There isn't a really cheap way to do it. Solar panels are cheap, li-ion batteries are cheap. A PWM/MPPT charge controller is still not cheap. You can spend $20-

There isn't a really cheap way to do it. Solar panels are cheap, li-ion batteries are cheap. A PWM/MPPT charge controller is still not cheap. You can spend $20-30 on the other components and still have to purchase a $70-100 charge controller.