The term SENTINEL identifies a battery that has the balance low voltage warning circuit built onto the pack. The SENTINEL replaces the older PLUS version.
Lithium-Ion's are available in a 7.4 volt SENTINEL version.
SENTINEL is reference to the low voltage warning option in which a high intensity LED can be plugged into the pack. The light is an option and does not need to be used for the balancing to work.
Other features of this SENTINEL circuitry is that it will allow a 3 amp charge. In addition, the balance feature is faster than the older PLUS version and will stop balancing if there is a cell that proves to be weak. This is to protect the pack from over discharge. The circuit balances the pack on its own and does not need the charger to be plugged in to initiate the balance process.
The SENTINEL version of DURALITE batteries is great for planes in which battery access is not convenient such as in jets, scale planes and helicopters as well as large aerobatic planes.