EV Battery Balancing: The Hidden Risk in Used EVs

A used EV arrives on your forecourt. The dashboard shows 95% state of health. The range estimate looks reasonable. You list it, price it, sell it.

Three months later, the buyer reports that the range is falling short of what was promised. The battery is degrading faster than expected. They want a resolution under the Consumer Rights Act.

What went wrong? The battery wasn't balanced.

What Battery Balancing Actually Means

A modern EV battery pack isn't a single unit. It's an assembly of hundreds of individual cells working together. A typical pack operates at 300-800V and contains a carefully engineered architecture:

• 3 cells wired in parallel form one cell block
• 108 cell blocks wired in series form the complete pack
• Total: 324 individual cells that must work in harmony

Here's the critical distinction. Cells wired in parallel naturally self-equalise — their voltages converge because they're directly connected. But cells wired in series do not self-balance. They operate independently, and over time, they drift apart.

This drift is called cell imbalance, and it's one of the most significant hidden risks in any used EV.

Why Cells Become Imbalanced

No two cells are identical. Even fresh from the factory, there are tiny differences in capacity and internal resistance. Over time, these differences compound:

• Manufacturing tolerances: Slight variations in chemistry and construction
• Temperature gradients: Cells in the centre of the pack run hotter than those at the edges, causing uneven aging
• Uneven load distribution: Some cells work harder than others depending on their position in the circuit
• Charging habits: Frequent fast charging stresses cells unevenly
• Calendar aging: Even parked, cells degrade at slightly different rates

The result is a pack where some cells have significantly less capacity than others. And here's the problem: the weakest cell determines the total usable energy of the entire pack.

Think of it like a chain. The pack can only charge as high as the weakest cell's maximum and can only discharge as low as the weakest cell's minimum. Every cell that drifts below the average effectively shrinks the usable capacity of the entire battery.

The Consequences of Imbalance

Cell imbalance isn't just a theoretical concern. It has real, measurable consequences that directly affect the vehicles you sell:

Reduced Range

The most obvious impact. If the weakest cell has 80% of its original capacity while the average is 92%, the pack's usable energy is constrained by that weakest cell. The driver gets less range than the state of health figure suggests.

Inaccurate SoC Readings

This is particularly dangerous. The Battery Management System (BMS) estimates state of charge based on pack-level readings. When cells are imbalanced, the BMS can overestimate remaining range. The dashboard says 50 miles remaining, but the weakest cell hits its lower limit at 35 miles. The car cuts power unexpectedly.

For a buyer who has just purchased a used EV, this is alarming — and it generates complaints.

Accelerated Degradation

Imbalance creates a vicious cycle. Weaker cells are pushed harder relative to their capacity, which causes them to degrade faster, which increases the imbalance further. Left unchecked, a moderately imbalanced pack becomes a severely imbalanced pack.

Increased Safety Risk

In extreme cases, cell imbalance can create conditions where individual cells are overcharged or over-discharged — both of which increase the risk of thermal events. While modern BMS systems include safety cutoffs, these are last-resort protections, not solutions to underlying imbalance.

As Patrick Schabus of AVILOO puts it: "For the driver, good balancing means more range, longer battery life, and maximum safety."

How the BMS Tries to Fix It

Every EV has a Battery Management System that attempts to keep cells balanced. The industry standard approach is called top balancing: during charging, the BMS bleeds energy from cells that reach full charge before others, allowing the lagging cells to catch up.

It works — to a point. But top balancing has limitations:

• It can only balance during charging, and only when the pack reaches high state of charge
• The balancing rate is slow (typically 50-100mA), so significant imbalances take many charge cycles to correct
• If the vehicle is rarely charged to high SoC (as many owners do to preserve battery life), balancing opportunities are reduced
• It cannot restore lost capacity — it can only equalise what remains

For cell-level diagnostics, the balancing state of a pack is a critical data point. A well-balanced pack with moderate degradation is a fundamentally different proposition from a poorly balanced pack with the same headline SoH figure.

The 800V Problem

The industry is moving toward 800V battery architectures (Porsche Taycan, Hyundai Ioniq 5/6, Kia EV6, and many more). Higher voltage means more cells in series — roughly twice as many as a 400V system.

More series cells means more opportunities for imbalance. The precision required for balancing increases, and the consequences of imbalance are amplified. As 800V vehicles begin entering the used market in volume, this becomes an increasingly important diagnostic consideration.

Why the Dashboard Won't Tell You

Here's the core problem for dealers. The vehicle's own systems report pack-level data. The dashboard SoH reading is derived from the BMS, which estimates based on aggregate measurements.

A pack-level reading can show 92% SoH while individual cells range from 85% to 96%. The headline figure looks healthy. The reality is a pack with significant imbalance that will cause problems for the next owner.

This is why state of health is not a simple number. The methodology behind the measurement determines whether you're seeing the full picture or a misleading average.

How Cell-Level Testing Reveals Imbalance

The AVILOO FLASH test, which powers Battery Health Check's service, measures individual cell voltages and state of charge across the entire pack. The results are presented as a heatmap visualisation:

• Green cells: Voltage and SoC within normal range, well balanced
• Yellow cells: Minor deviation, worth monitoring
• Red cells: Significant deviation, indicates a problem

This cell-level view reveals imbalances that pack-level readings completely miss. A three-minute test shows you exactly which cells are underperforming and by how much — before you list the vehicle, before you price it, and critically, before you sell it.

For NMC and NCA battery chemistries, the diagnostic requires a minimum 80% state of charge for accurate cell-level readings. LFP batteries require 95% due to their flatter voltage curve. These thresholds ensure the test captures meaningful voltage differentials between cells.

What This Means for Your Dealership

Every used EV on your forecourt has a balancing story. Some will be well balanced with healthy cells across the board. Others will have hidden imbalances that will cause range complaints, buyer dissatisfaction, and potential Consumer Rights Act claims.

The difference between a good acquisition and a costly liability often comes down to data you cannot see from the driver's seat.

A single AVILOO FLASH test takes three minutes and costs approximately GBP 35. It shows you the balancing state of every cell in the pack, not just the headline SoH number. That visibility is the difference between confident pricing and guesswork.

Ready to See What's Inside the Pack?

Battery Health Check brings TUV-certified cell-level battery diagnostics to UK dealers. No upfront hardware costs, full training, and certificates your buyers can trust.

Book a demo and see a cell-level heatmap for yourself. Three minutes is all it takes to know exactly what you're selling.