the charging current is controlled by the current control circuit.
http://www.hobby-elec.org/e_car2.htm
An advanced battery charger design based on Maxim's battery charger IC MAX712. This device can fast-charge an NiCd/NiMH rechargeable battery.
http://wiredworld.tripod.com/tronics/batt_chg.html
based on the UniTrode UC3906 Integrated Circuit.
http://www.geocities.com/vk3em/sla-charger/sla-charger.html
A very effective and simple sealed lead acid battery charger
http://www.myra-simon.com/bike/charger.html
http://www.electronic-circuits-diagrams.com/psimages/7.gif