The data logger is based on an Arduino, which uses an ATmega328. The max input for any of the I/O pins is slightly higher than VCC. A Uno uses 5V so you can't put much more than 5.5V into a pin without causing damage. (A 3V3 Arduino is I/O limited to around 3.7V).
The most common way to measure a 12V battery is using a voltage divider.
Since current must be measured in series, you actually measure a voltage across a shut resistor. 5A is pretty high and would require a very large power resistor. Another approach is to find a hall-effect sensor like an, Allegro ACS714, which will be more efficient.