This is a good detailed review that shows you how the Tenavolts work, but there are some mistakes so read the comments:

• Review & Teardown: Tenavolts AA Size 1.5V Li-ion battery : r/AAMasterRace

Correct me if I'm wrong, but last I checked the 30% discount applies to all of the Tenavolts packages:

• AmazonSmile: TENAVOLTS Rechargeable AA Battery Charger with 4 Counts Lithium 2775 mWh AA Rechargeable Batteries: Home Audio & Theater
• AmazonSmile: TENAVOLTS Rechargeable Lithium/Li-ion Batteries, AA Rechargeable Batteries, Constant Output at 1.5V, Quick Charge Less Than 2 Hours, 2775 mWh Electrical core Power- 4 Count …: Industrial & Scientific
• AmazonSmile: TENAVOLTS First Generation Rechargeable Lithium/Li-ion Batteries, AA Rechargeable Batteries, Micro USB Charger Included, Constant Output at 1.5V,Quick Charge,2775 mWh Electrical core Power- 2 Count: Home Audio & Theater

That last one is especially handy if you travel with AA batteries.