Discharge constant 2A-3A current from a battery

During the bulk charging stage the charger works in a constant current mode. That is, the charger supplies a constant current to the battery while monitoring the battery voltage. As the battery charges, the voltage increases even though the current stays constant.

No, you can't use Ohm's Law to calculate the current into the battery. It's important that you understand that in the bulk charging stage the charger does not try to set a certain voltage...the charger supplies a fixed current instead. It's incorrect to think of the battery as having some kind of resistance value.

Once the battery voltage rises to a pre-determined voltage (14.4 V is common for a 12 V lead-acid battery) then the charger changes its mode of operation from constant current to constant voltage. The charger starts providing a constant 14.4 V output while monitoring the current. As the battery charges it will draw less and less current. Once the current falls below some pre-determined level the battery is fully charged.

If you supply a constant current to a battery then the voltage will rise as the battery charges. If you supply a constant current and allow the voltage to rise to high then the battery will be overcharged and possibly damaged. If you supply a constant voltage to a battery then the current will gradually fall as the battery becomes fully charged. It's important to pick the appropriate voltage for this phase of charging so that the battery becomes fully charged but is not overcharged. Ideally, the voltage used in the constant-voltage phase will also be adjusted for temperature.

You can think of the battery like a water-bearing container with a small leak. If you have a constant current ("bulk") of water flowing into it at a greater rate than the leak, then the level of the water in the container will rise. Think of the 14.4 V set-point as a level marked on the inside of the container. Once the water reaches that level, you have to taper off the flow of water coming in (down to about the leak rate) if you are to maintain at approximately that level.

The only way to know the actual self-discharge rate of any given battery/cell, is by measuring from a fully charge state to a discharged state, over a given time period. Aside from that, a look at the datasheet, or an educated guess based on battery chemistry is required.

Standard NiMH Rechargeables for example, have a high discharge rate, noted of up to 4% per day, or 40% per month, when stored at room temperature. It is higher at higher temperatures, and lower at lower temperatures. That means a regular 2300mAh 1.2V AA sized NiMH cell discharges in the mid to high double digit milliamp range per day!! Alkaline batteries on the other hand, have a 5 year shelf life. The same 2300mAh AA battery but with Alkaline chemistry discharges at 1mA or less per day.