My toshiba Satellite P845-S4200 battery won't charge. It currently have 0% battery, but the laptop itself still works when connected to a charger. It also has a pop-up message whenever I turn on the laptop that says it has low RTC battery, change date/time. How can I charge my laptop battery? Does a low RTC battery affect my laptop battery?
Your laptop is somewhat old. It was released back in 2012 so that means the laptop was manufactured back in 2012 or 2013. For a perfectly working laptop, the battery is typically the first thing to die. If you want to use your laptop away from the AC outlet, then you simply need to buy a new battery. Below is a link to a replacement battery for your laptop. Note that this is not an official Toshiba battery, it is a "knock off".
The RTC battery is also known as a CMOS battery. It can be charged up when the laptop is plugged in, but as I stated above, for perfectly working laptops, the batteries are typically the first thing to die. If it does not charge, then it needs to be replaced. Doing a Google search, it seems it is hard to find a place that sells a replacement RTC battery for your laptop. The link below is a UK site that sells the battery internationally.