For a year, the red battery warning light would appear soon after riding at night with the headlights on. After several tests with a multimeter, this warning light only appeared when the battery voltage dropped below 12.0V volts. The battery was never more than 12.3 volts after riding it for an hour at night. This meant two things - the battery was dying or the charging system was faulty, so I investigated it.
I replaced my existing battery with a new Yuasa 12V, 8Ah, 135CCA battery. It was charged overnight before installation. After a few more night rides the warning light still came on. A quick battery test showed it had only 15% charge after riding for an hour. This meant that there was an issue with the charging system.
How to check the charging system on the DT300i:
1) Open the seat. remove the seat and then the seat bucket.
2) Locate the voltage regulator on the right hand side of the seat area, just in front of the right seat latch.
3) Disconnect the 3 yellow wires leading to the voltage regulator.
4) Use a multimeter to meaure the resistance between each of the yellow terminals of the connector coming from the generator. This should be around 0.5 ohms. If it is close to 0 ohms then there is a short circuit in the coil. If the value is high, then the coil is broken or burnt out. Also, there should be no continuity (conductive path) between the connector and the bike's frame/ negative terminal.
5) Start the bike on and measure the AC voltage between any 2 of the 3 terminals of the connector. At idle this should be 24-26 volts. At 4000 rpm this should increase to 60+ volts. The voltage should be similar across any two terminals, indicating healthy coils.
6) Turn off the bike and plug the connector back in.
7) Measure the voltage of the battery. Anything less than 12.0 indicates a flat battery.
8) Turn on the bike and rev it to 4000rpm and measure the battery voltage again. In my case it was only 13 volts. It should be 14-15 volts for sufficient charging.
After confirming that the generator was fine and the regulator was faulty, I replaced it with a new regulator from eBay. The regulator is easy to replace - there are only two 10mm bolts holding it to the frame and two connectors to disconnect. Now the battery light never comes on and the battery voltage after an hour-long ride (with headlights on, drawing 6 amps) is 13.2 volts, much better than the previous 12.0 volts. My battery tester confirmed that the battery was still 95% charged. Also, the charging voltage is at least 13.8 volts @ 4000 rpm and the headlights are brighter.
