Rechargeable Batteries and You

In previous blogs, we reviewed the pros and cons of various disposable battery technologies for effects pedals. This time around, we are going to study rechargeable batteries as an alternative.

The Contenders

Theoretically, rechargeable versions of the alkaline batteries would be a good choice for effects pedals. The chemistry is the same, but the battery is constructed so as not to explode when being recharged! Rechargeable alkaline batteries are inexpensive to make and only require a simple charger. They are non-toxic, and have a low self-discharge; left unused, they have a shelf life of up to 10 years. Unfortunately, few companies seem to make them these days and I couldn’t find a 9V at all. Newer technologies seem to have pushed them aside.

One limitation to rechargeable alkaline batteries is the high internal resistance which means they are not suitable for high current devices. Although this doesn’t matter for many effects pedals, technology had to evolve to support the high drain digital products we use today. New rechargeable chemistry had to be developed, and one of the first was Nickel Cadmium (NiCd).

The common chemistry used in the early days had drawbacks. Recharging a single battery a hundred or even a thousand times before disposal should be much more environmentally friendly, especially if you have access to domestic power from renewable sources such as solar. Unfortunately, the Cadmium used in NiCd rechargeables is highly toxic and requires special processing for disposal, undoing much of the environmental benefit of recharging. The use of Cadmium is now significantly restricted in the European Union under the RoHS and REACH programs, making these pretty much unusable in Europe.

Early NiCd cells suffered from an issue where a particular sequence of charge discharge events could cause the battery to apparently lose capacity. The story goes that this behavior was first observed on a satellite in space, but there was also a much more down to earth use case. Imagine you regularly drain a battery to a particular level, say 50% such as when using a laptop in a normal workday. In the evening you plug in the charger and leave it to charge slowly overnight. You do this for a week or so, then one day, you go on a long trip, you try to use all the batteries capacity: Although apparently fully charged, it dies at 50%, as if it ‘remembered’ it’s usual workday. For this reason it became known as the ‘memory effect’.

In reality what was happening was the cadmium-hydroxide crystals in the cells were growing as much as 100 times, increasing the internal resistance and causing voltage depression. The capacity was actually still there, but could no longer supply the voltage necessary to drive the device. The issue can be countered by exercising (discharge /charge) and reconditioning (slow discharge to below cut off voltage). Recent design NiCd’s have significantly reduced this behavior.

Nickel metal hydride (NiMH) is a good choice for effects pedals. They can last up to a thousand cycles with reasonable performance. They are prone to self-discharge which means they will lose some of their charge just sitting unused. However, advances have been made recently that improve this and good quality ‘low self-discharge’ 9V batteries with capacities of around 250mAh are available for under $10 each. A charger can be had for around $20.

The new kid on the block for 9v rechargeable batteries is lithium-ion, using the same chemistry as the batteries in smart gadgets like phones and computers, but in a 9V format. The specifications look attractive; the batteries are really light, and a 4 pack with charger can be had for less than $30. It’s early days for these. It will be interesting to see how they work out.


Pros and Cons

Alkaline
+ Low cost, very low self discharge, non-toxic
– Unavailable in 9V, high internal resistance

NiCd
+ High discharge rate, good over charge discharge tolerance, long cycle life
– Heavy, toxic, low energy density

NiMH
+ Light, non-toxic, good energy density, wide availability
– High self discharge, low over charge discharge tolerance

Li-Ion
+ Very light, non-toxic, very high energy density
– Limited choice, low over charge discharge tolerance, unproven in 9V form


Conclusions

Charging a rechargeable battery costs pennies, and with hundreds of recharges over several years, the extra initial cost is soon recovered. When they reach the end of their useful lives, disposing of one rechargeable vs. one hundred alkaline batteries is always going to be better for the environment. Music equipment such as effects pedals, wireless microphones, headphones, and portable recorders make great candidates for rechargeable batteries.

Apart from a few niche applications such as RC car racing, NiCd is on its way out. The low energy capacity and toxic contents are 20th Century battery technology.
NiMH is going to get the best purchase rating here. NiMH has a reasonable energy density, and should be able to provide about 20 hours of use per charge for a typical middle of the road analog effects pedal. Most of the common battery types, including 9V, are available from a wide range of different manufacturers, including the major brands. A top of the line 9V NiMH will cost about $10, with cheap ones for around $3. I’d steer clear of the real low end ones. There’s plenty to choose from reputable manufacturers for just a little more. Get a decent quality ‘smart’ charger rather than a ‘value’ or ‘dumb’ charger. The smart charger will reduce the likelihood of overcharging or shorting. If you don’t use the batteries regularly, take them out every few months and give them a full charge.

Li-Ion gets the Most Promising Newcomer award. The chemistry is well proven in numerous electronic gadgets, computers, power tools, medical and industrial applications, even cars and airplanes, but is somewhat new to the 9V. The choice is pretty limited, but the light weight and high energy density make them appealing.

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