A battery pack can show a healthy overall voltage while one series group is already near its safe limit. That is why you balance lithium cells at the group level, not by relying on the pack’s total voltage alone. In a 10S e-bike pack, for example, ten groups may add up correctly even though one group charges too high or drops away early under load.
Cell balancing keeps the series groups in a lithium-ion pack at closely matched voltages. It supports usable capacity, prevents repeated overcharge or over-discharge of the weakest group, and gives the battery-management system, or BMS, a fair chance to protect the pack. It is not a cure for worn-out, mismatched or damaged cells. If a group will not hold its voltage, balancing only helps reveal the underlying fault.
What it means to balance lithium cells
Most DIY lithium-ion packs use cells in parallel groups connected in series. A 13S4P pack, for instance, has 13 series groups, each made from four matched cells in parallel. The four cells within a parallel group naturally settle to the same voltage. The BMS monitors and balances the 13 series groups.
Balancing works by reducing the charge in the highest-voltage groups, usually through small bleed resistors in the BMS. This is called passive balancing. It commonly begins near the top of charge and may only move 30mA to 100mA, depending on the BMS. That is enough to correct small differences over time, but it is far too slow to fix a badly mismatched pack.
Active balancing is different. An active balancer transfers energy from higher-voltage groups to lower-voltage groups rather than burning the excess as heat. It can be useful in larger storage packs, but it adds complexity and does not remove the need for well-matched cells, sound assembly and a correctly specified BMS.
The key point is simple: balancing maintains a good pack. It does not turn a poor pack into a safe one.
Why series groups drift out of balance
New packs can drift if the cells were not checked and brought to a similar state of charge before assembly. A difference of only a few tenths of a volt at the start can create trouble if the BMS has a low balancing current and only balances at full charge.
Over time, the more common cause is variation in cell capacity or internal resistance. A group made with lower-capacity cells reaches full charge sooner and empties sooner. A high-resistance connection, weak spot weld or corroded nickel strip can also cause a group to sag under load, making it look like a cell problem when the connection is actually at fault.
Heat accelerates the issue. Packs used in high-current applications such as e-bikes, scooters, power tools and high-output torches need cells chosen for their continuous-discharge rating, not just their mAh figure. A high-capacity cell can be the wrong choice where sustained current demand is high. Repeatedly overloading it increases voltage sag, heat and uneven ageing.
Mixing cell models, ages, capacities or reclaimed cells is another frequent cause. Even cells with the same physical 18650 or 21700 format may have very different discharge behaviour. Build each parallel group with identical genuine cells from the same model and, ideally, the same batch and purchase date.
Check the pack before attempting balance
Do not start by connecting random cells together or bypassing the BMS. First, remove the pack from the device and work in a dry, non-flammable area away from combustible materials. If the pack is swollen, leaking, hot, physically damaged, smells unusual or has melted insulation, do not charge or repair it as a normal DIY job. Isolate it safely and seek appropriate battery recycling or specialist advice.
With the pack disconnected, measure every series group at the BMS balance connector or directly at the nickel tabs where safe access is available. Use a reliable multimeter and record each group voltage in order. Measuring at the BMS connector is useful because it can also expose a broken sense wire or poor connector contact.
A healthy, rested pack should show relatively close group voltages. The acceptable difference depends on chemistry, pack condition and state of charge, but a large spread is a warning sign. More importantly, look for one group that consistently sits lower than the rest after charging or drops much faster under load. That group needs investigation before any attempt to rebalance.
If the pack shuts down under load but group voltages look acceptable at rest, test again soon after a controlled load. A weak group may recover in voltage once the load is removed, hiding a capacity or resistance problem. Do not use a short circuit, improvised high-current test or damaged equipment to assess performance.
Inspect the cells and connections
For a serviceable pack, inspect nickel strip, spot welds, insulator rings, fish paper barriers, balance leads and BMS connections. Loose balance wiring can cause incorrect readings or stop balancing entirely. Never solder directly to bare cylindrical lithium-ion cells unless you have the correct process and understand the heat risk. Spot welding is the preferred method for pack connections.
If one parallel group is faulty, replace the complete group with properly matched cells rather than adding one new cell beside aged cells. A new cell and tired cells in parallel will share current unevenly. Where several groups show poor capacity or the pack’s history is unknown, rebuilding with matched new cells is usually the safer and more dependable option.
How to balance lithium cells safely
For a new build, test each cell first. Check physical condition and voltage, then use a suitable charger or analyser to confirm capacity where practical. Cells selected for one pack should be the same model, with closely matched measured capacity and similar resting voltage. Assemble parallel groups only after the cells are at nearly the same voltage.
Bring all groups to a consistent voltage before making the series connections. Many builders choose a middle state of charge for assembly rather than a fully charged state, which reduces stored energy while still making voltage matching straightforward. The exact target matters less than consistency: every group should be close before it is joined in series.
Connect the BMS exactly as specified by its wiring diagram. B-, pack negative, balance-wire sequence, charge port and discharge port arrangements vary between common-port and separate-port BMS designs. Connecting balance leads in the wrong order can destroy the BMS immediately. Verify each incremental voltage at the balance plug before plugging it into the BMS.
Once assembled, charge the pack with a charger matched to the series count and lithium-ion chemistry. A 10S lithium-ion pack requires a different full-charge voltage from a 13S pack. Do not assume a charger is suitable because its connector fits. Stay nearby for the first charge cycle and monitor group voltages near the end of charge.
If the BMS provides passive balancing, leaving the pack on its correct charger after it reaches full charge may give it time to trim small differences. This can take many hours because balancing current is low. Stop if a group rises abnormally, the BMS cycles repeatedly, or the pack becomes noticeably warm.
For an existing pack with a minor imbalance, a quality multi-channel charger or controlled bench supply can sometimes bring a low group back towards the others. This should only be done by someone comfortable working with exposed pack voltages and using current-limited equipment. Charging an individual group without monitoring it can push it beyond its safe voltage. In many cases, finding and replacing the weak group is the better repair.
Choose a BMS that suits the pack
A BMS should match the pack’s series count, chemistry, continuous current requirement and intended charging arrangement. Its discharge rating needs headroom above the real continuous load, particularly for motors and other applications with strong start-up current. It also needs adequate balance functionality for the pack’s likely drift and use pattern.
A basic BMS with passive balancing is suitable for many well-built 18650 and 21700 packs. For high-current builds, pay attention to the BMS’s thermal design, cable size, nickel layout and connector rating. A 60A BMS cannot make undersized nickel strip or low-drain cells safe at 60A.
The BMS is a protection device, not a substitute for correct cell selection. Start with genuine cells that suit the job, use appropriate insulation and spot-welded connections, then fit a BMS designed for the pack.
When balancing is not the answer
Repeated imbalance is a fault pattern. If the same group is always low, the cells may have lost capacity, a weld may be poor, or the group may be exposed to more heat than the rest. If every group drifts after a few cycles, check charger voltage, BMS wiring and whether the pack is being pushed beyond its design current.
A pack that has been deeply discharged, stored flat for a long period or assembled from unknown reclaimed cells deserves extra caution. Lithium-ion cells can suffer internal damage even when they appear normal externally. Replacing questionable cells costs less than dealing with a failed pack, damaged device or fire risk.
For Australian makers building or repairing packs, the practical approach is to measure each group, identify the cause of any spread, and only then decide whether the pack needs a top-balance, a BMS repair or matched replacement cells. TinkerTech AU stocks the cells and pack-building components needed for a properly specified rebuild, but careful measurement remains the first tool to reach for.