So the worst that could happen is that it charges slower? That would even be better for the battery, right? Just want to make sure that it doesn't charge too fast or something.
And can a USB cable ever affect the charging speed of flashlights or phones? Or only the charger itself?
First off, the factor coming into play here is voltage drop. Any conductor will have some resistance per foot. The higher the resistance the more the voltage drop, which has a direct effect on the charging time. The following is a very good explanation.
One can see the resistance in a circuit increases, the current flow decreases.
In a cable, resistance depends on the gauge, or wire size of the wiring. First off, remember that the higher the gauge number of a wire, the smaller the diameter, and the more resistance per foot. So a two foot cable will have twice the overall resistance of a one foot cable.
With a short cable (3 ft and under) you really won't notice any difference between cables unless you are using a REALLY cheap cable. For longer runs the size of the wire in the cable will make a difference. Basically, the larger the gauge number (and the smaller the diameter of each wire) the greater the resistance per foot. Using a longer cable has already upped the resistance of the total cable just because there is more wire.
In my experience, the wire gauge in the cables runs from 26 gauge to 22 gauge. It is usually hard to determine the gauge of the wire used from the description. I always look for the cables that claim 22 gauge wire. Unless you make your own or have a cable specially made, you won[t find any with thicker wire (remember, as the gauge number goes down, so does the resistance per foot).
I have actually queried the manufacturer when buying a cable. With Amazon, there is almost always a spot where one can ask a question about the product, in this case the particular cable of interest. Usually, they will respond with the wire gauge used. If they don't, or this particular product does not have a way to ask the manufacturer, forget the cable.