Battery Pack Basics: Series vs. Parallel Cells ⇱

A lithium-ion battery pack is made of many cells. Cells in series add up voltage (e.g. four 3.7 V cells in series give ~14.8 V). Cells in parallel increase capacity (more current/amp-hours) while keeping the same voltage. Using series cells lets us power high-voltage devices (like e-bikes or laptops), and parallel strings boost running time. But one rule is critical: all series cells see the same current flow. That means if even one cell has lower capacity or higher internal resistance, it will limit the whole string. In fact, a pack’s performance is always limited by its weakest cell – once that weak cell is empty, the whole pack is effectively empty.

• Series cells: Voltage adds up, but capacity is only as large as the smallest cell.

• Parallel cells: Current/capacity adds up, but all parallel cells must match well or the weakest drags the string.

Because of this, manufacturers initially match cells by capacity and voltage. Even so, tiny differences remain. Over time (with use and aging), these small mismatches grow. In practice, BMS (battery management system) hardware constantly monitors each cell’s voltage and state of charge (SoC) ⇱ to catch imbalances early. A good BMS will then trigger balancing measures to keep every cell in line.

Why Cells Become Imbalanced

No two cells are perfect clones. Even brand-new, same-model cells can have tiny differences. Factors that cause imbalances include:

• Manufacturing variations: Slight differences in capacity, internal resistance or chemistry mean some cells hold a bit more charge or age differently.

• Self-discharge ⇱ differences: Cells leak charge over time. One cell might lose 1% per month while another loses 0.5%, so SoC drifts apart during storage.

• Uneven temperatures: Heat accelerates aging. Cells running hotter than others will fade faster, leading to capacity gaps.

• Cycle aging ⇱: Over many charge/discharge cycles, some cells degrade (lose capacity) faster. Older or overworked cells end up with permanently lower capacity (irreversible imbalance).

These issues make cell voltages and SoC gradually diverge. For example, the fastest-charging/lowest-capacity cell might hit full voltage before others, causing the charger to stop early. Or during discharge, one high-impedance cell might sag in voltage first, cutting runtime short. In general, state-of-charge (SoC) imbalances (caused by charge/discharge cycles or self-discharge) are reversible: we can re-balance them by dumping or shifting charge. But capacity imbalances (due to permanent aging) are not fully fixable without replacing cells. In an aging pack, one cell may permanently hold, say, only 90% of the original capacity; no amount of balancing can give it more energy, it’s a lost cause.

Key Point: Even minor cell mismatches will hurt your pack’s performance. “Uneven cells in lithium batteries can lower performance and cause risks,” notes a battery engineering guide. Good practice is to use a robust BMS and balancing scheme to keep cells level and avoid one weak cell dragging down the pack.

Impact of Imbalance on Battery Performance

When a battery pack is well balanced, each cell charges and discharges evenly. This means: full use of the pack’s theoretical capacity, consistent voltage per cell, and all cells stay within safe voltage limits throughout the cycle. In other words, you get the full range and longer life you expect.

However, imbalance introduces risks. Consider what happens if cells diverge:

• Reduced Capacity (Range Loss): The pack can no longer use the energy in its stronger cells. If one cell reaches its minimum voltage (end-of-discharge) early, the whole pack must stop discharging even though other cells still have energy. Even if some cells still have charge left, the weak cell hitting empty forces shutdown, wasting usable capacity.

• Safety Hazards: Imbalanced cells also threaten safety. If a cell gets overcharged or overdischarged relative to its mates, it can overheat or suffer accelerated wear. Even when the pack current is safe, an outlier cell might cross its safe voltage or temperature limit, leading to thermal runaway or premature failure. That’s why a BMS continuously checks each cell’s voltage. When it spots a mismatch, it uses balancing to keep all cells in a safe range.

Passive (Dissipative) Balancing: How It Works

Passive balancing is the simpler, older method. It works by bleeding off excess charge from the high-SoC cells through resistors. In practice, the BMS uses switches to connect any “too-full” cell across a small resistor. The extra energy in that cell is dissipated as heat until its voltage drops in line with the other cells. This is like pouring out a little bit of water from an overfilled tank so its level matches the others.

Key features of passive balancing:

• Simple control: The hardware is straightforward. Each cell has a switch and resistor, and the BMS just turns on the switch when a cell’s voltage exceeds a threshold (often a few tens of millivolts above average). No special energy-transfer circuitry is needed.

• Predictable energy loss: All excess charge (100%) is lost as heat in the resistor. This wastes energy, so passive balancing is less efficient. However, the losses happen in short bursts only when cells drift apart, so overall impact on runtime is often acceptable for many devices.

• Widely used & low cost: Because of its simplicity, passive balancing dominates in consumer products. Many BMS chips (like Texas Instruments’ BQ77910A) include internal bleed circuits. For example, TI’s BQ77910A monitors up to 10 cells and provides 50 mA of bleed current per cell to equalize voltages. This integration keeps hardware cost down, which is why passive balancing is common in cars, e-bikes, laptops, power tools and other cost-sensitive packs.

In practice, passive balance usually runs only during charging (not discharge) to avoid wasting energy on the way down. Even so, it fixes SoC differences just fine. “Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell”. In effect, you use up to 100% of the lowest cell’s capacity in all cells. The trade-off? You throw away the rest as heat.

Active (non-dissipative) Balancing: Innovation and Limits

Active balancing ⇱ uses electronics to move charge from higher-voltage cells to lower-voltage cells instead of burning it off. In other words, extra energy isn’t lost – it’s transferred to “fill up” the weaker cell. This can happen via switched capacitors or transformers/inductors and converter circuits.

* Figure: Energy transfer path of capacitive balancing (cells connected through capacitors between adjacent cells).

How it works: A simple example is a bidirectional DC/DC converter (like a tiny flyback or buck/boost converter) connected between cells. When a cell has too much charge, the converter draws energy from it and pumps that energy into a lower SoC cell. The converter actively sends current back and forth under microcontroller control. Unlike passive balancing, active methods can even balance during discharge – top cells can feed charge to bottom ones while the pack is in use.

Advantages:

• Higher efficiency: Since energy is reused, active balancing wastes much less energy. MPS notes that passive balancing “simply dissipates energy; active balancing redistributes energy with a significant improvement in energy efficiency.” You keep that extra charge in the pack, improving the effective capacity.

• Works on both charge & discharge: Active schemes can balance while charging and discharging. This means balancing can happen continuously (even in the field), which is great for packs that cycle frequently.

• Faster balancing currents: Active circuits can often move more current. Passive is usually limited to a few hundred milliamps (often ≤0.25 A per cell), while modern active balancers can handle several amps. That means large or fast-cycling packs can rebalance quickly.

• Extended runtime: By equalizing during discharge, a pack can drain more of its total energy. The weakest cell empties at the same time as the others, rather than cutting off the pack early. In short, active balancing increases total usable battery runtime.

Drawbacks:

• Complexity and cost: Active balancing circuitry (bidirectional converters, extra switches, controls) is much more complex. It takes more parts and board space.

• Higher cost: Because of the extra components (transformers, inductors, MOSFETs), active solutions are pricier. For now, they’re mainly used in high-end or space-critical systems where efficiency and runtime are worth the cost (satellites, deep-sea equipment, large energy storage).

In short, active balancing is like using a pump and valves to circulate water between tanks – it keeps all levels equal without wasting water, at the expense of a bigger pump setup. Passive balancing is like simply opening a drain – easy but wasteful.

Passive vs. Active: A Side-by-Side Comparison

Below is a quick comparison of the two approaches:

Both methods aim to keep cell voltages aligned, but their trade-offs mean one may fit an application better. For example:

• Scenario 1 – SOC Imbalance (Reversible): When cells start with nearly equal capacity but diverge in state of charge (say from uneven self-discharge), either method can rebalance. Passive will waste some energy, while active will preserve it. In practice, both will equalize charge – passive by burning off the surplus, active by moving it. MPS observes that active balancing allows faster and more efficient equalization than passive.

• Scenario 2 – Capacity Imbalance (Permanent): If one cell has faded to lower capacity (aging), passive balancing hits a limit: you can only discharge the whole pack until that weakest cell is empty. Any extra capacity in other cells is unreachable (it’s “wasted” on paper). Active balancing shines here: by shifting charge from strong cells to weak ones, it can exploit some of that leftover capacity in every cycle, partially compensating for the weak cell. In other words, active can somewhat break the “weakest cell bottleneck,” whereas passive cannot.

Overall, experts note that passive is dominant in most consumer and EV battery systems because it’s cheap and works well enough. Active balancing, on the other hand, is used where efficiency is critical – think satellites, high-end drones, or large stationary batteries that need every bit of energy wrung out. As technology advances and costs fall, active balancing is gradually finding more applications (even DIY and home energy systems) due to its obvious benefit of less wasted charge.

Conclusions and Next Steps

• Balancing is essential. Keeping all cells at the same SoC is key to safe, efficient battery operation. A well-balanced pack delivers full capacity and avoids stress on any single cell.

• Passive vs. Active: trade-offs matter. Passive balancing is simple and cheap, ideal for most gadgets and vehicles. Active balancing saves more energy and can even extend the usable life of aging packs, but it costs more. The “right” choice depends on your application’s budget and efficiency needs.

• Plan for imbalance. Any real pack will develop imbalances over time. Design your system with a capable BMS and balancing strategy in mind. For example, Bonnen Battery’s Li-ion packs can be paired with advanced BMS chips that handle balancing for up to 16 cells or more. Chips like TI’s bq77910A include on-board balancing circuits (50 mA bleed per cell) and will auto-balance as needed.

• Contact Bonnen Battery for solutions. At Bonnen Battery, we specialize in high-quality Li-ion cells and cells/ BMS modules for balanced performance. Our engineers can help you select cells and a balancing approach that match your project, whether it’s a cost-sensitive EV pack or a high-end industrial system. To learn more or get a quote, visit Bonnen Battery’s website or reach out to our technical team. Let us help you keep your batteries running longer and safer!

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