Sounds like you have a faulty battery. Take it back to the carrier and have them replace it (if the unit is still under their warranty), or have them contact RIM to get a replacement if their warranty is up.
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They may not replace it since you are not an AT&T customer. To do a warranty exchange, they need an active AT&T account to process the exchange. Do you know anyone that has AT&T that would let you use their account to do the exchange?