scheme car battery charger
First off you would need a panel which can give you 15V and sufficient amps (2-5) to charge your battery. Automotive batteries are rated for 20 amphour. so for 45ah battery, the max charge current is 45/20. That is around 2.25A. The battery (fully discharged) would take 20 Hrs to reach full charge.You can ofcourse charge at double this rate, but not more than that as it is going to reduce the life of your battery.
Next you would require a charge controller between the panel and the battery. A parallel type is preferred as it is less dissipative.Also control circuit to follow the lead acid battery charging algorithm. Also a charge cutoff circuit when the battery has reached full charge.
A three stage charge control scheme would be fine.There are also advanced advanced technology like pulse charging, which you can consider and add later.