You need a diode if you will be using a non-rechargeble battery becouse the motherbaord will be trying to charge the battery when turned on. the phone battery can be recharged therefore you do not need the diode.
The diodes job is to let the current only move in one direction so the motherboard will not be able to deliver any to the battery, the battery will only be able to deliver to the motherboard.
Wires to a rechargable phone battery away off the motherboard is the best idea. Sometimes a 2032 lithium coin battery will still drop the RTC because they are only 3v and the Amiga's RTC chip expects 3.5v.
Not having a diode in the circuit and alowing a 2032 lithium coin battery to charge will lead to VERY undesirable results.
