Respironics BiPAP Focus Operator's Manual page 70

Care and Maintenance
b. Record the VBATT reading.
11. Press CHANGE SCREEN to return to the Menu screen.
12. Turn the blower on by repeating step 8, but setting Blower to On.
13. Wait 1 minute for the VBATT reading to stabilize. Record the
VBATT reading by repeating step 10.
14. Subtract the second VBATT reading from the first reading. If the
difference is greater than 2.5 V, install a new battery. If the
difference is less than 2.5 V, the battery is acceptable.
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BiPAP Focus™ Noninvasive Ventilator System
1027404 Rev K