Philips V24CT User's Reference Manual page 359

• The lead-acid batteries need not be fully discharged prior to
charging. Unlike Ni-Cad batteries, lead-acid battery capacity is not
affected by depth of discharge.
Note— If you insert only one battery into the monitor for charging, the
LED will flash continuously. The
Battery Charging
LED will not light even when the one battery is fully charged. This is to
remind you that both batteries are not fully charged. The one battery will
provide enough power to run the monitor. However, the operating time
with one charged battery is less than half the operating time with two
charged batteries.
When the unit is not being used for transport, connect it to AC
Note—
power with the monitor turned off. This maintains a full battery charge
and prolongs battery life.
Note— In the event of AC power failure, the instrument's battery is
automatically activated so that no data or product information is lost.
AC and DC (Battery) Operation
Battery Information (V24CT and V26CT only) 12-5
Battery Charged