Yuasa list two batteries for the GL1800:
Battery Family: High Performance, Maintenance Free
Battery Type: YTX20HL-BS
Voltage: 12
Capacity: 18
Dimensions: 6 7/8" x 3 7/16" x 6 1/8"
Weight: 12.4lbs.
Metric Dimensions: 175mm x 87mm x 155mm
Metric Weight: 5.6kg
Polarity:
Acid Volume: 0.93
Amps: 1.8
C.C.A.: 310
Battery Family: Maintenance Free
Battery Type: YTX20L-BS
Voltage: 12
Capacity: 18
Dimensions: 6.875" x 3.4375" x 6.125"
Weight: 11.2lbs.
Metric Dimensions: 175mm x 87mm x 155mm
Metric Weight: 5.1kg
Polarity:
Acid Volume: 0.93
Amps: 1.8
C.C.A.: 270