We think our battery is on the brink, we get nice voltage but as soon as we run the vestas bow truster underload the battery drops down to 5Volts. The connections are clean. Do I order a new battery or ......
Yes the battery is dead.
It does not do the bow thruster any good to run on those low voltages so change it before you use the thruster.
Check the charging to the battery to make sure faulty charging is not the cause of the batteries demise.