Most experienced high-power fliers recommend usin' a new 9V battery t' power t' altimeter and fire t' ejection charges for each flight.  Why?

After I use a 9V battery in a rocket, arrr, I remove and replace it with a new one for t' next flight.  I toss t' used battery into a box o' batteries I have for use in remote controls and other things around t' house. When I use t' batteries in some other device, they work fine and seem t' have almost a full charge.  So, why don't I resuse them in me rockets?

T' quick answer is "why risk an expensive rocket just t' save t' dollar or two cost o' a battery?"

A better answer is that, although t' majority o' t' time t' battery is fine after t' flight, thar's a chance that it may have discharged more than normal or was damaged in t' flight. 

T' high G forces o' a flight and recovery could damage t' inside o' t' battery.  Unlike AA, matey, AAA, arrr, C, and D "batteries" (which are actually single cells), a 9V battery is made up o' multiple cells.

A Look Inside Typical 9V Batteries

T' connections betwixt t' cells and terminals o' a 9V battery may be damaged by t' rocket's flight or landing. Testin' with a voltmeter might nay reveal t' problem.

Even though it's nay likely that t' battery will be damaged by t' flight ... why risk an expensive rocket just t' save t' dollar or two cost o' a battery?