The battery packs in your EVs are made up of hundreds (and in some cases thousands) of individual cells connected in series. The more cells connected, and the higher the voltage of each cell, the higher the overall pack voltage, which powers the vehicle.
However, keeping all those individual cells balanced – meaning each cell maintains a similar voltage – is more challenging than it might seem. Proper cell balancing is essential not only to maximise performance and extend battery life but also to ensure safe vehicle operation.
You may have heard of passive top balancing before. In this article, we’ll explain what passive top balancing is, how it works, and why it matters in your workshop or fleet.
Why battery cell balancing matters
In an ideal world, every cell inside a battery pack should be exactly the same. The same capacity, same voltage, same charge level, same state of charge (SOC) and same state of health (SOH). But the reality is that if you look deep enough (at anything), no two are exactly alike. Even from day one, there are tiny differences between cells, in things like internal resistance, chemistry or how they handle temperature. You won’t spot them without the right data, but they’re there.
And as the pack ages and gets used, those differences grow. Heat, load and repeated charge cycles put extra strain on the weaker cells, which fall further behind. To keep up with the rest of the pack, they either undercharge or over-discharge, while the stronger cells risk overcharging if the battery management system doesn’t intervene.
Left unmanaged, that’s what creates imbalance in an EV battery pack. And if the cells aren’t balanced:
- The pack’s overall capacity shrinks, limited by the weakest cell.
- Cells can overcharge (with thermal runaway risks!) or undercharge, leading to faster wear and safety risks.
- The vehicle’s battery management system (BMS) can’t accurately judge battery health, which affects charging and performance.
How passive top balancing works
So, balancing is extremely important. It means making sure all the cells reach the same voltage level, so the battery works as one strong unit.
And passive top balancing is an EV’s built-in way of balancing its own batteries.
All electric cars come with a BMS. It monitors and controls each battery cell to keep the whole pack safe, efficient and balanced. It uses passive top balancing to even out cell voltages, especially near full charge. Here’s how:
- The BMS monitors each cell’s voltage as the pack charges.
- When a cell hits its max voltage (say, 4.2 Volts for lithium-ion), the BMS activates a small resistor across that cell.
- This resistor safely burns off the extra energy as heat, so it doesn’t overcharge.
- Charging continues for the lower-voltage cells until all cells reach the same voltage.
It’s called ‘passive’ because it doesn’t shift energy between cells. It simply burns off the excess as heat. And it’s called ‘top balancing’ because it happens near the top of the charge cycle, when the battery is close to full.
In very simple terms, let’s say you have a group of friends trying to finish their pizzas at the same time. However, some eat faster than others. To keep things fair, you make the ones who are nearly done stop eating for a while, giving the slower eaters a chance to catch up.
Passive top balancing is one of the most common balancing methods used in EVs, but it’s not the only one. More advanced systems may use active balancing, which moves charge between cells to improve efficiency and reduce energy loss.
What does passive top balancing mean for the workshop?
Relying solely on passive balancing means trusting the vehicle’s BMS to manage battery health on its own. While the BMS handles passive top balancing by bleeding off excess voltage from higher-charged cells at the top of the charge cycle, your workshop must stay aware of what’s going on. In high-mileage fleet environments, assuming the BMS covers everything often leads to overlooked issues.
Passive top balancing is a simple, proven method that slows cell wear and improves safety. But it’s not a complete solution, especially as battery packs age or cell voltages begin to drift.
Think of balancing as just one part of battery health management. If you notice range loss, longer charge times or reduced performance, imbalance is likely a factor. Passive balancing can struggle when charging currents are high or when the vehicle is frequently charged before full discharge. In these cases, cells don’t have enough time to equalise, and voltages drift apart.
Your technicians need to test individual cell voltages, much like checking 12-volt battery pairs in 24-volt systems, using dedicated EV battery testers. Consistent voltage differences indicate the passive system isn’t keeping up, requiring deeper diagnostics or reconditioning.
Passive balancing works, but only when combined with regular, proactive maintenance. It’s one piece of the puzzle. The rest depends on your team, your tools and your testing routine.
Partner with Rotronics for smarter battery management
At Rotronics, we know batteries inside and out. For over 20 years, we’ve supported workshops and fleets with trusted diagnostic tools from brands like Midtronics and CTEK, plus our ROBIS Analytics portal for collecting and monitoring actionable battery data.
Among our range of products are tried and tested solutions for EV battery module testing and charging. We’re committed to helping clients maximise EV lifespan, range, performance, and safety. Your success is our success.
And we don’t just supply the kit. We’ll work alongside you to build practical, hands-on battery maintenance and management programmes that cover testing, balancing, charging and ongoing monitoring.
If you want to get serious about EV and fleet battery balancing, we’re here to help. Get in touch today to discuss your options and learn how we’ll tailor our solutions to your specific needs.
