I did a bit of 'net surfing this morning and it seems that 36 volt regulators are extremely rare, in fact almost non-existent as far as my searching went. However, in the way that sometimes happens when surfing around I came across this page
where the author gives brief instructions on how to convert a Solarex SC-18 regulator to handle 36 volts.
A word on regulators.
Elec-Trak tractors require 36 volts to operate. From a solar standpoint this can be a bit problematic, since most solar systems are either 12 volt, 24 volt, or 48 volt systems. Thus there aren't many regulators out there for 36 volts.
By chance I found a 36 volt model from a company called thelastplaceyoulooked.com. They apparently bought a lot of solar equipment from Australia, and in Australia they use 36 volt systems much in the way we use 48 volt systems here in the states. The Solarex SC-18 controller fit the bill perfectly: It could be configured for 36 volt operation by changing a solder-pad setting, and it was a two stage charger to boot.
Changing it to 36 volts is not a completely simple task. In order to do this you not only need to know how to solder, you need to know how to *remove* solder, since you will take the solder off the 12 volt pad while putting a new solder joint on the 36 volt pad.
Once the regulator was installed, I ran a 25 foot extension cord (outdoor rated) out to the tractor. I terminated it with a 20amp Hubbell twist lock connector, which plugs into the Elec-Trak's option power supply.
As soon as the tractor was plugged in, the charge light came on, and the tractor began to accept a charge. In checking with my Ammeter, I noted that the tractor was pulling almost the same amount of current that would be pulled from the panels in a direct short to ground, indicating that the system was functioning at maximum performance. Initial charging rates were up to 4 amp/hours.
Further, I found a local (Perth) link to a place which sells this item ($20).
http://www.gumtree.com.au/s-ad/perth-re ... /351281406
If the lady with the 36 volt bike contacts me again (I gave her my card) I'll suggest this to her, and maybe see if I can tinker something together.
Otherwise, I'm putting this up for those who have 36 volt systems and are wishing to build an "on-board" PV battery charging system. If you do attempt this yourself and it works, please post it up here, and I'll do the same.