As a busy mechanic or technician, you’ll know that a chain is only as strong as its weakest link. The same principle applies to EV battery packs. When one module falls out of sync, whether it’s overcharged, undercharged or running at a different voltage, it becomes the bottleneck that limits your entire system’s performance.
Think of it like a team of horses pulling a carriage. If one horse is tired while another is fresh, or one is pulling harder than the rest, the whole team suffers. The carriage veers off course, the strong horses tire quickly from compensating and, eventually, the weakest one gives out entirely.
Your EV’s onboard battery management system (BMS) tries to keep the modules balanced, but it only kicks in during the final stages of charging and has limited correction capability.
Once the modules drift too far apart, the BMS can’t catch up. That’s where manual battery balancing comes in. It’s preventative maintenance that keeps every module working in harmony, maximising performance and lifespan.
Why manual EV battery balancing still matters
EVs rely on passive balancing built into the BMS, which typically involves resistors that bleed off excess energy from nearly full cells, releasing it as heat. Since most manufacturers use this technique, you can rest assured that it’s a low-cost and tried-and-tested method. However, it only works under certain conditions (usually when charging has reached at least 95%) and only within certain tolerances.
In short, it’s reactive rather than proactive. And that’s where the problems begin.
Over time, cells inside a battery pack naturally drift apart in charge-holding capacity. This drift is caused by differences in internal resistance, temperature exposure, use patterns such as depth-of-discharge and so on. Known as ‘cell drift’, this process increases the state of charge (SoC) delta between the strongest and weakest cells. If the spread grows beyond what the BMS can passively correct, the system may reduce its usable capacity, flag diagnostic codes or, in some cases, even limit the vehicle’s performance. That’s why manual balancing is essential, especially in dealer, fleet and EV service environments.
Using a dedicated battery balancing system ensures that each module or string is returned to a common SoC before reinstallation. This restores pack performance and helps prevent range loss, SoC mismatch faults or premature cell ageing.
Why you need a specialist EV charger for balancing
You can’t manually balance an EV battery pack using just a multimeter and a bench power supply. Even though most EV cells use similar chemistries, every manufacturer builds their packs differently. They use their own layouts for series and parallel connections, different BMS setups and various methods for accessing the modules. Pack layouts vary widely, even within the same brand or model, depending on revisions and cell suppliers.
Balancing requires measuring and adjusting several parameters at once. That’s why general-purpose electrical tools aren’t enough for high-voltage battery balancing. You need a purpose-built system, like the Midtronics XMB-9640.
The Midtronics XMB-9640 and EV battery balancing
The XMB-9640 is designed for balancing modular EV battery systems. It’s simple to use, highly accurate and packed with features to help you spot issues long before they become problems.
Key features of the Midtronics XMB-9640 include:
- Live module diagnostics – real-time monitoring of SoC, internal resistance (IR), voltage spread and other key metrics.
- Stable current regulation – maintains precise current levels across all channels to prevent overshoot, thermal stress, or overcharge during balancing.
- Programmable cycling – allows simulation of real-world drive cycles by running controlled charge/discharge tests (ideal for identifying weak or degraded modules before reassembly).
- Thermal monitoring – tracks temperatures at both cell and module levels. Uneven heating might mean early-stage failure, high IR or potential internal shorts.
- Built-in isolation – protects users and ensures accurate measurements across high-voltage modules by electrically isolating channels.
- Per-channel voltage and current control – delivers precise charge and discharge control for each module, reducing risk and improving accuracy.
- Preloaded pack profiles – lets you select approved templates based on vehicle model and module type, with no guesswork required.
- Automatic process control – once a profile is selected, the tool handles balancing automatically within those parameters.
Many carmakers now require balancing in their service procedures, especially when replacing just one or two modules in an older pack. Without balancing, the new cells may not match the old ones. Ironically, this could worsen SoC drift, speed up ageing and lead to onboard BMS errors.
Ford, for example, is widely reported to require balancing to within ±1% SoC or 30 mV using approved tools (such as the Midtronics XMB-9640). This avoids triggering BMS lockouts or trouble codes.
Skipping these steps or using the wrong tools risks shortening the life of the pack or creating worse problems right after service.
How to interpret the charger data (beyond voltage matching)
Manual balancing isn’t just about equalising module voltages. If you stop at voltage matching, you may miss early signs of failure. The data captured during the balancing process is one of the most powerful diagnostic tools available, if you know how to interpret it.
- SoC and IR must be assessed together. For example, if two modules sit at the same voltage but one shows higher IR, that module could be masking cell degradation. It’s more likely to heat up, drift, or fail under load.
- Charge rate patterns are also revealing. If a module reaches the target voltage faster than the others, it may have reduced capacity. That’s an indication that the module will likely fall out of balance again during everyday use.
- Stored cycle data becomes more useful over time. The Midtronics XMB-9640 logs every charge/discharge cycle, helping technicians identify recurring patterns across modules or vehicle platforms. This makes it easier to spot early degradation trends, reduce repeat diagnostics and build predictive maintenance models in fleet operations.
- That same data also improves workflow standardisation and traceability. Documenting how each module or pack responds during balancing allows you to maintain consistent repair records, build trust with customers and support warranty claims with service logs.
Source your EV battery balancing tester from Rotronics
The long and short of it all? Battery balancing isn’t just about repairing problems. It’s about forecasting, quality control and diagnostic processes. And when all of those come together, it helps modern EV service centres extend battery life, optimise pack performance and reduce breakdowns, all of which means less stress and an improved bottom line.
If the Midtronics XMB-9640 is the perfect match for your workshop, and here at Rotronics, we’re convinced it is, give us a call for a friendly chat. We’ll answer any questions you might have, and we’d be delighted to help you enhance your EV battery servicing plan.
In the meantime, check out our range of 12-volt chargers and testers to keep on top of your other systems. We look forward to hearing from you.
