When Energy Storage Systems Need Active Balancing (and When Passive is Enough)

The Daily Cycling Problem Passive Balancing Was Not Designed For

A consumer e-bike pack cycles maybe once a day, often less. A power tool gets used in short bursts. Most general lithium applications give the pack plenty of time to sit at rest, and any small imbalance between cells gets corrected slowly in the background. Passive balancing — typically around 100mA shunted across the highest cell during the top of charge — works perfectly fine for that duty profile.

Energy storage is different. A solar-coupled home battery cycles deeply every single day, year after year. A commercial storage system may cycle multiple times a day. Over thousands of cycles, even small differences between cells — manufacturing tolerance, slight age differences, temperature gradients across a 16S pack — accumulate into measurable voltage drift. The pack stops being a uniform string and starts behaving like its weakest cell. The whole-pack capacity drops, the imbalance widens, and eventually the BMS has to disconnect early to protect the weakest cell — leaving usable capacity stranded.

This is the failure mode that pulls energy storage buyers toward active balancing. The question is not whether active is better in general — it is whether the duty cycle of your project is demanding enough that passive balancing cannot keep up.

What 100mA Passive Balancing Actually Does (and Where It Falls Short for ESS)

Passive balancing works by burning off excess energy from cells that reach full charge first, as small amounts of heat across a shunt resistor. A typical 100mA passive balancing current is enough to handle the drift that accumulates in lighter-duty applications — but it has two structural limits that matter for storage:

• It only acts at the top of charge. A passive system needs cells to reach the balancing threshold (usually high SOC) before it can equalize them. In partial-cycle storage operation that rarely sees a full charge, passive balancing gets less opportunity to work.
• Its rate is small relative to the drift that may accumulate daily. In some ESS duty cycles, imbalance can accumulate faster than a 100mA passive balancing system — applied only in a limited window at the top of charge — is able to correct, so the gap may widen over months rather than close.

For shallow-cycle applications, passive balancing matches the duty cycle well and adds the least cost. The problem for ESS specifically is the mismatch between drift rate and correction rate when the duty cycle is heavy.

What Active Balancing Adds (and Where the Real Value Lies)

Active balancing works by transferring energy from higher-voltage cells to lower-voltage cells — typically through an inductive or capacitive transfer circuit — rather than burning it off as heat. Two practical consequences follow:

• Higher balancing current. Where passive is around 100mA, dedicated active balancing in storage BMS is typically in the 1A range — an order of magnitude faster correction.
• It can work across more of the SOC range, not only at the top of charge. This matters in storage operation where the pack may rarely sit at 100% SOC.

The net result for an ESS project is that cell voltage drift can be corrected at a rate that more closely matches the rate it accumulates. Active balancing can help the pack stay closer to a uniform string over its service life, reducing the likelihood that usable capacity is stranded by the weakest cell. The qualifier worth keeping in mind: balancing performance in service depends on the rest of the system — pack-cell matching at the start, thermal spread across the string, and where in the SOC window the balancing is allowed to operate. Specific balancing data for a given pack configuration is something to confirm with the engineering team rather than to assume from the data sheet alone.

When Passive Balancing is Enough (Do Not Over-Spec)

Active balancing is not a default upgrade. For a wide range of applications, passive is genuinely the right answer:

• Light-duty backup systems that cycle infrequently
• Telecom UPS packs that operate primarily as standby and rarely deep-cycle
• Small consumer-scale storage where the project economics do not justify the added BMS cost
• Well-matched cells with tight initial tolerance, where drift accumulates slowly

Specifying active balancing for these applications adds cost without proportional benefit. A good supplier will tell you when passive is the right answer for your project — and a red flag worth noting is a supplier who recommends active balancing for every project without a clear technical rationale tied to your duty cycle.

When Active Balancing is Worth Specifying for Your Storage Project

The duty-cycle conditions that tip the balance toward active for energy storage are reasonably specific. If your project meets several of these, active balancing is worth specifying:

• Daily deep cycling. Solar-coupled storage that discharges meaningfully every day, year after year, accumulates drift faster than periodic top-of-charge balancing can correct.
• Multi-year service life expectations. The longer the system is expected to run, the more cumulative drift active balancing helps you protect against.
• Larger pack configurations. A 16S string has more places for drift to develop than an 8S string, because a larger number of cells in series increases the probability of cell-to-cell variation across the string. Storage packs at 48V (15-16S) and above benefit more from faster correction.
• Parallel pack architecture. Active balancing operates at the cell-to-cell level inside each pack — it does not equalize between parallel packs, but it helps each individual pack maintain internal consistency, which supports more predictable behavior when multiple packs operate together in a bank.
• Partial-cycle operation. If your storage profile rarely takes the pack to full charge (peak shaving, self-consumption optimization), passive balancing's reliance on the top-of-charge window becomes a real limitation.

Quick Selection Reference

To summarize, here is how active versus passive balancing typically maps to common applications. Treat this as a starting point for your RFQ, not a substitute for matching against your specific duty cycle:

Table is a starting reference; specify against your actual duty-cycle profile rather than the application label alone.

DALY Active Balancing for Storage Applications

For projects where active balancing is the right specification, DALY's 4th Generation Energy Storage BMS line carries it natively. The LK variant provides 1A active balancing for standard home storage configurations; the LM-B variant provides 2A active balancing for higher-current and larger-capacity systems. Both support 8-16S LFP and the parallel-pack architecture common in home and small-commercial storage, scaling up to 16 packs in parallel (around 160kWh per network) for projects that grow over time.

Two qualifiers worth noting in any pre-RFQ conversation: balancing performance in deployment depends on the rest of the system as discussed above, and specific configuration data — including balancing trigger logic, SOC window, and pack-cell matching guidance — is something the engineering team will work through with you on a project basis rather than something to assume from a data sheet.

Frequently Asked Questions

Q1Is 1A active balancing always better than 100mA passive?

Not always — what counts as better depends on what your duty cycle is doing to the pack. For applications where drift accumulates slowly (light-duty backup, shallow cycling), 100mA passive correction matches the problem and adds the least cost. For applications where drift accumulates faster than 100mA can correct (daily deep cycling in storage), 1A active correction matches the problem better. Match the balancing approach to your duty cycle, not the other way around.

Q2Does active balancing extend cycle life?

Cycle life is a property of the cells themselves, not something balancing creates. What active balancing does is help the pack reach the cells' rated service life by reducing the risk of imbalance pushing individual cells out of their safe operating window. The cells determine the ceiling; balancing helps you actually approach that ceiling rather than being limited by the weakest cell. Specific service-life data for your configuration is a project-level conversation with the engineering team.

Q3If I am unsure whether my project needs active or passive, what should I do?

Provide your duty-cycle profile to the supplier — daily cycle depth, expected cycles per year, target service life, pack size, and whether the system will reach full charge regularly. A supplier who specifies based on that information rather than defaulting to the higher-priced option is the one to take seriously. If you cannot get a specification rationale that ties back to your duty cycle, that is the data you need before the RFQ goes out.

About DALY

DALY designs and manufactures lithium battery management systems for OEMs, pack manufacturers, and integrators, with products used in 130+ countries. Founded in 2015, DALY operates under ISO 9001 / ISO 14001 systems with CE and RoHS compliance; the energy-storage line carries UL Recognized Component status (not full UL system certification — the distinction matters for North American projects), with documentation provided to support system-level certification at the pack or system level.

Specifying Active Balancing for Your Storage Project?

If you are scoping a solar storage, home battery, or small-commercial ESS project and want to specify balancing correctly, the DALY engineering team can review your duty cycle and help you match the BMS approach to it.

• Share your duty cycle: daily cycle depth, expected service life, pack size, parallel configuration
• Request 4th Gen LK / LM-B specification documentation
• Email: dalybms@dalyelec.com

Active balancing product page: https://www.dalybms.com/active-balancing-products/