People building lithium packs often ask whether they need a BMS, an active balancer, or both. They do different jobs, and only one of them keeps the pack safe.
What a BMS does
A BMS is the pack's safety switch. It watches every cell and disconnects the pack when something goes wrong:
- a cell goes above its maximum voltage (over-charge)
- a cell drops below its minimum voltage (over-discharge)
- the current is too high, or there is a short circuit
- the temperature is out of range (on models with temperature sensors)
Every lithium pack needs a BMS. Most BMS boards also do some balancing, but that is a side job.
What an active balancer does
An active balancer moves energy from the highest cells to the lowest ones, so all cells stay at the same voltage. It does not protect the pack: it cannot cut the power if a cell is over-charged or shorted. That is why an active balancer is always used together with a BMS, never instead of one.
Why balancing matters
A pack is only as good as its weakest cell. If one cell reaches full charge first, the BMS stops charging the whole pack, even though the other cells still have room. On discharge, the lowest cell trips the BMS first. Over months, a pack with drifting cells delivers noticeably less of its rated capacity. Good balancing gives you that capacity back.
Is the BMS's own balancing enough?
| Balancing type | Typical current / what it suits |
| Passive (most standard BMS) | Well under 0.1 A - fine for small packs of new, matched cells |
| Active, built into a smart BMS | 0.6 A on JK smart BMS models, 1 A or 2 A on some - fine for most home and inverter packs |
| Separate active balancer | 2 A to 15 A on JK balancers - for large packs, used or mixed cells, or a pack whose BMS balances slowly |
When to add a separate active balancer
- Large packs - 200 Ah and up, where a small balance current takes too long to even out a big difference.
- Used or second-life cells, or cells from different batches that drift apart quickly.
- Your BMS has only passive balancing and the app or a meter shows cell voltages spreading apart at full charge.
- Daily deep cycling - solar storage and e-rickshaw packs that are fully charged and discharged every day.
If your cells are new, matched and top-balanced, and you use a smart BMS with active balancing, you usually don't need a separate balancer.
How to choose a balancer
- Series count: the balancer must cover your pack - 4S, 8S, 16S or up to 24S.
- Balance current: 2 A is plenty for most home packs; 4 A to 15 A suits very large packs.
- Smart or not: a smart balancer shows cell voltages in an app, which helps spot a weak cell early.
Options include the JK Smart Active Balancer 8S 2A, JK Smart Active Balancer 24S 2A, JBD Smart Active Balancer 4S 5A and Daly Smart Active Balancer 16S 1A.
The short answer
- BMS: always needed - it is the safety system.
- Active balancer: optional - it improves usable capacity and cell life on large or mismatched packs.
Choosing the BMS itself? Read how to choose a BMS for a LiFePO4 battery, or browse all BMS & Balancers.