28 to a predetermined rest position with respect to case in which the ports 32 and 18 are misaligned the rotational inertia of the cells 12 the insulating case 16 and the outer housing 14 is substantially greater than that of the cylindrical valve 28 so that angular acceleration of the battery about its vertical center line rotates valve 28 with respect to the wall this causes ports 32 and 18 to come into alignment as shown in the drawings when the projectile reaches a constant rotational speed the spring 30 returns the valve 28 back to its rest position not shown in which the ports 32 and 18 are misaligned it will be appreciated that a fluid or mechanical damper may be added to the system if desired in order to slow movement of the valve from the aligned to misaligned position in operation acceleration of battery when the projectile in which it is located is fired breaks the ampul as in prior art reverse cells angular acceleration of the projectile causes the valve 28 to rotate with respect to the wall of the casing against the force of the spring so that the ports 32 and 18 come into alignment the liquid electrolyte 24 flows into the cells 12 activating the battery angular acceleration decreases as the projectile comes up to its final rotational speed and the spring 34.