AC Computer Warehouse's policy is to ensure that their reconditioned laptops are always sent out with a working battery but, as they (rightly) regard it as an expendable part, they don't offer any guarantees on it.
For that reason, I'd always add in the cost of a new battery when considering the purchase of a reconditioned laptop (whether from ACCW or from anywhere else). They're easily available on eBay, as here:
https://www.ebay.co.uk/itm/223201847135
Replacing a battery in a laptop should never take more than a few quick clicks. (After all, some people like to carry a charged spare battery or two with them when they're in remote locations, so changing a battery should never be a task that involves undoing screws, etc). In the case of the model I've suggested, you simply turn the laptop upside down, release the catches on the door at the back to remove it, lift the old battery out, slot the new one in and replace the door. (See page 54 here:
http://h10032.www1.hp.com/ctg/Manual/c04533763 ).