I still have an issue with the regen for the first few minutes of riding
It's probably because the high voltage of the freshly charged SLA's
(14.5V per battery) is allowing the regen to raise their combined voltage above 60V
(15V per battery), causing the controller to cut out because the GM controller's maximum 60V limited has been exceeded! As soon as it cuts out, the regen would stop, causing the voltage to drop back down below 60V, allowing the controller cut back in and repeat the cycle again.
After you have been riding for a few minutes, the battery will have become discharged enough to enable it to absorb the full regen current without the voltage exceeding the 60V limit, allowing the regen to function correctly.
Any chance you can try a test run on 24V to see if the regen still plays up?
If you did this, you would also discover that the regen is much stronger on 24V than it is on 48V.
Alan