I have 12V 18Ah lead acid battery. 15V 2Amp SMPS circuit and an auto cut of circuit. When i connect charger to battery with auto cut of circuit then battery becoming full charged within 1 hours which is not idel. I used a series resistor of 5ohms 10W to controll flow charge but it is unable to make full charge. How should i arrange it to make charging time more then 6 hour. Help please

 

SLA batteries need a constant voltage of 13.8V, the battery will take the current it needs to charge up, and it drops as the voltage reaches it's maximum limit, around 13V,.
Maximum charge is 0.2C , so your battery needs 3.6A max current.

Ideally you want a linear charger like a transformer and bridge rectifier circuit.

 

I used a series resistor of 5ohms 10W

Resistor value is too large, use 1/2 ohm resistor rated 5 watts. How is the auto cutoff circuit configured?

 

As sghioto mentioned, need to know what the auto-cutoff voltage is.