I have a old battery charge, which says its output is 6 - 12 Volt 4 Amps R.M.S. Does this mean the output is AC?
Those older chargers are very simple &mdash; just a transformer and a rectifier, no filter. The output is pulsating rather than what we would consider "pure DC". Therefore, it makes a certain amount of sense to rate the output current in terms of its RMS value rather than its peak or average value &mdash; mainly because that's how the internal transformer is rated.

While the voltage pulses are half-sinewaves, the actual current pulses are narrower spikes whose actual shape depends on the state of charge (i.e., terminal voltage) of the battery. It is the RMS value of the current that determines how much stress is being put on the transformer and rectifier.