P Series Cable Battery
The battery inside the battery case on the cable may be replaced with any ER6V/3.6V, 2000 mAh battery. The connectors that are on the cable are noted below. Parker doesn't sell these batteries or the battery with the wire and connector. If you purchase a battery on-line that already has a connector, you may want to check to make sure it is a Molex 5263PBT or equivalent. Alternatively, you can buy the cable and add your own wires and connector, or use the connector off the old battery. Another alternative, is just to remove the connectors and splice the wires directly together.
The drive will indicate when the battery starts to get low by throwing a battery 'warning' but not a fault (W-02). It will throw a fault later when the battery voltage drops further (AL-35). The warning is designed to be that signal that its time for you to replace it.
The drive will hold enough charge to allow you to swap the battery out without losing position as long as its done with an hour. That is why the Charge LED shows and why it stays on so long when you remove control power. The batteries are only used when the drive is powered down. The batteries are typically going to be ok for 4-5 years as a rule of thumb. Lots of different circumstances will affect that though so its really just a rule of thumb.
Note, the Pseries motors have absolute Biss-C multi-turn encoders built into them. The battery cable is optional; if not using the battery cable, the multi-turn data isn't retained but you still have the high resolution (over a single rev of the motor) available (19bit resolution for the 60 & 80mm sizes, 18bit on the 40mm size). The Pseries motors also are smart encoders telling the Pseries drive what motor's connected automatically.