It's probably just a transformer->rectifier->resistor->battery, set to charge at a C/10 rate or thereabouts.
So you just wait ~12 hours or so and you're guaranteed it's fully charged -- no guessing necessary. :-)
There was a time, not that long ago, when most battery chargers were this, um, "featureless!"
Sounds like it might be worth $15, actually...
I think Harbor Freight is a good example of (1) you tend to get what you pay for and (2) advertising leads many people to believe they need a lot more than they really do. Their "value for the dollar" is actually pretty good -- unlike a well-known brand name where the "image" can sometimes be much flashier than what you're really getting (counting on many people never recognizing as much -- see #2), with Harbor Freight it's pretty clear exactly how rugged (or not) the item you're purchasing is.
If you really want to impress your dad, get him one of these:
---Joel