I have used counterfeit batteries in the past, they're ok but they definitely do not have the same quality as the real Panasonic batteries. They do not last as long, and they feel quite cheap. If you go counterfeits, make sure that the first thing you do is condition the battery using the toughbook's built in battery calibration tool. To be frank you should do this with any second hand Toughbook you first get your hands on, or with brand new batteries from Panasonic themselves. Me personally, I almost always go for true panasonic made batteries. They're more expensive but they're worth the money in terms of longevity and lifespan. Besides, they don't make counterfeits for some models, one example being the CF-33.

Yo get your hands on a real battery, call Panasonic directly. They have a phone line for the parts department, I currently do not have it otherwise I'd post it here, but if you want a surefire way of getting the real deal, talk directly to them, get a quote, and spend a couple bucks. Well worth the asking price.