbalance worsens, the range becomes less predictable and the performance less stable. In commercial fleets, this often results in increased downtime, inconsistent daily operations and a higher total cost of ownership. And that’s bad for your brand, reputation and bottom line.
Diagnosing and managing imbalance in the fleet workshop
But don’t fear. Imbalance is measurable and, in many cases, reversible with battery balancing techniques, if caught early. Here’s what you need to know:
Diagnosis
Use EV-specific diagnostic equipment. These testers and chargers should be capable of reading individual cell or module voltages.
A healthy pack typically shows a voltage spread of within ±10-20 mV. If you find variations beyond 30 mV, especially under load, those are early signs of imbalance.
Here at Rotronics, we have just the tool for the job: the Midtronics xMB-9640. The Midtronics xMB-9640 is capable of full pack testing, including module-level scanning, without the need to dismantle the battery. Make scanning your EV high-voltage battery packs part of routine maintenance. Over time, you’ll build up a dataset, and this makes any anomalies even more obvious, as well as highlighting progressive imbalance or degradation.
Management
Almost all EVs on the road today rely on passive balancing, where resistors bleed off excess energy from high-voltage cells to reduce imbalance. It’s simple and reliable but inefficient, as the excess energy is lost as heat.
In contrast, active balancing transfers charge between cells using inductors or capacitors. Active balancing is, technically, far more efficient but remains vanishingly rare (except in hypercars like the McLaren P1) due to higher cost and system complexity. A few advanced systems, especially in commercial or high-demand applications, now employ hybrid balancing, which combines both methods. However, this approach is rare.
In some cases, external balancing equipment or a BMS update can restore cell balance without replacing modules. For heavily imbalanced packs, technicians may need to isolate and service the affected modules directly.
External balancing sounds complex, but it doesn’t have to be. All you need is the right equipment. Once again, we recommend the Midtronics xMB-9640, which comes with built-in balancing capabilities thanks to its advanced algorithms. It can detect and correct imbalances during charge or test cycles, so you get both testing and balancing done automatically at the same time.
For more information about the Midtronics xMB-9640, get in touch with our friendly team today. As electric vehicles become more advanced, so does the need for accurate, efficient battery maintenance, and that includes 12-volt and 24-volt systems still critical to vehicle operation. To learn more about our full range of chargers and testers, our team is on hand and ready to answer any questions you might have.
