Why are rechargable batteries not recommended for my Shure ULX wireless system?

The primary reason that we don’t recommend 9-volt size rechargeable batteries is that the operating time is usually too short for many purposes. The capacity of the typical 8.4V or 9.6V NiMH battery is only about 150mAh. (Energizer Rechargeable 9 volts are rated for 150mAh.) When compared to 500mAh for the typical alkaline 9-volt, the operating time is only about one quarter as long. Thus, if the alkaline lasts eight hours, the rechargeable will only last about two hours at best. Note that wireless transmitters are relatively high-current devices: 40-50mA is typical current draw.

As you are no doubt aware, the operating voltage is also not standard for NiMH 9-volt-size devices: six-cell version have only 7.2 initial voltage and will run only briefly while a seven-cell versions have 8.4 initial voltage and will run up to a couple hours. Eight-cell versions have 9.6 initial voltages but will still only run a couple of hours typically.

However there is one additional wrinkle unique to the ULX transmitters: They have a “latching” circuit governed by the battery voltage to prevent false “good ” battery indication due to brief voltage bounce when the unit is switched off in a low-battery condition. That is, once the battery voltage falls below 6.8 volts during operation the red low-battery indicator on transmitter), the unit will not display a “good” battery indication until the unit is switched off then the supply voltage is brought above 8.4V and the unit is switched off the unit supply voltage is brought above 8.4V an the unit is switch back on. This effectively forces the operator to put a new battery in the unit rather than continue with the nearly dead on. Note that the transmitter tonekey ceases at 6.0V and the unit turns off altogether at 5.5V.

The battery meter on the transmitter will still read the voltages correctly, but the expected remaining operating time as a function of the number of battery bars is no longer accurate since it is still proportional to the (expected) alkaline discharge curve. In particular, a “low battery” indication will be very near the end of operation for the rechargeable battery.

There is no risk to the equipment when using rechargeable batteries in the ULX product and no issues with warranty coverage. The only real risk is unexpectedly short operation times of the rechargeable batteries to provide satisfactory and reliable operation in regular service.

Knowledge Base