diff --git a/mergebot_test_utils/utils.py b/mergebot_test_utils/utils.py
index 82a127d3..1695244d 100644
--- a/mergebot_test_utils/utils.py
+++ b/mergebot_test_utils/utils.py
@@ -12,8 +12,8 @@ MESSAGE_TEMPLATE = """{message}
 closes {repo}#{number}
 
 {headers}Signed-off-by: {name} <{email}>"""
-# target branch '-' source branch '-' base64 unique '-fw'
-REF_PATTERN = r'{target}-{source}-[a-zA-Z0-9_-]{{4}}-fw'
+# target branch '-' source branch '-' batch id '-fw'
+REF_PATTERN = r'{target}-{source}-\d+-fw'
 
 class Commit:
     def __init__(self, message, *, author=None, committer=None, tree, reset=False):
diff --git a/runbot_merge/models/backport/__init__.py b/runbot_merge/models/backport/__init__.py
index 7be4652a..1812c222 100644
--- a/runbot_merge/models/backport/__init__.py
+++ b/runbot_merge/models/backport/__init__.py
@@ -83,11 +83,7 @@ class PullRequestBackport(models.TransientModel):
             self.target.name,
         )
 
-        bp_branch = "%s-%s-%s-bp" % (
-            self.target.name,
-            self.pr_id.refname,
-            secrets.token_urlsafe(3),
-        )
+        bp_branch = f"{self.target.name}-{self.pr_id.refname}-{self.pr_id.batch_id.id}-bp"
         repo_id = self.pr_id.repository
         repo = git.get_local(repo_id)
 
diff --git a/runbot_merge/models/batch.py b/runbot_merge/models/batch.py
index c65bd09e..c492d4b2 100644
--- a/runbot_merge/models/batch.py
+++ b/runbot_merge/models/batch.py
@@ -315,14 +315,8 @@ class Batch(models.Model):
             )
             return
 
-        # the base PR is the PR with the "oldest" target
-        base = max(all_sources, key=lambda p: (p.target.sequence, p.target.name))
-        # take only the branch bit
-        new_branch = '%s-%s-%s-fw' % (
-            target.name,
-            base.refname,
-            secrets.token_urlsafe(3),
-        )
+        refname = self.genealogy_ids[0].name.split(':', 1)[-1]
+        new_branch = f'{target.name}-{refname}-{self.id}-fw'
         conflicts = {}
         for pr in prs:
             repo = git.get_local(pr.repository)