battery question-how come I can't do this?

[Jack]

echo, some rechargeable batteries, as they wear down, produce a gradually declining amount of voltage. So, a 12 volt rechargeable starts (when fresh) giving out 12 volts, when the battery's 1/2 done, it gives off 8-9 volts. And less and less as it's discharged.
Other batteries tend to give the same voltage until they're dead. If you put some alkaline batteries on a tester just as the battery dies, you can see the tester's needle go 100% 0% 100%
....all power , or none.
The declining amount of voltage in a rechargeable battery may be fine in some things, but the declining amount of voltage may hurt some electronics.