71U means 71 volts.
When a normal battery is fully charged, it can show up to 20% of the rated voltage without load, i.e. the rated voltage of 60 volts can display up to 72 volts once. the load is turned on, the voltage returns to normal. For a 60V battery, the end of charge voltage is around 70-72. If it shows 71V right after fully charging, it's normal.