I presume that you intend to use a three stage charging. U didn't mention the battery capacity so i'll explain it in general.
Suppose you are using a battery of 12v and 7AH so the maximum charging current would be 7/10 = 0.7Ampere.
And at the same time the load on the battery will be drawing a current of 1.5 ampere . So the maximum current that your charger should supply should not be less than 1.5 + 0.7 =2.2 Ampere . Now let's choose a power supply of say 2.5A , 14.4 V rating, now you have to measure the input current going to the battery and output current that is drawn by the load. And according to that, you have control the charging current by some microcontroller or some analog circuitry.
Now suppose at some instance the load is drawing a current of 0.5Ampere, you have adjust the input current to (0.5+0.7) = 1.2Ampere.