1. If I use a single 5AH/75A@5s battery will I get any cranking at all?
yes
2. Will doing so damage the battery (or myself)?
Well, yes. Normal lead acid starter batteries are not designed for deep discharge cycle functions. This has to do with reformation of the internal metal plates during recharge and plate separator technology. Much depends on how quickly your bike starts at the push of the button, which relates to how deeply discharged the battery gets. The deeper the discharge cycles, the shorter the battery life, where the battery has diminished capacity as it nears end of life. It will wear out quickly, which is a form of damage, I suppose. It depends on your life cycle needs. A weekend? no problem! 5 years? I think not.
3. If I were to wire two of these batteries together in parallel to get twice the capacity will things work out alright?
yes... and no.
Batteries in parallel will share the load evenly during discharge, provided both are charged to the same capacity level. So, that works out at first. In reality, you have 12 cells. These cells are generally the same but not exactly matched to each other. Some cells will have just a little bit lower voltage than others. In a series configuration the cells expend the same power during discharge. In a parallel configuration they do also, only if the cells are well matched The lower voltage ones slack off during discharge in a parallel configuration and don't discharge as deeply. The problem comes in recharge and repeated drain/charge cycles. Some cells are more deeply depleted than others. Providing charge current to replenish those, overcharges the ones that were loafing. Charging enough to replenish the shallow depleted batteries, undercharges the what-were-once-strong-cells. So, the battery assembly doesn't get charged to full capacity, and the strong cells may begin to sulfate.
So again, it will work for a while.
Cheers,
