Sounds like a bad battery. How long have you had the bike? Where did you purchase it from?
The SLA (sealed lead acid) batteries on these bikes don't last long -- about 300 to 500 charges. But even if it hasn't been charged this many times, if you've had the bike sitting uncharged for a long period of time (or it's been sitting in a warehouse), it will impact the life of the battery. We sell replacements here:
http://www.eco-wheelz.com/catalog/rmb-battery-pack-ezip-izip-p-56.php
There's also a new Li-Ion upgrade battery available:
http://www.eco-wheelz.com/catalog/liion-lifepo4-battery-pack-izip-ezip-p-284.php
Unlike the SLA battery, this Li-Ion upgrade can last 1000+ charges -- it has a 3-year warranty! It gets about the same range, but it's over 10lbs less. It obviously costs more upfront, but in the long run it will be about the same.
HOPE THAT HELPS!