These are interested, actively shopping prospects, and anything less than 100% of abandoned shopping carts getting a reminder e-mail is criminal neglect of your business.
For the second reminder e-mail, add a small sweetener, a special offer that still allows for a profitable transaction, again with the link to their open cart.
Participants in each of those two groups received an e-mail explaining the HRA and what the incentives would be for completing it, followed by weekly reminder e-mails for each of the next four weeks.