I'd like to expand on what Paul said.
First, for those who don't already know, I'll give definitions of adapter and converter.
An adapter is a simple mechanical device that has one type of plug (ex. German/French Schuko) on one end and another type receptacle (ex. for a US plug) on the other end. It just allows us to use a US plug in a German receptacle. It does not change the voltage. Here is an example of an adapter.
A converter changes the voltage, for example from 220V in to 110V out. It probably also changes the plug configuration, like an adapter. There are two types of converters. A transformer can convert from any voltage to any voltage, but the ratio is fixed (ex 240V to 120V). For low wattage (ex 40W for battery charging) applications, transformers can be small and lightweight, but to power a 1600W hair dryer, a transformer would be prohibitively heavy. So electrical engineers "invented" a lightweight electronic converter, which uses transistors to limit the output to a some desired voltage (ex 120 volts). These can handle 1600W without being heavy.
However, the resulting wave form of the power is not appropriate for input into electronic devices, so for those a transformer is necessary.
In his post, Paul said,
If for some reason, the charger is limited to 110 or 120 Volts, charging batteries
is a low wattage application, so a very simple convertor will work
Technically (sorry, Paul), whether the input is 110/120 volts or 120/240 volts, charging batteries is still a low wattage application. For low wattage applications you can use a simple (transformer) converter.
There are three different types of voltage inputs.
Single voltage: 120V or 240V would be examples.
Dual voltage: Devices, like curling irons, that have resistive elements. have a switch to put them in parallel (for 120V input) or series (for 240V input). The inputs voltages are always in the same ratio for a specific device. They can be used either on US voltage or European voltage, depending on how the switch is set.
Multi-voltage: Most electronic devices today accept a range of voltages, from 120 to 240 (probably higher although I know no place where nominal household voltage is higher than 240V) with a fixed output voltage. I think today some high wattage devices, like hair dryers and curling irons, have an feflectronic converter built in, so you don't have to worry about having a switch set correctly for the input voltage. USB chargers are actually multi-voltage converters, accepting 100-240 VAC and converting it to 5 VDC.