Many people want to use rechargeable batteries with wireless systems. Unfortunately, there are downsides to using certain types of rechargeable 9 volt batteries. On the other hand, rechargeable AA batteries tend to work quite well in wireless microphones.

The first problem is the low capacity of Ni-Cd rechargeable batteries and Ni-MH 9 volt batteries. Ni-Cd and Ni-MH 9 volt batteries will last 1/4 of the time that an alkaline battery would. So, if your wireless bodypack lasts 8 hours with an alkaline battery, it would only last 2 hours with a Ni-Cd or Ni-MH 9 volt rechargeable battery.

The second problem is the voltage or rechargeable 9 volt batteries. Unfortunately, although batteries are packaged in the 9 V size, they may not have a potential of 9 volts. Obviously, batteries that are the 9 V size contain multiple cells. Since alkaline cells are 1.5 V, it takes 6 cells to make one 9 V battery. Six Ni-Cd cells, though, are only 7.2 V. It is possible to get a 9 V sized Ni-Cd with 7 or 8 cells to make 8.4 or 9.6 V respectively. This shows that when looking at batteries, attention must be paid to their actual voltage, not just their physical appearance. Wireless transmitters like to see 9 V, so the 7.2 V lifepo4 batteries are generally not acceptable. Also, the 9.6 volts of an 8 cell battery can be too much for a wireless transmitter.

The latest secondary cell is the rechargeable alkaline. The rechargeable alkaline is a good choice for a rechargeable battery. Unfortunately, no 9 volt version is available yet. While its initial capacity is within 10 percent of a standard alkaline, the discharge capacity fades with each cycle, with the majority of the fade occurring in early cycles.

The best choice for a rechargeable 9 volt battery is the polymer lithium ion battery. This type or rechargeable battery has a similar capacity and discharge when compared to the alkaline. AA rechargeable batteries can often have a larger capacity than their alkaline equivalent, making them an excellent choice. The Lithium non-rechargeable AA battery will last about twice as long as an alkaline for those events that need extended battery time.