Cells joined in parallel must be at nearly the same voltage before you connect them. A big difference causes a large current between them.
What the message means
When a charged cell meets a flatter one, current flows from one to the other limited only by their small internal resistances.
Fix it in this order
- Measure each cell with a multimeter before you join them.
- If the voltages differ by more than a small amount, charge the lower cells separately until they match.
- Use cells of the same type, age and capacity.
- Use a holder or a pack with a protection board rated for the pack.
- Do not join damaged or very old cells to new ones.
- Read the safety guide on lithium cells before you build a pack.
Why it happens
Two cells joined in parallel are forced to the same voltage. If one is higher, current flows from it into the other until they match, and the only things limiting that current are the small internal resistances of the cells and the connection.
Because those resistances are low, even a modest voltage difference can give a large current. Check the voltages with a meter and let the higher cell discharge a little on a small load first, as the page describes. Never join a cell that looks damaged.
If you see this, do this
| You see | Do this |
|---|---|
| Sparks or heat when joining | Voltage mismatch. Stop and separate them. |
| Pack runs flat quickly | Weak cell in the group. |
Related
LiPo safety, battery pack calculator, battery management systems.
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 joined in parallel share one voltage. At the moment of connection, a cell at the higher voltage drives current into a cell at the lower voltage. The only things that limit it are the internal resistance of the cells and the resistance of the wiring, and in lithium cells these are small, often tens of milliohms. So the current is the voltage difference divided by a very small resistance, and it can be large.
A large current heats the cells and the connections, can weld or burn a contact, and is a risk of damage or fire. Charging a cell that is flat this fast is also hard on it. The aim is to make the difference small before connecting, so the current is small.
How to confirm the cause
- Measure every cell with a multimeter, one at a time, with the cells not yet joined. Write the values down.
- Compare the cells of the same type and age. A cell far below the others may be old or damaged, not just discharged.
- Check the cells for dents, swelling, a damaged wrap or heat. Do not join any that look damaged.
- After charging the low cells, let them rest and measure again, because a freshly charged cell reads high at first.
- Check the labels for type and capacity. Do not mix different ones.
Worked example
An illustrative calculation, with an assumed resistance. Two cells measure 4.15 V and 3.60 V, and each has about 30 milliohms of internal resistance, so about 60 milliohms in the loop before the wiring. Real cells differ, so this only shows how large the current can be.
difference = 4.15 - 3.60 = 0.55 V
loop resistance = 0.030 + 0.030 = 0.060 ohm
current = 0.55 / 0.060 = about 9.2 A
with a 0.1 V difference:
current = 0.10 / 0.060 = about 1.7 A
Follow your cell maker's guidance on how close the voltages must be.Common mistakes
- Connecting first and measuring after.
- Joining cells of different chemistry or different age.
- Trusting a quick reading taken straight after charging.
- Using thin wire or crocodile clips to join them.
- Skipping a protection board on the finished pack.
If this did not fix it
- Cells that cannot be balanced because one is damaged or very old.
- The pack protection board and its rating for the total current.
- The charger setting and cell count if the pack is charged after joining.