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Collection Potential for Nickel-Cadmium Batteries in Denmark

Summary

Only half of spent cadmium batteries are collected separately

The collection potential for nickel-cadmium (NiCd) batteries, e.i. the amounts of NiCd batteries that are disposed of annually in Danish society, have been assessed at about the double of the NiCd battery amounts actually collected through the Danish separate NiCd waste collection scheme. Cadmium is a toxic heavy metal. To minimize cadmium pollution, Denmark has worked for more than two decades to minimize the inputs of cadmium to society. NiCd batteries are today by far the main contributor of cadmium inputs to Danish society. The results indicate that while much is being done to collect NiCd batteries, substantial NiCd amounts may still be disposed of with ordinary waste, resulting in increased cadmium pollution now and in the future. The estimation method, developed for the Danish EPA, seem quite robust to uncertainties associated with input numbers. The assessment is an update of a similar study from 1994.

Background and scope

The aim of this assessment is to estimate the collection potential of NiCd batteries, i.e. the total quantity of NiCd batteries that could be collected in a given year, if the users dispose of all NiCd batteries through proper collection schemes, and do not discard them with household waste etc.

This assessment is a 2004 update for Denmark, based on the methodology developed by Maag and Hansen (1994) in their first assessment of the collection potential for rechargeable batteries. The update was prepared for the Danish EPA.

The Assessment

The collection potential is here attempted estimated on the basis of the following factors:

• Annual consumption (sales quantities) of batteries for the individual applications, assessed on the basis of information from suppliers, reports and statistics. Uncertainties on input data were included in the calculation through the use of stochastic variables.
• Lifetime of the batteries for each individual application. Assessed on the basis of detailed information from producers etc. on battery characteristics, charging technology and use patterns.
• The time span in which the users keep defective batteries, before they are disposed of (designated the "hoarding effect"). Implemented in collection potential estimates through the use of 4 specified scenarios for the hoarding effect.

Main results

The collection potentials for NiCd batteries in Denmark in 1997-2004 were assessed to fall between the min and max values shown in table 0-1. The estimated collection potentials proved to be quite stable towards the applied hoarding effect scenarios.

For the years 1997-2002 the estimated collection potentials have been compared to the amounts of NiCd batteries which have actually been collected in Danish scheme for the separate collection of NiCd batteries. Estimated collection efficiency varies over the years but generally lies within about 30-70% of the estimated collection potentials.

Table 0-1 mean collection potential values, as well as the minimum and maximum considered plausible, across all 4 scenarios, in tonnes/y.

Note *1: Minimum and maximum among all quantiles across all four hoarding scenarios and all three lifetime options tested. *2: An alternative presentation of the uncertainty on the mean, e.i. the distance between the mean value and the interval limits. The numbers in the column express the "A" in the often used notation "Mean +/- A %".

Project resultsSealed nickel-cadmium 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.

Illustration of how consumption, lifetime and hoarding effect scenarios affect the collection potential

The relationship between consumption, battery defect rate and collection after the hoarding effect is shown for an example, professional power tools, in figure 3-2 below. It should be noted that the figure is only meant to illustrate the principles applied in the assessment, and discussion of the numbers themselves are given in other sections of the report.

The blue line is the estimated consumption of NiCd batteries in the assessed period. The consumption peaked in 1988 and 2000. The consumption before 1985 and after 2004 was not estimated. As such, the figure illustrates in principle how the situation would be if sales of this NiCd application did not continue after 2004.

The pink line illustrates how the defect rates are delayed compared to the consumption. The peak defect rates are observed after about 1 mean lifetime after the consumption peaks. The defect rate peaks are wider than the consumption peaks because the lifetime distribution applied spreads the battery defect incidents over a range of years around the average lifetime, reflecting the fact that not all batteries becomes defective at exactly the same time after purchase. The defect rates before 1990 are not shown, because the input consumption estimates before 1985 are not available.

The yellow line is the calculated annual collection potentials. It illustrates how the hoarding effect further delays the actual discarding of the defective batteries. In this case, the collection potential under hoarding effect scenario 3 is shown (see section 3.6). In this scenario, half of the consumption of professional power tools is assumed used by so-called "organised users", who discard their defective batteries 1 year after defect on average, while the other half is assumed used by "un-organised users", who discard their defective batteries 7 years after defect on average. The discarding time is delayed in time compared to the time where the battery becomes defective, and the compound hoarding effect model used in scenario 3 further spreads the discarding of the consumed batteries over time. If a uniform delay in time for all applications (scenario 4) had been shown, the yellow line would be a precise replica of the defect rate line (pink line), but would simply be delayed 4 years, compared to the defect rates.

Click here to see figure 0-1

Consumption estimates

A detailed overview of the consumption of NiCd batter over time and distributed on uses is given in table 2.8 in section 2.9.1. The same data are shown in figure 0.2 below. For the background of the individual data, please see the respective sections of the report.

Note that the category "other uses" reflects rough estimates for the period 1985-1993, as derived by Maag and Hansen (1994), interpolations for the period 1994-1996, and balances versus tax-derived NiCd consumption totals for the years 1997-2002, as described in section 2.9.2

The figure shows how consumption peaked around 1997-2000, and declined through 2002 as NiCd batteries were gradually substituted for by NiMH and Li-ion batteries. Only few uses remain, cordless power tools being the most important tonnage wise. Also for power tools however, substitution has set in over the last few years.

Click here to see figure 0-2

Assessment results

The assessment results are presented in detail in table 3.8 in section 3.8. A close look at table 3.8 reveals that the resulting collection potentials are rather robust to both the hoarding effect scenarios, and the different lifetime options tested, for the period 1997-2005, which is of most interest here. This is considered mainly a result of the consumption trends in the years influencing the values most, in combination with the "smoothing" effect of the battery lifetime distributions (not all batteries bought in "year 1" become defective within the same "year x", see illustration above). As shown in section 2.9, the consumption peaked in the years 1997-2000 and exhibits a declining trend from 2000 to 2002.

Table 0.2 below show the estimated mean collection potential values, as well as the absolute minimum and maximum among the presented quantiles, across all 4 scenarios. The table also show the calculated differences between minimum and maximum quantiles for each year in tonnes, and half of the same difference in percent of the mean value.

Table 0-2 mean collection potential values, as well as the minimum and maximum considered plausible, across all 4 scenarios, in tonnes/y.

Note *1: Minimum and maximum among all quantiles across all four hoarding scenarios and all three lifetime options tested. *2: An alternative presentation of the uncertainty on the mean, e.i. the distance between the mean value and the interval limits. The numbers in the column express the "A" in the often used notation "Mean +/- A %".

Conclusions

Though the assessment made do not fully include all associated uncertainties, it may be concluded that there is a high likelihood that the true collection potentials for NiCd batteries in Denmark fall between the min and max values shown in table 0.2.

For comparison, the collected amounts of NiCd batteries in Denmark each year since the introduction of the state-paid awards for collected NiCd's in 1996 are shown in table 0.3.

Table 0-3 Collected NiCd batteries registered in Denmark 1996-2003, tonnes/y (Danish EPA, 2004)

Note that some time passes between the NiCd batteries are originally collected and the time when the awards are paid and the amount therefore can be seen in the Danish EPA's statistics (so-called "pipeline effect"). In line with normal business principles, this time does most likely not exceed 1 year. The collection award was 120 DKK/kg NiCd batteries collected from 1996-1999, but was raised to 150 DKK/kg as from 2000. The award is the main driver behind this controlled system, and the numbers presented may be considered as precise.

When comparing the data in the two tables, the overview shown in table 0.4 emerge. Note that here, the collected amounts presented for 1997 are the amounts registered in 1998, to account for the pipeline effect. The table shows that the estimated collection potentials indicate that large amounts of NICd batteries have been collected, but a more or less equal part of the potential has not been collected.

Table 0-4 Comparison between estimated collection potentials and actually collected NiCd battery amounts

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Version 1.0 May 2005, © Danish Environmental Protection Agency