A cheap holder can drop a lot of voltage under load. Measure at the holder while the robot runs.
What the message means
Thin springs and thin leads have resistance. At motor current they drop voltage and heat up.
Fix it in this order
- Measure the voltage at the cell terminals and at the board with the load running.
- Clean the contacts and press the cell firmly.
- Replace thin leads with thicker wire.
- For high current use spot-welded nickel strip or a holder with solid contacts.
- Check the cells match in voltage, especially in series.
- Never solder directly to a cell without the proper method and protection.
Why it happens
Every contact and every thin lead in the path adds a little resistance, and the voltage lost there is the current times that resistance. A spring contact that touches the cell at a small point is a poor path for the large current a motor can draw, and the lost power turns into heat.
The cell has its own internal resistance too, so the voltage at the terminals also sags under load. Measuring at the cell and again at the board while the robot runs shows how much is lost in the cell and how much in the holder and wiring.
If you see this, do this
| You see | Do this |
|---|---|
| Voltage at holder is fine, at the board low | Wiring. |
| Low at the holder too | Contacts or the cell. |
Related
battery sag, cells in parallel.
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
Voltage lost in a wire or contact follows Ohm’s law: the drop equals current times resistance. A holder has several resistances in series: the contact between the cell and the spring, the spring itself, the solder tag, and the thin leads. Each may only be a few tens of milliohms, but they add, and a motor starting up can ask for several amps. The power wasted in that path is current squared times resistance, and it ends up as heat.
The cell has its own internal resistance too, which is why even a perfect holder shows some sag under load. A cell with low charge or an old cell shows more. To find the cause, measure at three points: the cell terminals, the holder terminals, and the board input. The difference between neighbouring points tells you which part is losing the voltage.
How to confirm the cause
- With the robot running under real load, measure the voltage across the cell itself, placing the probes on the cell body ends. This is what the cell can deliver.
- Measure again at the board input with the same load. The gap between the two readings is the loss in the holder and the wires.
- Measure directly from the cell end to the holder’s solder tag on the same side. A drop of tens of millivolts or more at a few amps points at the spring contact.
- Compare the unloaded and loaded cell voltage. A big sag between the two points at a tired cell and not at the holder.
- Feel the holder and wires after a minute of heavy use. A warm spring or thin wire is a sign of high resistance.
Worked example
A worked measurement, using illustrative numbers rather than results from a real holder:
Load current (from a bench supply or meter): 2.0 A
Voltage at the cell terminals under load: 3.95 V
Voltage at the board input under load: 3.62 V
Drop in holder + leads = 3.95 - 3.62 = 0.33 V
Resistance in that path = 0.33 / 2.0 = 0.165 ohm
Power lost as heat = 2.0 x 2.0 x 0.165 = 0.66 WCommon mistakes
- Measuring only the unloaded voltage, where everything looks fine.
- Replacing a bad cell with a new holder, or the reverse, without measuring first.
- Using thin hookup wire for a motor load. Choose wire for the current, not for convenience.
- Putting a multimeter in current mode in series with a motor load and blowing its fuse.
- Mixing cells of different charge in series.
If this did not fix it
- The cell may be at fault. Charge level and age change its internal resistance a lot.
- A protection board on the cell may cut off at a current limit, which looks like flickering power.
- A converter after the holder may drop out when the input sags below what it needs. Check its minimum input voltage.