Potentiometers are not power devices, they are not used to reduce voltage at anything beyond milliamps. At typical potentiometer is rated at 1/2 W.
You still haven't stated what battery chemistry you are trying to charge, so I will assume lead acid. These require roughly 14V for charging.
If you start with 30V and reduce it to 14V with anything other then a switch mode DC to DC converter (buck converter, as davenn called it), then, at 10A the potentiometer would have to drop 16V at 10A. That is 160W. Think how hot a 100W light bulb gets, now multiply that by 1.6. You will not find a 160W potentiometer. If you do, you will not be able to afford it.
But let's go back even further.
Can you tell us what you are trying to accomplish with a "variable voltage" battery charger? Have you wondered why you don't see "variable voltage" battery chargers commercially available? What is the problem you are trying to solve?