Has the laptop ever run off the battery and did it just suddenly stop working from the battery? if so it could be that the power switching/charging circuit is damaged.
it has been doing this for some time now, but it used to work just fine. is this something I could replace? or even better test before i buy anything. this is a fairly old laptop but except for this is very reliable, but i dont want to spend too much on it.
i think the only test is if it doesn't charge any battery you plug into the laptop, you could look on ebay for a charging circuit for your make and model but it depends on if you are comfortable with taking apart your laptop and doing the work.