Packard Bell Easynote Battery

My computer tells me that my battery is charged at 100%. This is as it
should be as I rarely use the computer via the battery. The gree 'fully
charged' light is also lit.
When I try to use the machine using the battery it is completely dead. No
lights, no power - nothing. The machine is still under guarantee. do I need
to replace the battery or is there something I haven't done. It worked fine
in the store 9 months ago and was still working well 3 months aga.
Help! Please

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Frank Pitt-Pladdy