question

Just based on the info you've provided, I would just use a Queue for batching instead of a Processor. Follow these simple steps to solve your problem:
1. Drag in a queue to perform your batching at.
2. Add a label "TotalWeight" to the Queue. Set it to 0 and check the "Automatically Reset" box.
3. Add the following code to the Queue's OnEntry trigger: (assume that the weight of the object is stored on item.Weight)
current.TotalWeight = current.TotalWeight + item.Weight; // Update TotalWeight
if (current.TotalWeight >= 50)
{
current.output.open(); // Opens the output port to release batch of items
}
4. Add the following code to the Queue's OnExit trigger:
if (current.subnodes.length == 1)
{
current.output.close();
current.TotalWeight = 0;
}
5. Add the following code to the Model's OnRunStart trigger:
Object queue = Model.find("QueueName"); // Change "QueueName" to whatever your queue is named
queue.output.close(); // Initially close the output port

That did it for me