I have done this with AP Invoices in C#.  Here is how I did it...

1. You will need to create a variable for your batch number and set it to nothing.  Make sure that the batch number is set to nothing outside of your invoice loop.

2. After you instantiate your AP/AR invoice business object, you need to check to see if your batch number variable is empty.

a. If empty, use the "nSelecteNewBatch" method to create a new batch.

b. If not empty, use the "nSelectBatch" method to use the existing batch and pass in the batch number.

3. Create your invoice.

4. If your batch number variable is empty, set it using the "sGetKey" method.

Here are some code snippets:

Do this right after you instantiate your AP/AR_Invoice_bus object...

//Check to see if Batches are enabled.  Will return 1 if enabled and 0 if not.
if (ap_inv_bus.InvokeMethod("nBatchEnabled").ToString() == "1")
{
    if (string.IsNullOrEmpty(batchNo))
    {
        //nSelectNewBatch - First argument is batch number, second is private (Y/N), third is batch comment.  Leaving batch number blank will get next batch number.
        ap_inv_bus.InvokeMethod("nSelectNewBatch", "", "N", batchComment);
    }
    else
    {
        ap_inv_bus.InvokeMethod("nSelectBatch", batchNo);
    }
}

Do this after you have successfully created your invoice....

if(string.IsNullOrEmpty(batchNo))
{
    //Set index to include batch number
    batchNo = ap_inv_bus.InvokeMethod("sGetKey", "KBATCH").ToString().Substring(0, 5).TrimEnd();
}