If you are transmitting at 50 watts, a regular brick (ca 10 Amp-Hours) may not remotely give you the 60-some watts it takes to transmit. And even if it could, and you assume a perfect system (spherical radios in a frictionless vacuum, perfect connections, perfect antenna, etc.), you'll be able to transmit for less than an hour, and you can only do it once, because running completely flat might not entirely kill a lithium battery, but it will drastically decrease its capacity (You'll likely have effectively destroyed the batteries by the second time).
Just get a low-end power station that's intended to support residential use. You can get little (100ish watt) ones for less than $200, and they have AC outlets, and both usb and lighter-12-volt ports. For the best efficiency, make your own 12 volt plug, because that usually has the least voltage conversions being done, so you get more time on your bank, and most "mobiles" run on 12v. Plus, if you run out of juice, you can charge it in your car during a beer-run!
Of course, if you only have to walk it a little way, you *could* just buy a Marine battery, and wire directly to it. Cheap ones can manage 80-90 Amp Hours (from which you can probably use 50-60 Amp hours without killing it), and there are thousands of charging options...