Why is Google Translate API giving me so many 403s? - google-api

Question

I've posted the relevant code below. I have a quote of 100 requests / second and a total quota of 50M characters daily (the latter of which I've never hit). I'm including 75 requests in each batch (i.e. in the below, there are 75 strings in each group).
I'm constantly running into 403s, usually after a very short time span of less than a minute of firing off requests. After that, no amount of backoff works until the next day. This is really debilitating and I'm very unsure why it's happening. So far, their response team hasn't been helpful for diagnosing the issue.
Here's an example error:
"Google Translate Error on checksum 48af8c32261d9cb8911d99168a6f5b21: https://www.googleapis.com/language/translate/v2?q=QUERYSTRING&source=ja&target=en&key=MYKEY&format=text&alt=json returned "User Rate Limit Exceeded">"
def _google_translate_callback(self, request_id, response, err):
if err:
print 'Google Translate Error on request_id %s: %s' % (request_id, err)
print 'Backing off for %d seconds.' % self.backoff
sleep(self.backoff)
if self.backoff < 4096:
self.backoff = self.backoff * 2
self._translate_array_google_helper()
else:
translation = response['translations'][0]['translatedText'] \
.replace('"', '"') \
.replace(''', "'")
self.translations.append((request_id, translation))
if is_done():
self.is_translating = False
else:
self.current_group += 1
self._translate_array_google_helper()
def _translate_array_google_helper(self):
if self.current_group >= len(self.groups):
self.is_translating = False
return
service = self.google_translator.translations()
group = self.groups[self.current_group]
batch = self.google_translator.new_batch_http_request(
callback=self._google_translate_callback
)
for text, request_id in group:
format_ = 'text'
if is_html(text):
format_ = 'html'
batch.add(
service.list(q=text, format=format_,
target=self.to_lang, source=self.from_lang),
request_id=request_id
)
batch.execute()