Tenergy 9V NiMH Battery, High Capacity 250mAh Rechargeable 9 Volt Batteries for Smoke Detector/Alarms, TENS Unit, Metal Detector, and More (4 Pack)

Customers find these batteries work well in smoke alarms and TENS units, with good capacity and value for money. The rechargeability and battery life receive mixed feedback - while some report they hold charge well and last long, others say they don't recharge properly and don't last as long as normal 9V batteries. The size is also a point of contention, with customers noting they're slightly larger than standard 9V batteries and don't fit in some battery compartments.