Yamaha RS90K Manuel D'atelier page 141

Charging method using a constant voltage charger
YES
Charge the battery until the battery's
charging voltage is 15 V.
NOTE:
Set the charging time at 20 hours
(maximum).
Measure the battery open-circuit voltage
after leaving the battery unused for more
than 30 minutes.
12.8 V or more --- Charging is complete.
12.7 V or less --- Recharging is required.
Under 12.0 V --- Replace the battery.
BATTERY INSPECTION
Measure the open-circuit
voltage prior to charging.
Connect a charger and
AMP meter to the battery
and start charging.
Make sure that the current
is higher than the stan-
dard charging current writ-
ten on the battery.
Volt meter
2-50
INSP
ADJ
NOTE:
Voltage should be measured 30
minutes after the machine is
stopped.
NO
This type of battery charger cannot
charge the MF battery. A variable
voltage charger is recommended.
Charger
APM meter