1Tired cells, left in for months
The radio is played now and then and otherwise sits in a cupboard. Its four alkaline cells are nearly used up: 4.5 to 5.5 volts at rest instead of 6, and their internal resistance has risen, so the voltage sags further whenever the amplifier draws current. One cell is always a little weaker than the others.
2Gas builds up inside the weakest cell
Inside an alkaline cell, zinc slowly reacts with the water in its potassium hydroxide electrolyte and gives off hydrogen, faster once the cell is nearly flat. The weakest cell can also be pushed backwards by the others in the series string, which makes more gas. The pressure builds until the plastic seal at the negative end splits open, a vent designed to stop the cell bursting.
3The electrolyte creeps out
Potassium hydroxide is a strong alkali, and it has an odd habit: it creeps along metal surfaces as a thin film, even uphill. It wets the spring and the plates beside the vented cell. Where it meets carbon dioxide in the air it turns into potassium carbonate, the white, fluffy crystals seen on old batteries.
4The contacts corrode
The crystal crust does not conduct, and it wedges between the spring and the cell. Where the thin nickel plating is scratched, the damp alkali gets at the steel underneath, which rusts, and the copper leads darken and turn green. The resistance of the contacts climbs from hundredths of an ohm to several ohms.
5The sound fades and crackles
Switched on, the radio now plays quietly and crackles. Every loud beat draws more current, and with ohms of resistance in the loop the voltage at the board dips each time. The supply capacitor bridges the shortest dips, but when the board falls near 3 volts the oscillator stutters and the sound breaks up.
6The radio goes silent
The crust finally separates the spring from the cell: the circuit is open, and the radio is dead even though the cells still hold some charge. The alkali has crept along the leads to the supply pads on the board, where it will go on eating copper tracks if it is not cleaned off.
7How it is caught and prevented
- Take the cells out of anything that will sit unused for more than a few months, and replace all four together, never mixing old and new or different brands.
- Remove flat cells promptly: a nearly empty cell gasses most, and a weak cell in a series string can be driven backwards.
- Signs of trouble: sound that fades or crackles when the radio is moved, cells that are hard to remove, a white crust or a sticky film round the springs.
- Cleaning: wear gloves, wipe the crust with a little white vinegar or lemon juice to neutralise the alkali, then rinse with isopropyl alcohol and dry. Rusted springs should be replaced.
- Lithium AA cells (lithium iron disulphide) don’t contain potassium hydroxide and rarely leak, which is why they are used in emergency radios.
- Designers help by sealing the battery box away from the board and running the leads out of the top rather than the bottom of the box.