It's better you use a transformer along with the triac or use SMPS circuits.
Since this is for a battery charger, isolation is a must. Otherwise, upon touching a battery terminal, you'll receive a nasty shock. This is not allowable for battery chargers, especially if the battery is not sealed inside a box or case.
You can use a transformer rated at 120V or 140V primary. Then, you can decrease the voltage around that level to control output voltage and thus current.
If you use SMPS, you can use a flyback, forward or half-bridge offline converter.
Hope this helps.
Tahmid.