Environmental Project, 1004 – Collection Potential for Nickel-Cadmium Batteries in Denmark

Collection Potential for Nickel-Cadmium Batteries in Denmark

1 Introduction

1.1 General characteristics of NiCd-batteries

This report covers sealed nickel-cadmium accumulators, also called NiCd rechargeable batteries. They are commonly referred to as NiCd-batteries and that is the designation used in this report.

Large box-type so-called "open" NiCd accumulators (with an appearance similar to lead starting batteries for vehicles etc.) are not covered in this report. Open NiCd accumulators are not very much used in Denmark (Drivsholm et al., 2000), and are not collected through the same channels as NiCd-batteries in Denmark.

1.1 General characteristics of NiCd-batteries

This section is largely based on descriptions from (Maag and Hansen, 1994).

Nickel-cadmium batteries constitute a substantial part of the cadmium consumption in Denmark as well as globally. The so-called closed or sealed nickel-cadmium batteries resemble ordinary primary batteries like alkaline cells and have also in many cases been used as substitutes for the same.

NiCd batteries are rechargeable, meaning that contrary to primary batteries they can be used and recharged many times.

Inside a NiCd battery

The NiCd cell is built up of one cadmium and one nickel electrode, in reality two plates separated by a plastic separator. The separator is permeable by water molecules and specific ions dissolved in water, and it is saturated with a solution of potassium hydroxide in water. Closed NiCd batteries are sealed in a close-fitting steel casing protecting the cell and simultaneously functioning as an electrical conductor.

Battery packs

The cell voltage of NiCd batteries is 1.2 V independent of the cell size. For most applications of NiCd batteries in appliances, several batteries are connected in series in order to achieve more power. The resulting voltage difference is 1.2 V multiplied by the number of batteries.

Such series-connected batteries can be either build into the appliances or mounted in a battery pack that can be released from the appliance by a simple release mechanism. In a battery pack the batteries are typically enveloped in a firm plastic shell (or sometimes shrink foil) and connected to the appliance via an electric connection mounted on the outside of the pack.

Battery sizes

NiCd batteries are produced as circular cells, button cells and so-called prismatic cells, all sizes known from ordinary primary batteries. Additionally a series of other sizes is produced, primarily used for building into appliances and as battery packs for appliances. In table 1.1 the most common battery sizes are shown. The weight and capacity of the individual sizes might vary dependent of the manufacture and the application.

Batteries for various needs

Some batteries are designed with an especially high capacity (for a low strength of current), others with high strength of current (and lower capacity), for fast charging or for extreme temperatures. NiCd batteries in appliances are generally adapted specifically to the need and function of the appliance in question.

Energy density

Especially up to the 1990's a general development towards a greater energy density has taken place. For certain applications this development has continued (power tools). The motivation is to achieve light hand-held tools with the longest possible operating time after recharging. The energy density is closely related to the cell weight, which means that also the weight of the batteries has been slightly increasing for some uses.

Table 1-1 Common NiCd battery sizes1)

Type	Dimensions		Weight	Capacity	Voltage difference
Type	Diameter	Length	Weight	Capacity	Voltage difference
R1, N, Lady	12 mm	29 mm	9 g	150 mAh	1.2 V
R 03, AAA, Micro	10.5 mm	44 mm	10 g	200 mAh	1.2 V
R 6, AA, Mignon	14.5 mm	50,3 mm	24 g	600 mAh	1.2 V
R 14, C, Baby	26 mm	49 mm	67 g	2.0 Ah	1.2 V
R 14, C, Baby	26 mm	49 mm	55 g	1.4 Ah	1.2 V
SC (Sub-C)	23 mm	42.2 mm	50 g	1.2 Ah	1.2 V
R 20, D, Mono	33.5 mm	61 mm	147 g	4.0 Ah	1.2 V
R 20, D, Mono	33.5 mm	61 mm	78 g	1.4 Ah	1.2 V
V 7/8 R	26.6/15.7 mm2)	48.5 mm	45 g	120 mAh	9.0 V
F	33.5 mm	91 mm	237 g	7.0 Ah	1.2 V

Notes:

1) Source: The manufacturer's "VARTA's"catalogue quoted in (Maag J, Hansen E. 1994)

2) Rectangular battery internally composed of 7 or 8 small battery cells

Quantities for disposal

The annual quantity of NiCd batteries to be disposed of can be assessed on the basis of the consumption of batteries, the lifetime of the batteries and knowledge of the consumer's disposal conduct. NiCd batteries must according to Danish law and the EU Battery Directive be collected and recycled because of the environmental toxicity of cadmium.

Lifetime of NiCd batteries

A NiCd battery can be recharged a certain number of times, before the maximum capacity per recharging is so low that the user exchanges the battery. The number of possible rechargings (so-called "cycles") may vary from less than 300 up to approx. 3,000, depending on technical quality, application, application pattern and charging unit.

Exposure to overloading, heating, and mechanical damage may result in instant function failure. Additionally the phenomena "memory effect" and "self-discharge" may give the user the impression that the battery is defective, although these conditions can actually be remedied.

Memory effect and self-discharge

At repetitive partial charging, i.e. charging when the battery is recharged to less than full capacity, or recharging is started before the battery is fully discharged, it is often seen that the maximum capacity of the battery decreases. The reason is that the battery "forgets" the part of its capacity that is not fully used (hence the name memory effect). Full capacity can however be recreated after repetitive, complete discharge of the batteries and subsequent, complete charging.

For NiCd batteries there is always an ongoing self-discharge. If the battery is not used, it will after a complete charging self-discharge completely within 1 - 3 months. After such a self-discharge the battery will not be able to obtain its full capacity after just a single charging. A regeneration as mentioned under memory effect is required.

Lifetime expressed in number of years

The lifetime of NiCd batteries measured in number of years is according to the above assessable on the basis of knowledge of the following parameters for each individual application:

Lifetime measured in the maximum number of cycles under optimal operating conditions
An assessment of the typical use pattern involving with the following aspects:
Typical number of discharges/recharging of the battery per day.
Applied types of charging devices.
Risk of defective battery because of self-discharge or memory effect. This depends on the users' knowledge of the optimal use of the batteries combined with their need for reliability - strict requirements of reliability can in connection with lack of knowledge result in much partial charging.
Risk of overloading and mechanical damage in use.
Importers' and distributors' consumer contact (consultancy, servicing, complaints).

Lifetime distribution

In (Maag J, Hansen E. 1994), the collected information on the mentioned conditions has been converted into a specific lifetime distribution for each individual use. The distributions expresses how, for the number of batteries sold "year 0", a1 shares of the batteries will be defective within the first year after the sale, a2 shares will be defective within the second year, and so on. Appendix A shows these lifetime distributions - see the principle there.

Substitution and development trends

NiCd batteries been substituted by other (newer) rechargeable battery types for many applications since the mid 1990's. The reason is partly the environmental toxicity of cadmium - and consequent public regulation of the sales in Denmark - partly the technical development over the recent years. For certain applications substitution has been initiated commercially only within the period of 1997 to 98. The new types of batteries are the so-called nickel-metal hydroxide batteries (NiMH) and lithium-ion batteries (Li-ion). The current state of substitution is described in detail for the individual applications.