Oh boy, yes, to Trevor's point, you can't get a half-decent battery on the cheap.
Good makes cost money but are much better value. I was quoted 230 Euros for a good new one for my modern, although a Chinese thing could be had for half that, and it's the same all the way down the range. I've had good service from Bosch ones (wherever they might be made), and from Japanese ones (ditto), and I always get the biggest amp-hour capacity that'll go in the space. Can't remember getting one that seemed any good for less than about 60 Euros, call it 50 GBP or, these days, an enormous sum in USD.
I have never used Cyclons myself, but for no good reason and friends have them. I do believe, though, that their internal resistance is very low and that they can therefore load a dynamo's field coil rather heavily for a bit if part-discharged. Quite what their ability is to withstand a good flattening, I don't know - but sounds good Marcus!
Was being polite TT about running electronic ign on a dynamo machine - a Boyer or equivalent will stretch early alternator systems to the limit and beyond, let alone a dynamo: where I've got elec ign I have high-output single phase alternators and appropriate regulation/rectification. Horses for courses. Maybe one of these expensive BTH spark generators is the best non-K series option for a dynamo bike, but hell of a price. As are Altons if you look at it the other way about and start with a decent charging system.
My recipe for happiness is: dynamo and DVR2 (preferably at 12v) and a mag plus 35W halogen light; standard alternator (110W or so) for standard coil ignition and halogen 35 max 45W light; high output stator plus Podtronics reg/rect for electronic ign, with any light you like.
Apart from a recent unexpected condenser failure on one mag that had been expensively rebuilt (Sod's law but sorted fast with a ?/I hope better ceramic condenser fingers crossed touch wood), no electrical problems in a lot of miles. There's just no virtue in running at the outer end of the capability of fragile systems, IMHO. The memsahib (the putative rescue trailer driver) agrees. It's worth a modest investment to avoid that *****-off look that comes over the fair features when she sees my mobile calling up reinforcements.