DIY 18650 battery pack: cells, BMS, spot welding and charging

Series for voltage, parallel for capacity and current

Cells in series (S) add their voltages; cells in parallel (P) add their capacity and share the current. A 10S3P pack is ten groups in series, each group three cells in parallel: 30 cells in all.

The same figures as in our battery pack calculator. A “36 V” e-bike pack is 10S.

Choose S from the voltage your device expects: the motor controller, the converter or the tool states its input range, and the pack must stay inside it from full (4.2 V per cell) to empty (2.5–2.65 V per cell under load, depending on the cell). Then choose P for two things: the capacity you want, and the current the cells can deliver between them.

Our example is an e-bike-style 36 V pack for a motor controller limited to 20 A: 10S. With three LG MJ1 cells per group the calculator gives 36.35 V nominal (42 V full, 25 V at the cell’s 2.5 V cut-off), 10.5 Ah and 382 Wh. At 20 A each cell supplies 6.67 A, below its 10 A rating, and the whole pack can deliver 30 A.

The example in the pack calculator: 10S3P, LG MJ1, 20 A load.Open the pack calculator

Choosing cells: the real discharge rating

The figure that matters is the continuous discharge current in the cell maker’s datasheet. Seller listings often quote a peak or a pulse figure instead, or a number with no source at all. Cells come in two broad kinds: high-capacity cells, around 3.5 Ah with ratings of 8–10 A, and high-drain cells, around 2.8–3 Ah with ratings of 30 A or more.

From the datasheets (Molicel’s on its own site; LG and Samsung don’t publish theirs, so the links in the sources are copies hosted by others) and Molicel’s product pages. LG and Samsung don’t state on their sites whether these models are still in production: buy from distributors who can show the datasheet and the batch.

If the controller pulled 30 A instead of 20, each cell would carry 10 A: right at the MJ1’s rating and over the 35E’s 8 A, which the calculator flags (see it). A high-drain cell such as the P28A carries it easily, but stores less: the same 10S3P holds 8.4 Ah instead of 10.5 (P28A pack; P30B pack). Pick the cell for the current first, then add P for the capacity.

Buying genuine cells, and matching them

• Implausible capacity. The highest-capacity 18650 cells in the datasheets we checked are rated about 3.5 Ah. A “9900 mAh” 18650 is a fake, usually a much smaller cell in a big wrap.
• Weight. Genuine 18650 cells weigh about 45–50 g (46 g typical for the P28A, 50 g maximum for the 35E). Weigh a few on a kitchen scale: a cell at 30 g is not what the label says.
• Wrap and markings. Rewrapped cells hide their origin. Buy from distributors that sell the maker’s own cells, show the datasheet and can tell you the production batch.
• Measure on arrival. Cells from one batch should arrive at a similar resting voltage; one much lower than the rest has been abused or is faulty.
• Never mix used and new cells, different models or different batches in one pack: the weakest cell sets the pack’s limits, and it is the one that gets overcharged and overdischarged.

For a pack with parallel groups, match the cells: measure capacity (and internal resistance, if your charger can) and build each group from cells that are close, so that every group has nearly the same total capacity. A charger with a capacity test, such as the XTAR VC4SL (0.25 to 3 A, 3 A only with a single cell; a real capacity test; USB-C power), does this a few cells at a time.

The BMS

A battery management system protects the pack: it cuts the current if any cell group goes above its full voltage or below its empty voltage, on a short circuit or an overcurrent, and usually on temperature; and it balances the groups. For our 10S pack and 20 A the calculator asks for a 10S BMS rated for at least 25 A continuous, a quarter of headroom over the load.

• Continuous rating. The figure that counts is the continuous current, not the peak. JBD, for example, rates its 30 A version for 90 A for one second.
• Balancing. Most BMS boards balance passively by bleeding a little current from the highest groups, a few tens of milliamps: JBD gives 20–55 mA for its SP14S004. That keeps a good pack in step; it can’t fix badly matched cells.
• Common or separate port. A common-port BMS charges and discharges through the same pair of terminals, as in our figure below. A separate-port BMS has a charge input with its own, often lower, current rating: check it against your charger.
• Temperature. A thermistor (NTC) on the cells lets the BMS stop charging when they are too cold or too hot. JBD’s BMS has one on the board and one external.
• Smart or not. A smart BMS reports each group’s voltage and lets you change the limits from an app (Bluetooth) or over UART, RS485 or CAN. Not needed, but it makes a weak group easy to spot.

From the makers’ pages. On any BMS, check the chemistry (Li-ion, not LiFePO4), the series count, the continuous current ≥ the calculator’s figure, the over-charge and over-discharge thresholds per cell, and whether the charge path has a lower rating.

Welding, insulating and wiring the pack

Nickel strip and spot welding

Cells are joined with nickel strip spot-welded to the ends: a short, very high current pulse fuses a small spot in milliseconds, so little heat reaches the inside of the cell. A soldering iron has to hold the can hot for seconds, and that heat reaches the seal and the separator; that is why packs are welded, not soldered. If a cell has to be soldered, it should come with factory-welded tabs.

Use pure nickel strip, not nickel-plated steel, which is sold under the same name, conducts worse and rusts at the cut edges. Pure nickel conducts about five times worse than copper (8.5 µΩ·cm for Nickel 201, against 1.72 for copper), so thickness matters:

Computed from the resistivity of Nickel 201 (AMETEK). Copper of the same 0.15 × 8 mm section: 0.14 mΩ per cm. Heat goes with the square of the current: twice the current, four times the heat.

In a 3P pack the current between two groups flows through three strip bridges, about a third each: 6.7 A at 20 A. Keep bridges short; for more current, add a second layer or use thicker strip, as long as your welder can still weld it.

From the makers’ pages; both are small-batch kits you finish yourself, and often sold out: Malectrics shows its prebuilt V4 kit, and so the bundles that include it, as out of stock, and kWeld’s page gives no stock status, so check availability first. In general, look for a welder powered by a car battery, a large LiPo or capacitors (supercaps) that can deliver well over a thousand amps for a few milliseconds, with adjustable pulse time or energy, and rated for the strip thickness you need. Cheap handheld welders with built-in batteries are everywhere; we found no maker data to check them against, so test any welder on scrap strip and a spare cell first and pull the strip off: a good weld tears the nickel rather than lifting cleanly.

Insulation and holders

• Insulating rings on the positive ends. On an 18650, the edge around the positive cap is the negative can. A paper insulating ring (fish paper) on each positive end stops a strip or a burr from shorting the two.
• Holders or spacers keep the cells apart and the pack rigid; fish paper between groups and over the strips, and a tough outer wrap, keep the nickel from rubbing through.
• Don’t cover the vents. The vent is at the positive end. Hot glue or potting over it stops a failing cell from venting where it should.

Balance leads, main leads, fuse and connector

• Balance leads. One wire from B−, one from each of the nine junctions and one from B+, in order. Before plugging the connector into the BMS, check with a multimeter that each pin reads about one cell voltage more than the one before; follow the connection order in the BMS manual.
• Main leads. For 20 A over 1.6 ft (0.5 m) at 36 V, our wire calculator gives 10 AWG with a 25 A fuse; with metric sizes and their own ratings, 2.5 mm² (check it). It uses ratings for PVC-insulated wire; for silicone wire, use its maker’s rating.
• Fuse. A fuse in the positive lead, close to the cells, rated above the load and below the wire: 25 A here. Check its DC voltage rating: a full 10S pack is 42 V, more than many automotive fuses are made for (Littelfuse’s MEGA, for example, is rated 32 V). Littelfuse’s MINI 58V blade fuses go up to 30 A at 58 V DC.
• Connector. An XT60 carries 20 A comfortably; for 30 A and more, an XT90, whose XT90-S version has an anti-spark resistor. Make the pack side the female plug, so its live contacts can’t touch anything.

Charging

Li-ion cells charge at constant current until they reach 4.2 V per cell, then at constant voltage while the current falls (CC/CV). A 10S pack needs a CC/CV charger made for 42.0 V; the BMS is a safety net, not the charger. For a longer life you can stop at 4.1 V per cell, 41 V: by Battery University’s table (90% of the energy at 4.13 V, 81% at 4.06 V) that keeps about 85–90% of the capacity, and Grin, which makes the Satiator charger, notes that lithium packs last far longer when not always charged to full.

• Current. The calculator gives 5.03 A as the maximum standard charge for this pack (the MJ1 datasheet’s 1.675 A per cell, times three). A 2 A charger fills the 10.5 Ah pack in about five to six hours and is gentler.
• Temperature. Charge between 0 and 45 °C for the MJ1 (0–60 °C for the Molicel cells), and never a cold pack straight from a frosty garage.
• Connector and polarity. Charger plugs are not standard: check polarity and voltage before the first charge.

From the makers’ pages.

Reading the charge

A Li-ion pack’s resting voltage says roughly how full it is: our 10S voltage chart turns a reading into a charge level, with the spread between the makers’ tables. Measure after the pack has rested, not under load. About 38 V at rest (3.8 V per cell) is around half charge; that is also the range for storage, 3.80–3.85 V per cell, 38.0–38.5 V for 10S.

The worked example

In the calculators: the pack, the main leads.

The pack holds 382 Wh, so it can’t travel by air as a spare battery: IATA allows spare lithium-ion batteries up to 100 Wh in the cabin, 100–160 Wh with the airline’s approval, and none above 160 Wh. Shipping it is regulated too; check with the carrier first.

Safety, storage and when not to build

• Charge where a fire can’t spread: on a non-flammable surface, away from anything that burns, not overnight in a bedroom or by the only exit. A fire-resistant bag or a metal box helps for small packs; it won’t contain a large one.
• Store it part-charged (3.80–3.85 V per cell), cool and dry, and check the voltage every few months.
• Retire damaged packs: a pack that was dropped hard, crushed, got wet, swelled or got hot should not be reused. Take it to a battery collection point.
• Keep the cells’ limits: charge at 0–45 °C (MJ1), never above 4.2 V per cell, never below the cut-off.