Automatic Universal Battery Chargers

Whilst almost anything is possible - and more will be - some information is required in advance by the charger. This starts with the voltage and to some extent the capacity. There are quite a few chargers that can cope with varying capacities and to a lesser extent voltages. They do this by measuring the temperature of the cell(s) under charge. Indeed many battery packs have an inbuilt temperature sensor.
When Ni-Cad & Ni-M-Hy types cells are fully charged and/or cannot chemically convert the energy anymore, their temperature rises quite substantially. This can be used to "assume" that a fully charged state has now arrived
Another, though less reliable way is to monitor the rise in cell voltage. There's a snag here! If one or more cells in a pack become shorted (as can happen with dendrite growth in Ni-Cads), the expected final terminal voltage can never be achieved. This is what makes the temperature measuring method much safer. Another issue in its favour is that even cells that have lost their original capacity will be properly detected for a fully charged condition.
Here you see why it is so important that the cells in a battery should be evenly matched. We need them all to be charged/discharged at the the same rate & time.
POSTSCRIPT - Another method of detecting when a Ni-Cad or Ni-M-Hy battery is fully cahrged has come to my notice. It is called the "minus Delta U" calculation which in this case works thus: When above battery types are charged with a constant current their voltage rises continuously to amaximum which falls slightly if the charge is maintained. This fall can be used to terminate the charge.
UPDATE - whether covered elsewhere here or not there's another point to make concerning pulse battery charging. This was a common method on older cars equipped with a dynamo that delivered DC at a voltage that varies with RPM. A "Regulator" was fitted to ensure that charging currents did not exceed a sensible level for too long. The period of each pulse was controlled by voltage & current operated solenoid which interrupted the current flow often diverting it via a resistor. Thus were two levels of cuurent applied in pulses the duration of which depended on the battery state of charge. This practice gave something else that later alternator circuits did not give which concerns the formation of sulfate on the plates. This happens when the battery is in a wholly or partially discharged state and since it effectively insulates the area of the plates to which it has adhered, it effectively lowers the battery capacity. It now seems that the older pulsed charging systems could somehow dissolve any such formation more effectively than a constant current/voltage system and this has brought us to the electronic pulse charger. In a car it is the job of the ECU. However, external to the car specialised electronically controlled pulsed lead-acid battery charging and conditioning brings those advantages back.
Nor does the story end there. Other battery types, in particular Ni-Cad & Ni-M-Hy types can be charged up much quicker with the pulse method. It seems that the chemistry conversion is more effiicient when charged in pulses. Of those chargers I have seen the pulses are about 1A with a duration of 25 to 50% or in other words something like 1 to 2 secs in 4