Cold weather, slow crank, won't start. Battery? Block Heater?

I have a Komatsu PC-50, recently acquired, so I'm still getting to know it.

This morning was cold (5-10 F). I did the normal glow plug warm up, then tried cranking it. It cranks very slow, and no start.

Usually/Nomarlly - (when cold) it will crank slow a few times, then speed up and then start. But, this morning, it would never crank fast enough to get started.

I have been using Diesel Power Service in the fuel.

Any suggestions? I was going to run out and grab a block heater. How about the battery?
I was trying to avoid buying a battery if it's not the actual problem.

thanks