On the Electrac site I read about Harry Landis "Charger controller" designed to replace the timer on the rudimentary chargers installed on the tractors. He had an ad but you have to sign up for the Eletrac list to view it. With Harry's permission, here is the ad:
Principle of operation:
The controller measures the voltage of the battery pack, and turns on the charger if the measured voltage is below the setpoint. After approximately 5 minutes it turns off the charger and monitors the battery voltage. If the battery pack voltage falls below the setpoint, the controller again turns on the charger and repeats the process.
The setpoint is adjustable between about 36 and 42 volts by means of an onboard potentiometer. The red LED illuminates if the battery voltage is LESS than the setpoint. The controller is supplied with the setpoint set at 38 volts. It is recommended that the setpoint be left at 38 volts for best results.
But if you want to adjust the setpoint, there are several options. The simplest is to use a well regulated variable power supply such as a 0-50 volt bench supply. Adjust the supply to the desired setpoint, and then adjust the pot so the LED just illuminates. Then vary the supply voltage to verify that the LED goes on and off at the desired voltage.
If such a supply is unavailable, you can use the ET itself. You will need a good digital voltmeter. Connect the controller’s red and black wires to the + and – of the ET battery pack. . As an illustration, let’s assume you want to adjust the setpoint to 39 volts. Measure the battery voltage with your digital voltmeter. If it is less than 39 volts, charge the batteries with whatever charger you have handy until the voltage is a little over 39 volts. This should only take a few seconds. Then disconnect the charger. The battery voltage should then slowly drift downward. As the voltage gets close to 39 volts, adjust the pot so the LED is just barely off, or flickering. Then watch the voltmeter and see if the LED goes on just as the voltage passes 39 volts. Then give the batteries another very quick charge to a little over 39 volts. Disconnect the charger, and monitor the voltage as it falls through 39 volts. If the LED illuminates exactly at 39 volts, you are done. If not, readjust the pot and try again.
If the ET is left unattended for long periods, the controller will automatically keep the battery pack fully charged and equalized. The controller will turn on the charger a couple of times a day to make up for self discharge and the small amount of current consumed by the controller (about 10 milliamp).
The red wire should be connected to some convenient point that is always connected to the + end of the battery pack, You can directly connect it to the most positive terminal of the battery pack if you like, but for reasons of convenience when changing batteries and also to prevent corrosion problems, it is preferable to find some appropriate place inside the tractor. Likewise, the black wire should be connected to some point which is always connected to the – end of the battery pack. On at least some models, the accessory plug is a good place.
The two large terminals on the solid state relay interrupt the 110 volts AC which goes to the charger. If you remove the original ET timer, you can simply connect the wires from the timer switch to the two terminals.
The controller should be mounted somewhere reasonably clean and dry. One option is to mount it where the ET timer was with a couple of screws. You can put it inside some kind of box for even more protection if desired. The controller consumes less than one half watt of power, however, the solid state relay may dissipate up to 15 watts when energized. So it is probably a good idea to mount it firmly to a flat part of the sheet metal to keep it cool. I have never had or heard of any problems from overheating, but it can’t hurt to take precautions.
I contacted Harry and he built me a controller with a setpoint of around 155v. This eliminates the need to guess at how long the batteries will need to be charged. A strength of this controller for the Electric Tractor is that it will keep the batteries charged during extended periods of inactivity. Tractors might be used once a week or much less during the winter. This could be the case with some EVs as well. (Not mine.) Keeping the batteries on a float charge of 13.5v is a usual approach to this, but it reduces the capacity over time.
Anyway, I adjusted the setpoint to hold the pack at 156.8v. I got this value by fully charging the pack and letting it rest overnight.
So far it seems to be doing a good job. I can't seem to get the picture of it in here! Here is the URL: http://picasaweb.google.com/stormconnors/SparkyTheElectricSamurai/photo?authkey=br5eIwNQ3i0#5159898365802713010