240v Inverter/Battery Issue?

[smokeysworld]

Ok so Incar PC is working however now having another issue....when I switch my inverter on it automatically puts the warning light on (either loo much current (not likely) and battery low indicator) Now is this because the battery sucks, or is it because I only use the car daily for short journeys or is there a way to charge up the battery or what? Any help would be appreciated as this is getting annoying. It does work sometimes but usually when I'm stationary!
Thanks in advance!