The charger is in three stages; the power source, current limiting and float charge stage.
In this design, 240/24Vac(rms) transformer was used. After rectification and filtration i got about 30Vdc.Therefore the transformer and diode rating should be well chosen to meet your demand
At the current limiting stage , transistor U1,Q1 and resistor R1 plays the vital roll in the current limiting; When U1 switches on it short circuits the gate voltage of Q1 thereby restricting current flow through the battery under charge. U1 will only switch when Voltage across it's base-emitter Vbe is up to 0.7V. the voltage drop across R1 determine this,if the voltage drop across R1 is up to 0.7V then U1 switches on then, I(current thru batt)=0.7/R1(sense resistor). Ohms Law.
The float charge stage come to play when the Battery Voltage builts up to 28Vdc. In this design once the voltage gets to 28Vdc, it is considerably large enough to make the zener diode reach it breakdown voltage. And once this is done U2 switches on and consequently short circuit gate voltage of Q1. The charger is now said to float charge.
The LED comes on when the battery is full.
Note: In most cases D6 is chosen to be 10v but, 6v can be chosen for logic MOSFETs. And this current limiting circuit only allow N-channel MOSFETs