Do I need to keep a minimum cash balance before investing?

No. You can keep a cash balance of $0 in your non-investment Lively account. While most banks try to earn money off of you in any which way possible, our goal is to ensure that you use your HSA funds in the best way the you see fit (not the best way we see fit). If you want to invest all of your funds, you should be able to. Please be sure to consult with an investment professional when making these decisions. Investments carry risk. 

If you do want to invest all of your funds, please be mindful of two things:

1. Any transaction with your debit card will not go through (it will pull funds from your non-investment cash account). 
2. Reimbursements will not be processed until there are enough funds in your non-investment cash account to cover such reimbursement.

The above being said, it is easy to move funds from your TD Ameritrade account back into your Lively cash account. Simple log into your account, go to Investments, click on transfer funds and schedule it. So long as funds are scheduled to be transferred by 7pm PT, the funds will be available in the morning of the next business day.