Fully waterproof PMNN4424 I.S. battery. Talktwoway.com suggests using this with an Impress charger to gain full benefits of reconditioning. IMPRES batteries when used with an IMPRES charger provide automatic, adaptive reconditioning, end-of-life display, and other advanced features. Data is stored in the battery and communicated to the charger via a unique IMPRES communication protocol which is designed to maximize talk time and optimize battery cycle life — all automatically. Automated maintenance and a six-month extended capacity warranty when used with IMPRES chargers.