verifying voltage of wall chargers--all types (and Olivetti Logos 9)

[dxaren]

Charger output is commonly stated at nominal load condition. The voltage is usually within +/-5% at nominal load.

If the charger is a simple transformer (often combined with a rectifier) the no-load voltage can be up to 10-30 % higher. This might be a calc-killer (!) if no batteries are in place. The same goes for open-circuit batteries! Beware of old batteries!

If the no-load voltage is lower (10% or more) than the spec label, the charger is damaged!

Batteries might get internal shorts (more or less severe). This may drain such a load, that the charger output might drop to zero!. If the charger is designed properly it will survive due to its internal cut-off circuits. It might become warm, but not critically.
However, badly designed charger might be damaged and if so, usually sends out lower voltage, that drops drastically under full load.