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.
Battery Charge Regulators
Samples were available to me of both the BattPro regulator designed by Mark Hansen and sold by Wilde Evolutions and the Rudman Regulator designed by Joe Smalley of Manzanita Micro is sold by Rick Rudman, E-Car and others. Since my batteries are placed in tight spacing, and with some on their sides, it was not practical to use the + battery post as the heat sink for the Reg as required by the BattPro. Also living in California with no freezing temperature my battery cases are open to the weather under the hood and in the rear trunk area where the gas tank used to be. Also I wanted to see all the LEDs at once. The blinking LED easier to see on the Rudman than the gradually increasing intensity glow of the BattPro.
At a electronic surplus store I found an IBM accessory box and mounted all the Rudman Regs in it. A fused lead from each battery junction connected through a multi-contact plug and socket connects to the regs. A pair of wires from each Reg then go to an external 5 ohm 25 watt resistor on a common heat sink. For final equalization battery charge every 2 weeks or so the cable connector can be disconnected to remove the regs from each battery and then fully equalized Optimas charged according to Optima recommendations for the final full charge of 2 A for 3 hours. I will not do this for more than 1/2 hours so as not to over gas any of the sealed batteries.
In operation the Rudman Regs operate only while the LED is flashing slowly at first, then bypassing that battery by sending the current through the other Regs through the on board power resistor and the heat sink and the external shunt to next batteries in the series string. Rich Rudman says 'You don't want the LEDs to almost lock full on...you are driving the Regs beyond their bypassing abilities. A good fast flash is ok, about once a second. While the Regs LED is off ALL the power is going through that one battery, and no current is being bypassed. When the batteries voltage rises to the Regs trip point then the Reg flashes on to bypass another pulse. '
'The Charger should reduce it's charge current when the entire strings voltage is just at the combined Regs set points...it should trickle charge at this voltage, and not over amp the batteries and the Regs. If the Regs are no longer blinking...they are no longer regulating...they must blink or else you battery voltage will be above 15.5...or about 1 volt over their set point. With a loadless Regs and 5 ohms of bypass on a external heat sink, the Regs are safe from overload...but they can't keep the battery voltage under control if you drive them to full on. To test this put a volt meter across a Reg protected battery and watch the voltage as the Regs start to work... Once you have them blinking...if the charge current is still to high for the Regs..the voltage will stay in the regulation voltage area for a while and then the blinking will increase in rate and duration, at a certain point the flashing will increase and the the voltage will then start to rise again. At this point reduce your charge current, or double up your 5 Ohm external load banks. The stock Regs will stand about 10 amps of continuous current...or 120 watts of waste heat. This would take 4 or 5, 5 ohm 25 watt resistors. Better yet crank down your charger to stay below 2 amps of finished float charge current.'
The charger must be set to turn off just before all the LEDS are full on and no longer blinking. That is exactly the way my 220 Vac Zivan charger operates. l am beta-testing a RUSSCO separate on-board 110 Vac for charging on the road when 220 Vac is not available.