is it possible to approximate the functionality of an intelligent
charger, simply by charging one battery at a time?
I am mostly interested in domestic batteries with NiMH chemistry.
No. A 'dumb' charger can do several cells several in series (with all cells getting the same charge) because it charges continuously at a low enough rate that Nicad and NiMH cells can be overcharged without damage. Applying the same technique to individual cells has no advantage other than being able to charge a single cell.
To charge at a faster rate the charger must have some 'intelligence', ie. a method of monitoring the battery's state of charge and cutting off when it is full. The reason such chargers do each cell individually is that each cell could have a different state of charge when inserted, so they must be monitored and charged separately.
There are two methods of detecting full charge in Nicad and NiMH batteries:- temperature rise, and 'delta-peak' voltage measurement. The charger may also have a timeout, which is not very 'intelligent' but serves as a backup in case the main method fails.
The graph below (taken from ST application note An Intelligent One Hour Multicharger for Li-ion, NiMH and Nicad Batteries) shows typical curves and how the charger responds to them.
Delta-peak is usually easier to implement than temperature change because it is purely electronic. However it does have some issues that increase the 'intelligence' required for reliable end of charge detection.:-
When charge current is first applied there is often an initial 'hump' in the voltage which might be incorrectly detected as end of charge. This 'false peak' can be avoided by delaying the start of delta-peak detection by a few minutes.
The voltage drop (-DV on the graph) at full charge is only a few millivolts and happens slowly, so a high resolution low drift voltage measuring circuit is required. 'Smarter' chargers don't just look for a voltage drop, but take the derivative of the curve to determine the inflection point.
The small signal can be swamped by voltage variations due to poor battery contact. To combat this the charger can stop charging while measuring, to reduce the effect of current passing through the varying contact resistance.
This adds up to quite a lot of 'intelligence' being required to get reliable and safe fast charging.