Most cars won't push a high number of amps for a long period of time to avoid damaging the battery in normal operation.
Lead acid batteries take a long time to charge. With a typical 6 or 10 amp charger, a totally dead automotive battery will normally take several hours or more to reach about 70% charged which is called the bulk charge period. The last 30%, or the saturation period, takes about as long as the first. If your battery is fully discharged, you can drive for hours and not reach a fully saturated state of charge. That's why a lot of batteries die a young death. They spend most of their life only partially charged. This is especially true in cold weather with mostly short trips. Partially charged lead acid batteries are more prone to sulfation.
The best way I've found to try to bring back a really dead battery is a prolonged slow charge. You want one of the old school trickle chargers, not a modern electronic controlled one, for really dead batteries. Preferably one that has a setting for one or two amps. You should also get a battery hydrometer.
First, pull the caps off if they are removable and verify that all the plates are fully submerged. Then test each cell with a hydrometer and record the values. What you want to see is that each cell is fairly close to all the others. If one is significantly lower than the rest, you probably won't be able to save the battery. Then, charge at the lowest setting. Check the voltage as it is charging. As the battery charges, the voltage will begin to rise. Once (and if) it reaches about 14.6 volts with the charger on, it is fully charged. If it hasn't reached 14.6 after 24 hours, remove the charger and test again with the hydrometer. If you see some cells that have a specific gravity of 1.26 and others that are 1.20 or lower, then you have bad cells. You can try charging further but chances are it won't help. If they are all less than 1.26 and all near the same values, then put the charger back on and continue monitoring voltage.
If you have some weak cells but most of the cells have reached 1.26 specific gravity, this is the point at which it might be worthwhile to try messing with desulfation gizmos. If you do it before fully charging the battery you won't know if it's helping or not without some solid before and after numbers. A lot of the time the bad cells are shorted internally and no amount of "desulfating" is going to help.
(
