When preaching the gospel in the open air, usually in Dublin or Enniskillen, I use the Aker MR2800 voice amplifer. I have recommended it to two other preachers who have also availed of its usefulness.
It's advantages are many:
1) Light (422 grams), small (12.5 x 9.2 x 4 cm) and very easy to carry.
2) Very clear and distinct sound.
3) Powerful volume when fully charged.
4) Internal chargeable battery from mains. The light flashes from green to red while charging and then goes to green when it is charged. I usually let it charge a further hour after that.
5) Reasonably priced. If I can get two years out of it, I'll be happy and probably buy another.
6) Good follow up service. Unfortunately my first one developed a problem and I got a no quibble replacement very quickly. They are obviously mass produced, but hey! even the most expensive toys for the boys can go wrong. Don't let this put you off.
7) USB port and mp3 player plus a SD Card port. I use this to play gospel music and someone reading the AV Bible (free mp3 downloads for Bible here) especially when I am in Dublin.
8) I also use it when travelling in my car to listen to the mp3 sermons stored on my mobile phone. Ear phones aren't much good with the roar of the engine. Rolls Royce engines may purr like a kitten, but they are beyond my current and forseeable budget.
(For those interested, the Aker also has a FM radio)
That's all. No more lugging heavy car batteries and equipment about any more. Even this, still in my possession as a back up, seems awkward now.