Discharging 100 A battery
Your calculations are correct. You need one more calculation - the load power rating.

From \$ P = VI \$ we can calculate \$ P = 12 \times 5 = 60~W \$. 

Use a 55 W car headlamp bulb - it's close enough and you probably have one. This has the added advantage of built-in visual indication of state of battery.

I would recommend that you put in some protection to shut off the test load when the battery voltage droops to, say, 10 V. This will reduce likelihood of damage.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Discharge circuit.

If you have a spare car relay you could try the circuit of Figure 1. 


R1 is added so that the relay drops out at the required voltage. (Relays require less voltage to hold them on that they do to pick-up in the first place.) 
SW1 provides full 12 V to energise the relay and is then released. When the voltage drops to the drop-out voltage the relay will switch off and the discharge will cease. 
This circuit is not very good in that the relay drop-out is very slow and you may see some arcing on the contacts as they lose pressure.