So, the first order of business is to decide how fast the battery is to be recharged and, from there, calculate how much charging current is required. There are limits, however, to the maximum charging current a battery can accept without damage. This, in turn, determines the minimum charging time. These limits are determined by the battery's construction and materials.
The maximum battery charging rate is often specified by manufacturers as C/3 - where C refers to the ampere-hour (Ah) rating of the battery. Using this terminology, a 6Ah battery being charged at a C/3 rate corresponds to a 2.0A current. Exceeding this maximum rate may damage the battery and should be done only with the manufacturer's permission. Some particularly robust batteries may be charged at a 1C rate (or even higher). However, this usually requires a charger that is custom designed for the battery. The size of an off-the-shelf charger should generally be chosen so that the C/3 rate is not exceeded.
The plot thickens, however, if the charger is a 2-mode or enhanced 3-mode type, that will also be used to power an external circuit while charging the battery. In this case, the circuit will draw whatever current it requires, leaving the rest for the battery. If the circuit draw is fairly constant, then the size of the charger may be increased to make up for the shortfall. If however, the current draw by the circuit varies a great deal, the current that is left over when the circuit load is at a minimum should not exceed the C/3 rate.
In this situation, even though the current draw may occasionally exceed the capabilities of the charger, the battery will make up the difference. As long as the average circuit load-current is less than the current rating of the charger, the charger will be able to recharge the battery.
The previous paragraphs discuss the largest charger that may be used. In many applications there is no need to recharge a battery quickly, and a smaller charger may be used to reduce size and heat and to save money. If a 2-mode or enhanced 3-mode charger is used, the unit need only be large enough to supply the leakage current of the battery (plus any draw from an external circuit). A 0.5A charger will eventually recharge and maintain a 50Ah battery (for instance).
If a standard (non-Ibex) 3-mode charger is used, it needs to be large enough to charge the battery at a C/10 rate or faster. If too large a battery is charged by a standard 3-mode charger, the extra leakage current of the too-large battery may cause the charger to lock-up in the absorption mode. This will overcharge the battery and shorten its life when used in a battery-maintenance application.
Note that these cautions do not apply to Ibex enhanced 3-mode chargers - they cannot "lock up" under any conditions. See App Note 4 for a discussion of 2-mode, standard 3-mode, and enhanced 3-mode chargers. App Note 6 discusses charger "lock up" and resultant battery destruction.