For making your own flywheel steel should do fine. You're probably not aiming for an 80,000+RPM flywheel that rides on magnetic bearings. So composites shouldn't be needed. The best material, eventually, for flywheels should be carbon nanotubes. They have huge tensional strength, but lousy compression strength. Tensile loads is what a flywheel has, so they'd work great.
It is important to note that not all batteries associated with these computer models are affected. The company is directing all users to its HP Battery Recall website to go through the online validation and replacement process.
January 2017 Expansion of the 2016 HP Notebook Computer Notebook Battery Safety Recall and Replacement Program â Additional Batteries Affected. In January 2017, HP announced an expansion of its ongoing worldwide voluntary safety recall and replacement program, for certain notebook computer batteries, which was announced in June 2016.