How important is the battery in a wireless mic?

The Duracell TV commercials would have you believe that batteries are crucial. Of course, they are right.

When I started in church audio, wireless microphones were very rare. They were expensive and churches did not know anything about them, much less how to budget for batteries.

One of my mentors had a solution. At the time, Radio Shack had a promotion called “battery of the month club.” In order to promote foot traffic, they would give one free battery to card-holders each month. Our city had four Radio Shack stores and Dan (name changed to protect the guilty) had a card with each one of them. Dan, Daniel, Danny and DJ! Each Saturday Dan would go to a different Radio Shack and buy one cassette tape to record the sermon and get his free battery to power the wireless mic. The tapes worked great, but the batteries were an adventure. Sometimes they were so bowed out in the middle that the door on the wireless transmitter would not close and we had to tape the battery in place. Some weeks the battery gave out in the middle of the sermon. Everyone accepted the challenge as part of that new-fangled technology. Years later, I still laugh at the little green batteries.

More than twenty years later, churches of all sizes have multiple wireless microphones and even wireless monitors, we are still wondering what to do about batteries. Our company just designed a system for a church that included ten wireless microphones and two mixes of wireless monitors. The church worship techs turned ghostly pale when we did the math. Twenty-four AA batteries per week, times 52 weeks in a year plus special events, weddings and dramas— equals a bunch (highly advanced mathematical term) of batteries.

One of the worship techs pointed out that the sales brochure for this line of microphone boasts nine hours of life from one set of AA batteries, therefore one should only need one set per month for each mic. I attempted to explain to these guys that a fresh battery every week is a small expense. AA batteries are between forty-five and forty-nine cents each when you buy in bulk from a store such as Sam’s Club or Costco Wholesale. I warned them that transmitters go on inadvertently. I also cautioned that marketing material does not always give the full details.

A condenser microphone capsule has a stronger preamp stage, using more battery power. Should there be a loud sound transmitted, the internal limiter would kick in, draining more power from the batteries. Also, the battery life test in question had squelch set to zero, which seldom happens when you use multiple frequencies in one room. There were no people soaking up RF energy, so the last drop of energy transmitted actually made the trip from transmitter to receiver, which never happens in the real world. Marketing says nine hours of life from one set of batteries, but I say five hours of use is more realistic.

Never to be defeated in the quest to save God some money, this pastor quickly moved on to the next great idea—rechargeable batteries! Pay a little too much the first time and use them over and over. The church bought two sets. One set to charge and one to use. As I see it, there are only two problems with that idea, as I explained to the minister of music— even the new smart chargers can get confused. If the batteries are left on the charger too long, they will be destroyed. Also, to properly charge, the batteries must be fully discharged, and no one wants to hang out at the church and watch batteries die. Third, (okay three problems) and most importantly, rechargeable batteries are mostly dead even when fully charged.

Because magazine articles are not supposed to allow opinion pieces, I had to do a little research to back up that last statement. An industry colleague told me that wireless microphones consider 7.0 volts to be lowest useable voltage in a nine volt system, therefore a 7 volt battery is dead. Duracell Procell tested new at 9.6 volts. Published info for Duracell and Energizer consumer batteries both are in the 9.2 volt range. In the real world, one can use these batteries for several minutes and they are still fully charged. Contrast with the Energizer Nickel Metal Hydride (NiMH) rechargeable battery fully charged is published at 7.2 volts. The owner’s manual for a Shure ULX series wireless states a NiMH battery has only fifteen minutes of useable energy. Rechargeable AA batteries fare even worse. Energizer NiMH AA published fully charged is 1.2 volts. AA alkaline batteries publish at 1.5 volts and tested at 1.6 volts. Wireless microphones should always get alkaline batteries. “Heavy duty” batteries are fine for flashlights, but not wireless microphones or other electronics.

Voltage is important, but it is not the only thing to consider when selecting batteries. Current capacity determines how long the battery can continue to produce the required voltage. Current is measured in milli-amp hours. Higher numbers mean that all else being equal, a battery with more current capacity will last longer. Home electric bills are computed in kilo-watt hours, this is the same idea. Energizer standard 9 volt has a 625mAH rating while the Energizer Ultra has a 655mAh rating, and the rechargeable has only a 150mAh rating. Energizer’s standard AA has a very useable 2850mAH rating while the Ultra has a little more punch at 2900mAh. The chargeable lags behind at only 2500mAh.

Twenty years of using batteries has led to my Three Simple Rules of batteries. One, forget the rechargeable batteries. Two, stick to Duracell Alkaline, Duracell Procell, or Energizer Alkaline standard batteries, no heavy duty or cheap imports. Third, change the batteries every week.

And for good measure, here’s a fourth rule: never put a nine volt battery in the same pocket as your keys.