troubleshooting
Why does my battery get hot?
Short answer: A wire from + straight back to − is a short circuit. Nothing limits the current, so the battery heats itself up. Disconnect it and find the stray wire.
See it happen
This is the broken circuit, simulated live. The readout under the board tells you what the parts are doing.
simulating…
What's going on
Every loop needs something that limits the current. Wire doesn't — a jumper is close to 0 Ω. With only wire between + and −, the only resistance left is inside the battery itself (about 1 Ω for three alkaline AAs together), so Ohm's law gives 4.5 V ÷ 1 Ω = about 4 A. That's four hundred times the current of an LED loop, and all of it turns into heat inside the battery.
Alkaline cells get hot in seconds and can leak; a lithium pack can do worse. This is the one mistake on a battery-powered breadboard that can actually hurt you — but only if you leave it connected.
The fix
Disconnect the battery first — pull one wire from the pack. Then look for the loop that has no part in it:
- A jumper landed in the wrong row and bridged the two power rails.
- A part's legs are touching each other or a neighbouring row.
- On some breadboards the power rails are split in the middle; on others a jumper you added joins + and −.
Put a resistor and an LED back in the loop and the same battery is happy at 11 mA.
simulating…
Also check
- A motor draws a lot but isn't a short — 100–300 mA for a small one. Warm battery pack, fine. Hot enough to hurt, not fine.
- Warm resistor? Check its wattage: 2.5 V across 22 Ω is 280 mW, over a ¼ W resistor's limit. Bigger resistance or a bigger resistor.
- Never try this with mains power, a car battery or a bare lithium cell. Battery packs of AA cells are the only supply this site uses, for exactly this reason.