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18650 cells in series drift apart: matched cells and a BMS with balancing

Use matched cells and a BMS rated for the cell count.

What the message means

In series, one weak cell reaches its limit first. Without protection it is over-discharged or overcharged and can be damaged or dangerous.

Fix it in this order

  1. Use cells of the same make, type and age.
  2. Bring them to the same voltage before joining them.
  3. Use a BMS made for the number of cells and your current.
  4. Check the pack voltage and each cell regularly.
  5. Never mix old and new cells in one pack.
  6. Fit a fuse on the pack output.

Why it happens

Cells in series do not share charge equally. Over time one cell ends up higher or lower than the others, and the weakest cell can be overcharged or over-discharged, which is dangerous with lithium-ion.

Use a battery management system (BMS) rated for the cell count and the current, use cells of the same make, capacity and charge level when building the pack, and charge with a charger that matches the pack. A lithium-ion cell is fully charged at about 4.2 V.

If you see this, do this

You seeDo this
One cell much lowerUnbalanced. Use a BMS.
Pack cuts off earlyWeak cell or current limit.

18650 holder, BMS cut-off.

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

Why this happens

Cells connected in series carry the same current but never have exactly the same capacity or internal resistance. During discharge the weakest cell reaches empty first, while the others still have charge. If the load continues, the weak cell is pushed too low, which damages it. During charging, the cell that fills first reaches 4.2 V earlier, and without protection it keeps being charged by the pack’s total voltage.

A pack-level voltmeter hides this. Three cells at 4.18 V, 4.19 V and 4.05 V add up to 12.42 V. That averages 4.14 V per cell, which looks normal while one cell is 0.14 V behind the others. A BMS watches each cell and, with balancing, bleeds off the higher ones during charging.

How to confirm the cause

  • Measure each cell separately at rest, with a multimeter, after charging and again after use.
  • Write the values in a table. A difference that grows over several cycles shows a weak cell.
  • Check the BMS label for the cell count (3S, 4S) and the maximum continuous current.
  • Check for heat at the holder or joint under load, which points to a high-resistance connection.

Worked example

A cell log in this form makes a drifting pack easy to spot.

3S pack, cell voltages at rest (illustrative)
Cell 1  4.18 V
Cell 2  4.19 V
Cell 3  4.05 V
Pack    4.18 + 4.19 + 4.05 = 12.42 V
Spread  4.19 - 4.05 = 0.14 V   -> balance before use

Common mistakes

  • Mixing old and new cells or different brands.
  • Joining cells at different voltages; match them first.
  • Using a BMS rated for a different cell count.
  • Relying on a small balancing current to fix a large gap; many boards bleed only tens of milliamps, so it is slow.

If this did not fix it

  • Check the weak cell: charge and discharge it alone to compare its capacity.
  • Check the BMS is actually balancing and not just protecting.
  • Check the cell holders or spot welds for resistance.
  • Check the pack current against the BMS and cell rating.