If you’ve bought an electric vehicle, a home energy storage system, or dabbled in model‑car batteries, you’ve probably heard the fancy term “BMS battery balancing.” It’s often touted as the “magic key” to extending battery life. But is that really the case?

Today, we’re going to break down battery balancing into its component parts and explain it clearly—what it actually is, what types there are, and… The most crucial point: when is it completely useless? 。

I. Why is “balance” necessary?

A battery pack is assembled by connecting anywhere from a few to hundreds or even thousands of cells in series. However, no two leaves in the world are exactly alike, nor are any two battery cells identical. Manufacturing tolerances, temperature variations, and differing rates of aging all cause the voltage and capacity of individual cells to gradually diverge.

The result is The law of the barrel The entire battery pack’s capacity is bottlenecked by the weakest cell. When it’s fully charged, the others haven’t reached full capacity yet; when it’s discharged, the others still have some charge, but the system must shut down early to ensure safety.

Balance is precisely intended to address this issue of “mismatched pacing.”

II. Two Mainstream Equilibria: Passive vs. Active

🔻 Passive balancing — the clumsy “shortening the long” approach

Passive balancing is the earliest and most widely used technique. Its logic is straightforward: Whichever voltage is higher, just connect a resistor to it, turning the excess electrical energy into heat and “burning it off.” , until everyone has the same voltage.

Advantages : The circuit is simple, low-cost, and technologically mature.

Disadvantages ：

Energy is wasted as heat, resulting in low efficiency.

The balancing current is very small (typically 20–100 mA), so high-capacity batteries require a long balancing time.

The heat generation is severe, requiring effective heat dissipation, and it can only operate during the final stage of charging or when the device is at rest.

Applicable scenarios: electric tools, light‑weight electric vehicles, small‑scale energy storage systems, and other cost‑sensitive applications requiring good consistency.

🔄 Active Balancing — A Smart Solution That “Shortens the Long and Complements the Short”

Active balancing, on the other hand, is far more advanced: Transfer the energy from the high-voltage cell to the low-voltage cell via the circuit. , enabling energy recovery and reuse.

Advantages ：

Energy utilization efficiency reaches 85–95%, with virtually no waste.

The balanced current can reach 2 A or even higher, with much faster response.

It can operate in any state—charging, discharging, or at rest.

It can effectively slow the progression of cell-to-cell variation, thereby extending the overall service life.

Disadvantages ：

The circuit is complex, with numerous components, leading to a significant increase in cost.

The design is highly challenging, and the control algorithms are demanding.

Standby power consumption is slightly higher than that of passive balancing.

Applicable scenarios: electric vehicles, large-scale energy storage systems, high-end consumer electronics, and other fields that prioritize performance and longevity.

III. Understanding the Core Differences at a Glance

IV. ⚠️ Important Reminder: A balanced diet is not a panacea!

This is the most easily overlooked point. Many people assume that once they’ve installed a high‑end equalization system, their battery is completely safe—nothing could be further from the truth.

Equalization can only correct “mismatched charging rates”; it cannot revive cells that are already beyond repair.

In the following cases, balance is essentially… Useless , and even harmful:

1. Permanent degradation of cell capacity (“the bottom plank of the barrel has broken”)

If a cell’s physical capacity has already degraded by 20%—for example, dropping from 100 Ah to 80 Ah—no amount of active balancing can change the fact that it can no longer hold that much charge. During charging, it will reach full capacity first; during discharging, it will empty first. The overall pack’s range will still be dictated by that cell. Balancing merely increases energy consumption and heat generation without any real benefit.

2. Internal micro‑short circuits or severe self‑discharge (“funnel effect”)

When micro‑short circuits occur inside the cell due to dendrite formation or other issues, the voltage continuously “leaks away.” Even if you supply energy, the cell behaves like a leaky bucket—charging while simultaneously losing charge. The balancing current may not even match the leakage current, and after compensating, the voltage can drop again after a short rest. Repeated, futile cycles only waste energy.

3. Severe internal resistance degradation (“dynamic pseudo‑mismatch”)

The typical basis for balancing judgment is the static open-circuit voltage. However, if a cell has extremely high internal resistance, its voltage will fluctuate sharply during high‑current charging and discharging—spiking upward on charge and dropping abruptly on discharge—triggering the protection circuit. Yet once the current ceases, the voltage returns to normal. When the balancing system sits idle and measures what appears to be a “normal” voltage, it fails to activate at all. As a result, in real‑world use, faults are reported repeatedly, leaving the balancing system powerless to resolve them.

4. Physical connection error (poor contact)

Loose or cold‑soldered connections can create contact resistance. Under high current, the voltage drop at such a node becomes substantial, leading to severe distortion of the voltage readings captured by the BMS. The balancing system, relying on this “false data,” may make incorrect decisions—potentially discharging a healthy cell as if it were a high‑voltage cell. Not only is this ineffective, but it can also exacerbate cell‑to‑cell imbalance and even cause localized overheating at the contact point, posing a safety hazard.

V. Conclusion

At its core, balance is the work of a “fine-tuner,” not a “miracle healer.”

On the premise that the cells themselves are in good condition, with normal internal resistance and reliable connections, it is responsible for correcting minor deviations in state of charge, ensuring the team moves in greater synchrony. However, if a particular cell is already “seriously ill”—exhibiting capacity degradation, internal short circuits, a sharp increase in internal resistance, or abnormal connections—then… The only correct course of action is to replace the faulty battery cell. , rather than relying on the equilibrium system to work a “miracle.”

Remember: Balance can address “unevenness,” but it cannot fix “failure.” Only by approaching technology with rationality can we make the most of every kilowatt-hour and navigate each journey with confidence.