The Battery charger is the device which is responsible for charging the multiple batteries. It may include or be a part of the overall Battery Management System (BMS). Chargers can range from a manual operated variac perhaps with a timer, sometimes called a bad boy charger, to a semi or fully automatic chargers which may also be intigrated into a fully automatic BMS.
Using a towed generator
Manzanita Micro PFC
The Italian company Zivan makes a range of modestly priced 3-stage chargers in various voltages. They are based on switchmode power supplies, so they are fully transformer isolated, but are much lighter than the typical boat-anchor ferroresonant or conventional transformer charger.
The Zivan NG1, available in outputs from 12 to 48 volts, is reportedly a very well-built unit with the price to match -- $573. It's a 1000 watt charger (about 27 amps at 36 volts). The older K5 is less robust, perhaps not as nicely built, with less capacity (15 amps at 36 volts) -- but it's cheaper at $461.
See http://www.mcn.org/a/innEVations/chargers.html for information on the Zivan chargers.
Ken Olum points out that three 12v chargers are usually cheaper than one 36v charger. That's true for simple, inexpensive utility chargers, but not so true for sophisticated 3-stage chargers. (Most 12v chargers you can buy in a hardware or auto parts dealer for $40 - $75 each are simple taper chargers.)
One very interesting charger I've heard of is made by Statpower (see http://www.statpower.com/tc20.htm for one example). It's a true 3- stage charger that delivers 20 amps (they also have 10 and 40 amp models).
Each charger is supposed to be able to charge up to three 12v batteries. It has 3 separate diode-isolated output terminals, though it's unclear to me whether the charge control actually senses and controls the batteries separately. Also, all 3 outputs share a common negative, so you'd have to disconnect your batteries from the tractor's series string before charging -- a real pain. You would probably want to use one separate unit for each 12 volts of the tractor's battery string.
I don't have any personal experience with Statpower chargers, but they look pretty sophisticated.
Statpower has an online store where they sell older refurbished units at about half price (I suppose one might wonder just what the failure rate is if they have lots of warranty returns to refurbish). A 20 amp dual- output charger is priced at Can$215 (US$139), pretty reasonable for such a unit. The 10 amp refurbished charger, which does not have the dual output feature, is priced at Can$152 (US$98).
You would probably need three of either unit to charge 36 volts worth of batteries. If these were not refurbs, three of them would cost more than a 36 volt Zivan NG1 (but they would do a better job of keeping the different batteries at an equal state of charge than a single charger for the whole series string).
There are 3-stage chargers made by Guest Corp. which actually have three separate chargers in one box. However, I have heard of reliability problems with these chargers. Too bad.
Sorry that I also have no personal experience with the pulse units. One person whose judgement I value points out that nearly *every* charger is a pulse charger, because they all rectify AC and put out pulsating DC at 120 pulses per second. However, I have heard reports from other people I respect who say they have had good results from units like the Canpulse. I get the impression that these units operate independently of the charger.