To my knowledge Project Charter should only be prepared once for a project. A project can have many phases and even many sub projects within it, but the PC should remain the same as PC documents the project business decisions (SoW, Business case etc).

Let's see what other expert has to say here.

Regards,
Sudanshu

 this sentence from PMBOK, PG 41 is interesting and perhaps explains clearly:

When a project is divided into phases, the Process Groups are invoked as appropriate to effectively drive the project to completion in a controlled manner.

In multi-phase projects, processes are repeated within each phase until the criteria for phase completion have been satisfied.

Also if we see page 44, 1st line:

The Initiating Process Group consists of those processes performed to define a new project or a new phase of an existing project by obtaining authorization to start the project or phase.

As part of the Initiating Process Group, many large or complex projects may be divided into separate  

phases. In such projects the Initiating processes are carried out during subsequent phases to validate the

decisions made during the original Develop Project Charter and Identify Stakeholders processes. 

Pg 45

Develop Project Charter: In multi-phase projects, this process is used to validate or refine the decisions made during the previous iteration of Develop Project Charter.  

So I guess Project charter should be made once for multiphase project but can be improved during subsquent phases

 No it is very clear each phase has its Project charter. Without project charter how it can be initiated.

See Figure 3-1 P 40, and Last Line of page 40.

P 44 first para is also clear about this part .

P 45 3.3.1 is very clear. 

For phase or project - Inetiating processes are necessary and every IPG, require PC.

For a project a main (original) PC is prepared ,(para 2, 3rd line p44) then for each phase , seperate PC are developed, which should not be considerd as improvement of earlier but a sub part like phases are sub parts of a projects.

PMBOK says in 3.3 INITIATING PROCESS GROUP

 The Initiating Process Group consists of those processes performed to define a new project or a new phase of an existing project by obtaining authorization to start the project or phase. Within the initiating processes, the initial scope is defined and initial financial resources are committed. Internal and external stakeholders who will interact and influence the overall outcome of the project are identified. If not already assigned, the project manager will be selected. This information is captured in the project charter and stakeholder register. When the project charter is approved, the project becomes officially authorized. Although the project management team may help write the project charter, approval and funding are handled external to the project boundaries.

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See the bold lines - it is clear indication that phases require their own charter.

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Little think where you will authirised the phase, Where you will refine or validate , or analyse the high risk , success criteria, high requirements, related to phase, where you will specify the statement of work (SOP) of that phase, and what should be that document then called. certainely it is Project charter.

Though in some cases even in projects - Project charter is not prepared and only - on behalf of documentation of this information as per the need - work is authorised. It depends on organization what methodology , is to be followed.