I have regularly been using an Airflow conditioner, and all was well until I went away for a month recently and left it on my 914. When I came back the conditioner showed the battery to be fully charged, and it was - started first time, and I went off for a celebratory cruise -about 40 miles. I noticed that my voltmeter on the centre console was stuck hard against the stops over 16 volts. Of course that must have been a gauge error, as the battery was charged!!!! However when I stopped there was this ominous hissing sound from the engine compartment - odd for an air cooled car, and I opened it up to find the battery boiling. I am still pursuing whether it is the regulator at fault, but I am left wondering whether long term use of the conditioner could affect the functioning of the regulator or alternator. I enquired on UK914 group at yahoo, and consensus is that it is a regulator failurebut there is still an element of doubt whether the conditioning cycle could have side effects!!
Incidentally CTEK have a new super-sophisticated intelligent charger AND conditioner which looks good
http://www.roadpro.co.uk/productItem.asp?ItemID=1382 is the link, although I expect they have their own site. I have seen them cheaper than this one.