Today's cell phone batteries typically provide hours of talk time and days of standby timeusually more than enough for a one- or two-day tripbut it's best to be prepared.
Smaller travel chargers may be available. Car adapters let you recharge quickly between meetings. And higher-capacity lithium-ion (LiON) batteries keep you going longer. Packing a spare, charged-up battery can also ensure that you don't run out of phone power.
Other power-up options include a multi-device charger such as those from APC, iGo, or Targus; or a notebook USB charger cable. For some cell phones, when there's no external power to charge from, there's Futai's TalkOn Emergency Cell Phone Charger, which uses 3 AAA cells (it's also a flashlight) or SideWinder's Emergency Cell phone charger, a 2.5 oz. hand-crank chargerjust the thing for a multi-day power outage!
My suggestion: Pack an extra (charged) battery, and an AC/car charger.