Tech's Files: Battery Power—Conventional or Rechargeable?

Jan 1, 2010 12:00 PM, By Eddie Ciletti

GOING GREEN MIGHT BE EASIER THAN YOU THINK

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Figure 1: This comparison of AA cell life shows the conventional alkaline (red) battery maintaining power and then suddenly dropping off. The rechargeable NiMH (green) battery shows a gradual voltage reduction over a longer period of time.

Figure 1: This comparison of AA cell life shows the conventional alkaline (red) battery maintaining power and then suddenly dropping off. The rechargeable NiMH (green) battery shows a gradual voltage reduction over a longer period of time.

Three “discoveries” in the rechargeable 9-volt category help define the expectations. Powerex has a true 9.6-volt/230mAh NiMH battery; Powerizer's 7.6V/400mAh Lithium Ion version has a built-in PCB to prevent overcharging and over-discharging (notice the trade-off of more capacity, but less voltage), and IPower US has a 520mAh “9-volt” with a working voltage range of 8.4 to 6.5 volts.

Ottimista

With rechargeable batteries, the working voltage may not be too far above the most basic of energy-management systems — the “low battery” warning. The major difference is rechargeable batteries hold the bulk of their charge longer than disposable batteries. Figure 1 details a simple test using a 1.5-volt battery driving a Mini Maglite bulb consuming about 200 mA. The disposable battery is in red; the rechargeable is in green. As you can see, the red dots make an “S” curve, while the green dots go longer and are nearly horizontal, after which the voltage drop-off is severe.

It pays to have a good charger and to know the battery's history. For example, consider a set of batteries that get exclusive use in a digital camera. Digital cameras are very energy-conscious — they need to accurately write and preserve image data — so these batteries will experience the same charge/discharge routine and as a result, should have a similar life expectancy.

Under the best circumstances there will always be the exception, a “surprise shut-off” that's most likely caused by only one of the batteries. Testing each battery under load — that is, with a current drain similar to the camera's requirements — will reveal the culprit. Similarly, inside a disposable 9V are six AAAA batteries — yes, smaller than an AAA! Inside a defective unit, one leaky cell was dead, the other cells ranged from 800 mV to 1.2 volts.

Experience and Observation

I've used rechargeable batteries in all sorts of conventional battery-powered gear — from kids' toys to test equipment, flashlights and student lab kits — and always had plenty of charged spares. After testing several readily available appliances, I learned that an aged digital camera (using four AA batteries) sucked 800 mA when running the motor to extend the lens, then settled in at 500 mA with the screen on. A basic wireless mic, a simple oscillator and an Ibanez Dual Chorus consume 30 mA, 10 mA and 92 mA at 9 volts, respectively.

Modern electronics designed for battery operation consume about one-tenth the current of their cabled cousins. I recently helped a student repair the electronics in an active bass guitar. Inside was a surface-mount TL062 operational amplifier (a dual op amp), the low-powered version of Texas Instruments' TL080 series, the former consuming 200 µA (micro amps) per amplifier compared to 2 mA (milli-amps)/amp for the latter.

To obtain the most consistent results, I recommend one of the most complex and labor-intensive procedures known to scientists around the world: routine inspection, cleaning and preservation of everything — the battery, device and charger contacts. When using test equipment, it's amazing how the slightest amount of contamination increases contact resistance, affecting both charge/discharge performance as well as how much pressure must be applied to get stable readings.

The essence of the scientific method is to observe, minimize variables, collect data and correlate the information. This is great when you have time but can be a nuisance when the show must go on and there's no time for failure. The pressure to have a glitch-free show may put our “green-ness” to the test — it's common to replace batteries for wireless mics and stomp boxes before every show. Used batteries are often recycled into less critical applications. That said, a little research and experimentation go a long way.


Eddie's mental battery is long overdue for a recharge in Italy. Send him a postcard via tangible-technology.com.






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