While an inverter will power your "mains" type electrical appliances straight off a battery bank an inverter/charger will do a lot more. As an inverter, an inverter charger works identically to the inverters described on my Inverter page. Beside being an inverter however, an inverter charger differs greatly in that besides having a battery connection, there is an input for generator or mains power or both.
It is these power inputs and what happens when the device detects an input that make an inverter charger an entirely different machine from the plain inverter. Typically, after detecting a power input to the generator or mains connection, an inverter charger will switch over to a device that passes on the input from the generator or mains power to the devices connected to it. This is done by in in built device called an automatic transfer relay. Now that the input is no longer provided by the batteries, this frees up the inverter part of the inverter/charger and frees it up for another use. This use is battery charging.
While this may seem like the ideal solution to battery charging, creating an uninteruptible power supply and combining two expensive items into a single unit, there is a bit more you should know before rushing out to buy an inverter charger.
A Quality Inverter Charger
A quality inverter charger will have some adjustable parameters, provide a seamless transfer of power and generally work correctly if it is connected to a clean stable form of input. In most cases this will mean a quality generator.
Assuming you have a quality generator, an inverter charger will then have a delay (often adjustable) that will determine how long after detecting an AC input, the unit will switch power through and become a battery charger. A delay here is often required so that a connected generator can warm up and stabilise before being called upon to provide serious power.
A good inverter charger should also have a phase matching mechanism so that when power is transferred there is no interruption to the power supply from the inverter output. Ideally it will match its output to that of the generator before transfer takes place.
Finally, a good inverter charger will be able to detect load on the connected generator and reduce power to the battery charger if the generator is going to be overloaded. The battery charging part should also be adjustable to suit a range of battery sizes and types.