Yeah, that's the option I was referring to. I believe it's 10€ per day for luggage. No idea how much it is in Chamartín.
The bus is absolutely doable with carry-on, it takes an annoyingly circuitous route through Alcobendas, though. There is a small area where you can leave luggage on the bus, IIRC (It's been about 2 years since I've used it). Just go out to the same area where the long distance buses are and the terminal transfer bus and find the sign with 827 in green and make sure it's toward Tres Cantos and not Canillejas.
Here is that schedule
http://www.empresamontes.es/index.ph...ea-827-horario