Your fleet’s EVs rely on hundreds of individual battery cells working together as one high-voltage traction battery pack. In an electric car or van, the crucial thing is that each of those cells stays consistent, delivering similar performance and charge acceptance as all the others.
But as with any component, this consistency doesn’t last forever. When your vehicle’s cells drift out of balance, the whole pack is limited by its weakest cell group.
However, this is a manageable problem with a relatively simple solution. In this guide to EV battery consistency and balancing, we’ll help you stay on top of your charging schedules by keeping your EV’s cells balanced. It explains what that involves and why it’s important.
What is EV battery imbalance?
EV batteries fall out of balance when the cells or cell groups inside the traction battery pack no longer have the same state of charge (SoC), voltage, capacity or condition. In simple terms, some parts of the battery hold more charge than others.
Why does that matter? Because an EV battery pack is managed as a complete system. If one cell group reaches its maximum safe voltage before the others during charging, the battery management system (BMS) has to slow or stop charging to protect it. If one cell group reaches its minimum safe voltage before the others during driving, the BMS has to limit discharge to prevent damage from overcharging.
And so, in both cases, one weak or inconsistent cell group restricts the performance of the whole battery. In your fleet, this leads to reduced usable range, slower or interrupted charging, unexpected SoC drops, reduced EV performance, more battery warnings, more diagnostics time in the workshop and less confidence – from all parties – in your vehicle availability.
Now, a small amount of imbalance is normal. EV batteries aren’t perfectly uniform on a molecular level, and cells always age at varying rates. However, when the gap gets too large, it reduces the usable capacity, increases stress on weaker cell groups, and makes the vehicle harder to manage in daily operation.
Onboard EV charger balancing
The vast majority of EV battery balancing is handled by the vehicle itself, through its onboard systems. Its BMS monitors cell-group voltages, temperatures, SoC and other battery data. Here are a few key terms you need to know when it comes to onboard EV balancing.
Passive balancing
Almost all production EVs use a built-in passive balancing process. During charging, the BMS bleeds a small amount of energy from cell groups that have charged faster and therefore have a higher voltage. By bleeding it off (using individually activated resistors), lower-voltage cell groups can ‘catch up’.
Passive balancing is simple and widely used across almost every mainstream EV manufacturer (at the time of writing). It is, however, relatively slow, and also less energy-efficient, as you lose that excess energy to heat. It’s very useful for correcting small imbalances on a daily basis. But it can’t fix a degraded cell or restore lost battery capacity. That is where further diagnostics or specialist battery equipment may be needed (more on those below).
Active balancing refers to when the BMS actively moves energy from cells with a higher charge to cells with a lower charge. It sounds great, but it’s very complex, and still in the R&D phase in almost all EV contexts. You might find it on certain battery tools or reconditioning systems. In the future, we expect more EVs to come with onboard active balancing.
Top balancing
Passive balancing usually uses a top-balancing approach. The BMS corrects small cell-voltage differences near the upper end (‘top’) of the battery’s usable charge range.
As a rough guide, passive top balancing is most likely to activate in the upper 80% to 100% SoC range, often closer to 90% to 100%. However, this depends on the vehicle, battery chemistry, BMS settings, temperature, and the level of imbalance. The BMS usually looks at cell voltage, voltage spread and safety conditions to know when to activate passive balancing.
Top balancing happens near the upper end of the charge cycle, so if you never let an EV reach a high state of charge, the BMS may have fewer chances to complete it. That doesn’t, however, mean you should charge every EV to 100% every day. Follow OEM guidance, especially with LFP batteries, which may require occasional full charges for BMS/range-estimate accuracy.
In contrast, bottom balancing means equalising cells near the lower end of discharge, so they reach their minimum voltage at roughly the same time. It’s more relevant to specialist battery maintenance, testing or repair work than routine EV fleet servicing.
How to test EV battery consistency
Don’t rely on the EV’s dashboard range estimates alone, even if you notice it dropping. Estimated range can vary an awful lot, as you’ll be aware. Driving style, temperature, payload, route type, tyre pressure and even the driver’s heavy use of air conditioning can all have a significant impact.
Instead, have your technicians conduct scheduled EV battery tests to gather diagnostic data. This allows you to compare the battery’s condition over time, making it much easier to notice anomalies. A professional EV battery consistency check should include the following:
Cell voltage comparison
At rest, under load, during charging and near a high SoC, your technician compares the highest and lowest cell-group voltages in the pack. A small spread may be normal, but a large or persistent spread can indicate imbalance, weak cell groups or battery degradation.
Internal resistance or impedance
A cell group with higher internal resistance performs worse under load, heats up more quickly and reaches voltage limits earlier. This reduces performance and makes the pack less consistent, even if the resting voltage looks acceptable.
Capacity testing
Where appropriate, capacity testing can show how much usable energy the battery can actually deliver. This gives a more meaningful picture than voltage alone.
Temperature data
If one area of the pack runs hotter or cooler than the rest, those cells are likely to age differently. Uneven thermal behaviour can also affect charging speed, performance and diagnostic accuracy.
Diagnostic trouble codes
Battery-related fault codes identify issues with cell voltage, module communication, sensors, isolation, charging, cooling or BMS function. However, never read the codes without conducting accompanying tests. They need to be interpreted alongside live data and the vehicle’s operating history.
How to balance an EV battery pack with specialist equipment
If onboard balancing isn’t enough, the battery may need specialist workshop attention. This is usually the case when testing shows persistent voltage differences between modules or cell groups, imbalance-related fault codes, or inconsistent range and charging performance.
A trained EV technician can use equipment such as the Midtronics xMB-9640 to balance EV and HEV battery modules. It uses advanced algorithms to quickly and safely charge or discharge individual modules and bring the pack into alignment.
This is different from normal onboard passive balancing. Specialist equipment like this lets your technicians balance individual modules under controlled workshop conditions. Note that it’s not quite the same thing as active balancing either, as it doesn’t ‘transfer’ charge between modules/cell groups.
Module balancing can help restore pack alignment and correct minor imbalance. However, balancing isn’t a cure for a failed battery. It can’t restore a badly degraded or damaged module. In those cases, further repair or replacement may still be needed.
How to support ongoing EV battery consistency
EV batteries naturally deteriorate with age, like all vehicle components. However, you can reduce avoidable stresses and spot issues sooner with a few simple maintenance processes.
Follow manufacturer charging guidance
Different EVs use different battery chemistries and recommended charging routines. Some vehicles are happy with regular high-SoC charging. Others are better kept below 100% for daily use, with occasional full charges only when recommended.
Avoid unnecessary time at very high or very low charge
Leaving vehicles sitting for long periods at 100% or very low charge increases stress on the battery. Where possible, schedule charging so your vehicles are ready when needed.
Give vehicles enough time to complete charging routines
If a vehicle needs to balance near the top of charge, repeated short charging windows might not give the BMS enough opportunity to fully balance the pack. This is especially important for those vehicles you always have to rush back into service as soon as possible.
Monitor range and charging trends
A single low range estimate doesn’t mean much. When a pattern of poor performance starts to emerge, pay attention. You can spot these patterns by tracking the following:
- Actual range against route type
- Charging time
- Energy added
- State-of-charge changes
- Battery warnings
- Temperature-related performance changes
- Driver reports
Use proper EV diagnostic equipment
Base your fleet battery decisions on real data from your own vehicles and batteries. Professional EV diagnostic tools help your technicians compare cell groups, read battery codes, assess SoH and identify whether the issue is imbalance, degradation, cooling, charging or another system fault.
Investigate changes as soon as possible
Don’t leave potential issues until the last minute. That’s when something’s likely to go wrong, throwing the entire day into chaos. Schedule routine checks on a regular basis, and reactive tests whenever you notice anomalies in your data. Early checks reduce downtime and protect your investment.
Balance your EV traction batteries with Rotronics
Rotronics supplies professional EV and battery diagnostic equipment to help your workshop or fleet teams test battery condition, identify imbalance and make better maintenance decisions based on real data.
If you manage electric vehicles, now is the time to build EV battery testing into your maintenance programme. Contact Rotronics to learn more about professional battery diagnostics for modern fleet workshops.
