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You are correct, but you'll need more than 5v if you want to fully charge your devices. This depends on the true voltage of the devices when fully charged. Although 5v is indicated, most of the time, when fully charge the voltage will be something like 6v or so. So you might need a 7V charger (for example) to fully charge your batteries.
Imagine that your battery (what you wanna charge) is an empty bottle of water which measures say 10cm in height..
Next to this bottle of water you have a water tank that operates by gravity (no pump if you like) with the water dropping from the top. A pipe connects the top of the tank to the top of the bottle.
If the thank is 1m high, you'll fill your bottle to the top with no problems. The speed at which you fill the bottle depends on the flow of water (so it depends on the diameter of the pipe and the aperture of the opening of the bottle).
Now, if the tank has the same height as the bottle, there will be no "slope" and the water won't be able to fill the bottle.
The heights are voltages and flow are currents (amps). The aperture of the pipe is the amp of the source, the aperture of the bottle the amp of your battery.
You have to respect voltages, but amps only define charging speeds.
In practice it is slightly different as the voltage of the battery changes with its level of charge. An empty battery might have a voltage of 4v while a full will display 6V. So in practice, you'd be able to charge a bit the battery with a charger that has the nominal voltage (5v) of the battery.
Now, if you use a 5v charger that you bought in a shop, it is most likely that it will account for the need of charging a bit over 5v, so you don't need to worry about it. If you are doing it yourself, well then you need to know the max voltage your batteries are charged up to and then set the charger's voltage slightly higher.
As far as current go, 1amp will charge the 0.7A faster and 3A slower.
Hope it makes sense.
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