If you jump your battery from a donor vehicle, make sure it is not running. You only need the 12.6V battery reserve, not the charging pulses and transients it sends to your unprotected bike if it's battery is discharged (high resistance). There is usually plenty of loss in the small contact areas of the jumper cable clamps that you won't get as much current thru them as you would expect. But jumping is not a substitute for a low reserve old battery.