I have this same problem on a 2009 street bob. Let me know if you find an answer. Mark
Something is on draining your battery overnight. Any light or any relay. It can also be your battery has a dead cell draining it overnight.
Either you have a bad battery or you have something draining it down overnight.
Either something is draining the power - or the battery is at the end of its useful life.
To remove the speedometer sensor on a 1999 Harley Davidson 1200 Sportster, first, disconnect the battery to ensure safety. Locate the speedometer sensor, which is typically found on the side of the transmission. Use a socket or wrench to unscrew the sensor from its mounting position, then gently pull it out. Make sure to check for any wiring connections and disconnect them if necessary before fully removing the sensor.
on the left to the rear of the seat (sportster)
Can be a dead cell in the battery. If so the battery will have to be replaced. Can also be any light that is pulling power from the battery. Check all lights. Under hood, glove box, dome light, trunk, etc. Disconnect the negative battery cable overnight and if the battery is dead then the battery has a dead cell and must be replaced.
To reset the dash on your HD Sportster after a battery change, first, turn the ignition to the "On" position without starting the engine. Then, press and hold the trip reset button on the speedometer until the odometer flashes and resets. Release the button, and the dash should return to normal operation. If you're experiencing any issues, consult your owner's manual for specific instructions related to your model.
Then either something is on draining the battery or the battery has a dead cell draining all the rest and must be replaced.
12.
Just like the battery on any other vehicle, connect the charger's clips to the battery terminals, red to positive and black to negative. However, because the Sportster has a relatively small battery, if charging at a high amperage (ten amps or more), it's important to monitor charging carefully unless the charger is fully automatic. Don't leave a small battery charging at a constant 10 amps overnight. For peace of mind and foolproof charging, I recommend using a Battery Tender, which you can leave connected to the battery whenever the bike is parked. It cannot overcharge yet keeps the battery at full capacity.
It can be any light in the car. Dome, glove box, trunk, under hood, etc. It can also be a relay that is stuck closed. It can also be a dead cell in the battery which will cause it to loose it's charge overnight. Have the battery tested at anyplace that sells batteries.
Have you had the alternator tested? Often a bad internal diode can drain the alternator overnight.