Rechargeable Batteries—Why You've Been Burned

This has been one of the most fascinating topics I’ve ever written about. As I’ve been Twittering and reading conversation threads on the CTDRT boards, there seem to be three camps of people in the rechargeable battery debate. The first group has used them for years, loves them and wouldn’t consider going back to alkaline batteries. The second group has never used them, though most have heard the horror stories and are leery. The third group has tried them, and had bad experiences. The phrase, “I’ve been burned too many times, I’ll never trust them,” has been thrown around dozens of times just in the last few weeks. This post is for the second and third groups. If you’ve been burned, I’m going to explain why, and if you’ve heard the stories, I’ll show you how to avoid them.

By way of introduction, know that I have been using rechargeable batteries in wireless mics (and a lot of other stuff) for almost four years. I’ve tested them, and have gone through hundreds of services using rechargeable batteries without a problem. Yes, hundreds of services. Far too many people have one bad experience, don’t investigate the cause and swear them off. The fact is, I’ve had just as many ProCells die on me at the wrong time as I have rechargeable cells--and in each case, it was always been my fault—I simply forgot to change them.

There are four things you need to know in order to get the most out of rechargeable batteries. Failing to understand and implement these things will guarantee bad experiences. However, trying to stretch a ProCell for 2 weekends worth of services is a bad idea, too. Keep in mind that rechargeable batteries behave differently than alkalines. Expecting them to be the same will only frustrate you. However, follow these guidelines and you’ll have great success; and save a ton of money.

Use Good Batteries

This is one of the key factors in getting good results. A lot of people went down to K-Mart 10 years ago, picked up a $10 pack of four NiCd AAs and a charger, tried them in their mics and went down in flames. That explains 50% of the “bad experiences.” To use rechargeable batteries in wireless mics, you need to use a more modern chemistry, Nickel Metal Hydride, or NiMh. The batteries need to be rated for at least 2000 mAh (I like 2500 and higher). mAh stands for milliamp hours and is provides a guide as to how much energy the battery can store. If you pull open your junk drawer and find a bunch of 1300 mAh NiMh batteries that are 5+ years old, that may explain the other 50% of the “been burned” statements.

A ProCell is rated for somewhere around 1800 mAh. The modern, high quality batteries from Sanyo, Powerex and Ansmann (with ratings from 2700-2850 mAh) absolutely blow ProCells away; as I showed you the other day with the test results. Even a 4-year old Ansmann 2700 beat a ProCell by almost 4 hours. And these newer batteries are rated for between 500-1000 charge cycles. Figuring one or two cycles a weekend, that could easily mean 5 years or more on one set, provided they are taken care of.

If your experience with rechargeable batteries does not include these newer batteries, you simply don’t have enough experience to make an informed decision. Sorry, but it’s true. You really need to check out the winners of the battery shoot out before making a decision.

So good batteries are important, but there is another component.

Proper Charging

The second key component to proper use of NiMh batteries is the proper use of a  good charger. Using a cheap “rapid” charger will not fully charge the cells, will overheat them and shorten their life. Modern, smart chargers are readily available, easily affordable and will charge the batteries at the correct rate to fully charge them, while avoiding over-heating. They will then switch to a trickle charge mode to keep them at peak capacity.

So what is a “proper charge rate?” Most battery manufactuers recommend a charge rate that is between 0.5-1.0c. That is to say, the charge rate should be one half to full capacity of the battery. So, if a battery is rated at 2000 mAh, the recommended charge rate would be 1000 mAh, and max charge rate 2000 mAh. A full charge will take from 1-2 hours at those rates, respectively.

If you have the time and want to extend the life of the cell, charge at .25c. Sanyo recommends a charge rate between 300-500 mAh for their Eneloops (rated at 2000 mAh). Charging at those rates will give you somewhere between 500-1000 cycles, according to Sanyo. The other batteries I tested have similar ratings.

Personally, I like chargers from BTI, Maha or Ansmann; especially the ones that have selectable “soft" charge rates. I buy enough batteries that I can spend 4 hours charging them, so I charge at 500 mAh. Once the batteries are well charged, it’s important to know how to utilize them properly. And that brings us to...

Proper Cycling of Batteries

To ensure good results (“good” defined as the mic not dying mid-service), it’s important to use the batteries properly. Fully charged batteries should always go straight from the charger to the mic. Once the charger is empty, it should be re-filled with another set of batteries. When the service or event is ended, remove the batteries from the mics, and charge. If you have enough charging bays, you can simply alternate from one set to another. However you do it, you always want to go from charger to mic.

The reason is that NiMh batteries will self-discharge over a period of 30-60 days. So while you may not lose a lot of capacity from Sunday to Saturday, you'll be down 10-20% or so. Why push it? Charger to mic, and charge immediately afterward. Keep a set on the charger all week and you'll always have fully charged batteries to work with.

Some are concerned about the “memory effect,” the loss of capacity that happens when NiCd cells are recharged before being fully depleted. NiMh cells have no significant memory effect, so charge them when you're done using them. Don't stretch them farther than needed.

Good chargers include a refresh cycle that will fully discharge each cell, then fully charge it again. It's a good practice to do this every 3-4 months. This procedure will prolong the battery’s life and ensure top performance.

Understanding Discharge Curves

Another main factor in the “I've been burned" phenomena is the different discharge curves between an alkaline and NiMh battery. An alkaline battery drops off in a pretty linear fashion. A NiMh, on the other hand, quickly drops from full voltage to something less than that and holds there for a long time. When it drops off, it drops of the cliff very quickly.

Discharge curve of the Eneloop (blue), a Sanyo NiMh (black) and an Alkaline (pink). I grabbed this from Sanyo's Eneloop website. Most battery meters in wireless mics are calibrated to the discharge curve of an alkaline battery. As the voltage drops off, the meter can predict approximately how long the battery is likley to last. However, with a NiMh, the voltage holds, then falls off very quickly. This is why it’s not uncommon to see a NiMh battery go from 4 bars to 0 in 5 minutes. The meter has no idea how to know where the battery is.

Some newer mics have battery meters that can be switched between alkaline and NiMh (the Shure UHF-R does for sure), and those can be a help. But there's still no substitute for doing some testing and finding out how long they last in your mics. Once determined, you know how often you need to change them, regardless of meter reading.

For example, I know that the Powerex and Sanyo 2700 mAh batteries will run a good 12-14 hours in our mics. So, I can feel confident putting them in Saturday afternoon for rehearsal and letting them run through end of service (about 4.5 hours total). I can then put in a fresh set Sunday morning and be fine through the end (about 5 hours total). I don't expect to ever have one go down, unless it's a fluke. But again, that happens with ProCells, too.

Those are some basic principles for the proper care and feeding of NiMh batteries. As I said, if you haven't tried them lately, you're throwing money away.