We have used Streamlight rechargeables on our apparatus for years with excellent service from them. The average life of the Ni-Cad batteries is approximately 18 months to two years depending on the duty cycle, use and recharge. One tip that I will offer is to connect the power cord so as to only charge when the vehicle is running or a dead vehicle battery may be the result. Overall, I think you will be well pleased with the service and durability. YMMV.
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Forever... A long time to be dead!
Staunch advocate of the First, Second, and Fourth Amendments