2S and 3S BMS Boards Explained: Charging Lithium Cells in Series

One lithium cell gives 3.7 V. Many projects need more: a robot with 6 V motors, a 12 V LED strip, a 7.4 V servo. Put two or three cells in series and you get 7.4 V (2S) or 11.1 V (3S), but you also get a new problem: the cells drift apart, and a single over-charged or over-discharged cell can be damaged or become dangerous. A BMS (battery management system) board watches every cell in the pack. This guide explains what it does, how to wire it and how to charge the result.

What "2S" and "3S" mean

The number is how many cells are in Series:

Pack	Nominal	Fully charged	Empty (about)
1S	3.7 V	4.2 V	3.0 V
2S	7.4 V	8.4 V	6.0 V
3S	11.1 V	12.6 V	9.0 V
4S	14.8 V	16.8 V	12.0 V

Packs are also described like "3S2P": three groups in series, each group made of two cells in parallel. Cells in parallel behave like one bigger cell, so a 3S2P pack still needs a 3S BMS.

What the BMS does

A BMS sits between the cells and the outside world, with a set of MOSFET switches on the negative side. It disconnects the pack when:

• Any cell is over-charged (above about 4.25–4.3 V).
• Any cell is over-discharged (below about 2.5–3.0 V, depending on the board).
• Too much current flows (over-current), or there's a short circuit.

Boards sold as "balance" or "with equalization" add small resistors that slowly drain the fullest cell near the end of charging, so the cells stay at the same voltage over many cycles.

What the BMS doesn't do: it doesn't charge. It only cuts off when something goes wrong. You still need a proper charger, covered below. The difference between BMS boards and charger modules is explained in battery charging modules compared.

Before you build: match the cells

A BMS can't fix badly matched cells. For any series pack:

• Use cells of the same model, capacity and age, ideally from the same batch.
• Charge each cell individually to the same voltage before connecting them (within about 0.05 V). A single-cell TP4056 charger is perfect for this.
• Never mix new and used cells, or cells from different brands.
• Avoid soldering directly to cells. Use series battery holders or cells with welded nickel strip.

Pads on a 3S BMS

Pad	Connects to
B−	The negative terminal of the first cell (pack negative)
B1 (or BM)	The junction between cell 1 and cell 2
B2	The junction between cell 2 and cell 3
B+	The positive terminal of the last cell (pack positive)
P−	The negative output: to the load and the charger
P+	The positive output (often the same point as B+)

A 2S board has B−, B1 (or BM) and B+. Some boards have separate C− (charge) and P− (load) pads; most cheap boards use one common P− for both.

Wiring order

The order matters: connecting the wires in the wrong order can momentarily put the full pack voltage across a part of the board meant for one cell.

1. Assemble the cells in series in their holder, but don't connect the BMS yet.
2. Check each cell voltage with a multimeter. They should match.
3. Connect B− to the pack's negative terminal.
4. Connect B1, then B2, working up the pack.
5. Connect B+ (and P+ if separate) to the pack's positive terminal.
6. Connect your load and charger between P+ and P−. Never connect the load directly to the cell's negative, or the BMS can't protect anything.

Activation: why the output reads 0 V

Many BMS boards start in a protected state after the cells are first connected: the cells read fine, but P+ to P− shows 0 V or a very low voltage. Connecting the charger for a second "wakes" the board and the output appears. If yours reads 0 V after wiring, check the voltages between each B pad first, then briefly connect the charger.

Choosing the current rating

BMS boards are sold by current: 10 A, 20 A, 40 A and so on. Pick one rated above the highest current your project draws, including motor start-up and stall current. If the rating is too low, the BMS cuts power every time the motors start, and the robot "dies" for no apparent reason. On the other hand, a huge rating means the over-current protection rarely helps, so don't oversize it ten times.

For a small robot with two gear motors, a 10 A board is plenty. For a 3S pack running a brushless motor and ESC, check the motor's maximum current and choose a board above it.

Charging the pack

You need a CC/CV charger set to the full pack voltage:

• 2S: 8.4 V, for example a TP5100 module set to 2S, or a dedicated 8.4 V Li-ion charger.
• 3S: 12.6 V, a dedicated 12.6 V Li-ion charger, or a CC/CV buck module (such as the XL4015) carefully set to 12.6 V and a safe current with no load connected. A buck module doesn't stop charging by itself, so disconnect it once the current drops to about a tenth of the set value.

Set the charge current to about 0.5 C or less (half the capacity of one parallel group in amps: 1.5 A for 3000 mAh cells).

A plain 12 V adapter isn't a 3S charger. It's below 12.6 V (so the pack never fills), it has no current limit (so the first minutes can overload the adapter or the BMS), and some "12 V" adapters actually output 13 V or more with no load. Hobby LiPo balance chargers (the kind used for RC models) are an excellent alternative: they charge through the balance leads and show each cell's voltage.

Testing the finished pack

1. Measure each cell (B− to B1, B1 to B2, B2 to B+). They should be within 0.05 V.
2. Measure P+ to P−: it should equal the total.
3. Charge fully once, then measure the cells again. A cell that ends much higher or lower than the others is weak; replace it.
4. Watch for heat during the first charge and the first discharge at full load.

Safety

• Build packs on a non-flammable surface, with no metal tools lying across the terminals.
• Insulate every exposed connection with heat shrink or Kapton tape, especially B+.
• Don't charge below 0 °C or above 45 °C, and never leave a new pack charging unattended.
• If a cell gets hot, swells or hisses, disconnect it and move it outside on a non-flammable surface.

Troubleshooting

• Output 0 V after wiring: the board needs activation; connect the charger briefly.
• Robot cuts out when motors start: BMS current rating too low, or a weak cell drops below the cut-off under load.
• Pack never reaches full voltage: the charger is set below 8.4 V or 12.6 V, or the cells are unbalanced and one hits the cut-off early. Use a balancing BMS or a balance charger.
• One cell always lower: that cell is weaker. Balancing only hides the problem for a while; replace it.