Non-Rechargables

Rechargables

In this category, the most important point to note is that while the form factor of the batteries are for 9 volts, no rechargeables actually reach a full 9 volt capacity due to the nature of rechargeable cells.

Standard 9 volt batteries are actually constructed of six individual cells. Alkaline cells are 1.5 volts each, and Nickel Cadmium and Nickel-Metal Hydride cells are 1.2 volts. Lithium-Ion cells are 3.6 volts each. So simple math tells you 6 x 1.2 = 7.2, which is why most rechargeable 9 volt cells are actually 7.2 volts! Some manufacturers are able to cram seven cells in their batteries, which get you the 8.4 volt varieties.

Most wireless' batteries should be replaced at about 7.5 volts, and even that is cutting it close. Each manufacturer has their own cutoff voltage (usually around 7.2v, I've found). So a rechargeable 7.2 volt battery doesn't last too long!

Now, while the cells are rated at 1.2 volts, when charged, they actually exceed that voltage, so a 7.2 volt cell may get you about 8 volts fresh out of the charger.

Nonetheless, my personal opinion is, rechargeables should never be used in a wireless.