Some guys on the Web DIY old PC 火牛 to be standalone 牛 for chargers and the procedure is not complicated. Usually there are one or two old PC 火牛 in people's homes so if you have the time, you can DIY one.
I'm lazy so I just connect the 12Volt line out to the charger from the PC. In general, there should be a lot of capacity left for a quality PC 火牛 so hooking up a charger should be safe enough if you PC draws very little amp. You can even monitor the fan speed of the PC 火牛 and voltage on the PC when you're charging
Perhaps other members may provide their expert advice here regarding PC 火牛 for charger as I know little about electrical stuff.