Here’s an article compiled from a series that has been running in our audio e-newsletter- which you can sign up for by clicking www.tfwm.com/newsletterform. The series garnered much attention from readers and so we decided to repurpose it in our digital version, for those of you who may not have seen it. Happy reading!
Nearly all churches today employ at least one wireless microphone, in order to provide mobility for the pastor while retaining clear sound reinforcement and enable the congregation to hear every word. Many churches employ multiple wireless microphones for the worship leader, backup singers, dramatic actors, and other service elements. Wireless microphones, wireless in-ear monitors, production intercom equipment, some types of hardwired in-ear monitors, and infra-red or radio hearing assistance systems all require batteries. Some churches have multiple venues which operate simultaneously on Sunday mornings. This can add up to a significant amount of battery usage every week, and a high potential for disruption of the worship experience should one of those batteries fail.
Most of the devices mentioned are shipped from the manufacturer with alkaline batteries or cells, in order to allow them to be set up and tested, and to avoid false failure reports due to the use of incorrect battery types or bad batteries. Most users simply continue using these devices with the same type of battery that came with the unit. Some users begin to notice the annual cost to operate these devices with alkaline batteries, and/or the substantial pile of spent batteries that the church will contribute to the local landfill every year. Some seek out alternative battery types on their own, and either experience success or failure, often by chance. Some have heard rumors that “rechargeable batteries are unreliable,” and never investigate alternatives.
Most manufacturers do not publish runtime data on their products even using alkaline batteries, much less any other type, and few if any publish technical data that would allow a qualified technician to evaluate their performance with different battery technologies. In short, as with many topics in audio, there are few sources of reliable information about battery technology for audio equipment, and many sources of misinformation.
Most small portable audio devices run on one of two styles of battery: the “9-volt” battery, or some number of AA-size cells. A few operate from custom rechargeable battery packs, and come with a charger intended for these packs.
Nearly all modern battery-powered devices have circuits in them which detect supply voltage and automatically turn them off when a certain “cut-off” voltage is reached. This is done to prevent poor or unpredictable operation, which often occurs when a device is “starved” for power. Some devices draw a constant current from start-up to cut-off, while others draw constant power, which means that they draw progressively more current as battery voltage falls. These parameters – the cut-off voltage, and the current draw versus input voltage curve – determine if a given type of battery will power a device at all, and which battery type can power it the longest.
Each model of each type of battery-powered device used at a given church may present different requirements that drive appropriate battery selection. Here we will summarize the powering requirements of common device types, the types of batteries currently available for small portable audio devices, and draw some conclusions about matching batteries to applications. It should be mentioned before going further that battery selection is part of the design of a sound system, and should be specified by a competent professional systems designer.
When considering the power requirements for battery powered audio devices in assembly facilities, we must first identify the reliability requirements for that device, given the role that it plays in the worship experience. Another consideration is how much runtime is required between battery changes.
Most wireless devices use one of two battery styles – the “9-volt” battery and the AA cell. Some AA-powered devices employ a single cell, some use two in parallel for approximately twice the run-time at the same operating voltage range, and some use two in series for a higher operating voltage range. In general, the trend in recent years among manufacturer has been to migrate towards the AA format, due to its greater energy storage capacity, lower cost, and broader worldwide availability. A few devices employ custom rechargeable battery packs that are essentially unique to that device, and preclude any selection of alternative power sources.
In general, portable audio devices are designed to operate from the maximum voltage available from the intended battery type, down to some minimum voltage, at which point the device warns the user, and then turns itself off.
Some devices draw the same amount of current regardless of the voltage level of the battery, whereas others employ a regulator that progressively draws greater current as the battery voltage declines. These regulators allow devices to operate down to lower voltages, and thereby extract more of the energy stored in the battery. The author has measured current draws for “9-volt”-powered devices ranging from a constant 13mA for a hardwired IEM bodypack, to well over 100mA for a device that employs a regulator, in the latter stages of discharge. Cutoff voltages range from about 7v down to around 4.5v. Most AA-powered devices employ regulators, and draw currents in the range of 150-300mA.
Electrical storage batteries are available in two general categories – primary types, which are not rechargeable, and secondary types, which are. Both 9v batteries and AA cells are available in primary alkaline and primary lithium types, as well as various rechargeable types. At present, the 9v rechargeable options include Nickel-Metal-Hydride (NiMH) and Lithium Ion Polymer (LiPo) types. Rechargeable AA cells are available in NiMH and Nickel-Zinc (NiZn). To briefly summarize a considerable amount of data, the longest-operating 9v type in most devices is the primary lithium, followed by the LiPo rechargeable, followed very closely by the best primary alkalines, with the best NiMH batteries offering slightly more than half the runtime of a high-quality primary alkaline. In AA batteries, primary lithium is typically the runtime winner, followed by the NiMH, followed by primary alkaline, followed by NiZn. Different battery and device combinations can yield results that vary from the above in cases of low battery starting voltage and high device cutoff voltage.
The foregoing results can also be significantly affected by the quality of charging that rechargeable batteries receive.