I guess I need a second battery?

I went out this morning and my NH575E would not start at 55*. It would turn over consistent but it was slow. I did some testing and found the battery was normal at 12.63 volts. I connected my volt meter to the positive terminal on the alternator and ground to the engine block to see what was going on and had the same reading I had at the battery. It dropped to around 12.35 while warming up the glow plugs then dropped to 5.8 when I engaged the starter. Checking my truck battery it too was at...

I guess I need a second battery?

I guess I need a second battery?