the bulb works as a ballast, because it's resistance is nonlinear. it's resistance rises with temperature. with 24V across the bulb and battery, the bulb will "soak up" the difference between the 24V and the battery voltage. in doing so, it will also act as a current limiter. as the filament temperature rises, the resistance rises, until it reaches an equilibrium point and levels off. then as the battery charges, the current drops, the temperature drops and so the resistance drops, and the bulb maintains equilibrium. with the 24V source, you will want to monitor the battery voltage often. i have found it more accurate to actually disconnect the battery from the charger, and then measure the battery voltage. batteries with weak cells will measure much higher with the charger connected, because of increased internal resistance in the battery. i have a 10A open frame power supply i use occasionally for charging gel cell batteries. the supply has foldback limiting, which means if a preset current is exceeded, the regulator will start dropping the output voltage until the current is at the preset level. i find that by setting it to about 5 amps or so, the power supply will source the charging current and at the same time the regulator output will be folded back to the present battery voltage. as the battery voltage rises, it can be monitored with a meter. there is one problem, however. if the battery has a weak cell, the charging current won't trigger the limiter, and so the battery's state of charge can't be accurately monitored with the supply attached. a small 300mA charger with a lamp is going to behave similarly to my foldback limited supply, but if there is a weak cell it will read higher than the actual battery voltage with the charger connected.