You don't seem to understand what current (or atrial fibrillation, but that's another topic) or current capacity is.

Current is pushed through a resistance by a voltage, at the most basic level. The resistance of unbroken dry human skin is 100 kohms. This does vary, but 100 k is a decent ballpark when working out safety. I've just measured the resistance across my body with two probes to get 1.2 mega-ohms, but we'll use 100 k.

Using the very simple equation I = V/R we get:

I = 12/100000 = 0.00012 A = 0.12 mA. = 120 uA

So no, they are not capable of lethal current (and "it's the current not the volts" is only true for AC or where the voltage changes rapidly, it gets far more complicated than that, to the point where you can put five amps through someone without harm). Your "capable of lethal current" is four orders of magnitude out. That's somewhat like saying a pencil is the same size as the moon.

A "100Ah" rating tells you the duration for which a current can be maintained, it is a measure of capacity, not capability. In our case, it could maintain that 120 microamps across your arms for a few years. You'd probably get very bored. A capacity rating doesn't tell you anything about how much current you can pull at any one time.