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D-cell battery capacity testing

Introduction
Single-use D-cells bought in foreign countries were tested to get a rough idea of battery capacity (in mAH) in order to compare the overall value in purchasing disposable batteries as opposed to rechargeable ones. As battery capacity is a function of the C-rate at which the current is drawn, one type of circuit for a typical current rate was chosen for all tests. Each battery was placed into said circuit, and voltage and current data was monitored until the battery discharged to well beyond the set cut-off voltage (described below).

Testing
After building a few different circuits, a simple constant-resistance circuit was chosen for all subsequent battery testing. A simple arduino sketch logged the voltage (and subsequently, current) data onto a microcard.  The circuit was designed to draw an average of 100mA of  current when the battery was fresh. This value would decrease naturally as the power available was slowly depleted.

schematic

schematic, showing a simple bjt switch for fast ‘high current’ pulses, in an attempt to gather state of charge information from internal resistance calculations circuit

circuit

circuit, meant to allow fast high pulses

A current of 100mA is a rate that is typical for standard uses of D-cells – for example, the LED flashlight bought in Mali uses two D-cells and pulls roughly 100mA of current, when the D-cells are fresh (still at ~1.5 volts). The light from the flashlight dims as the power supply voltage is decreased, shutting off at ~0.8 volts. These values are in line with the industry standard of turning off battery capacity testing at 0.8 volts and confirm our practice of counting mAH’s accrued to the same point. The foreign-bought D-cells brought into the lab were all designated “R20”, while the U.S.-bought Duracell batteries are “LR20”. This signifies a difference in battery composition. While the Duracell is alkaline-based, the foreign D-cells were all of a zinc-carbon chemistry, that is known to have a lower typical capacity (~8000 mAH instead of the alkaline’s 12,000 mAH, presumably at a current around 100-200mAH). Zinc-carbon batteries have low energy-density, short shelf-life, and also commonly leak through their outer casings, as these are the negative zinc terminals. All of these typical characteristics were noticeable during testing, as high C-rates fried the R20’s and leaks occurred often, during and after testing. Below is a picture of the zinc-chloride paste leaking through the outer casing of one D-cell:

leaking D cell

Results
The foreign-bought zinc-based D-cell batteries delivered on average less than 25% of the energy delivered by the U.S. Duracell battery. Although 75% capacity would be ideal (if one were to use the 8000mAH vs. 12,000 mAH alkaline capacity approximation), when battery prices were taken into account, most R20 brands performed on par with the U.S. D-cell, and a few brands even surpassed the Duracell in performance per cost (see Figure below).

battery-graph

Conclusion
Although foreign zinc-based D-cells are prone to leaking (perhaps damaging equipment), have short shelf-lives and low energy-density (all of which are likely worsened by high temperatures), if used for specific applications (such as low-power devices), most of the brands tested in this study delivered at least 80% as much energy (mAH) as the U.S. Duracell when relative costs were taken into account. A few brands even outperformed Duracell in this capacity/USD comparison. Converting these energy/cost values from mAH to kwh (assuming average voltage was around 1.1 volts), the Duracell delivers 0.0076 kwh/USD for a relatively expensive $131/kwh. The best performing foreign R20 D-cell, the ‘Panasonic Special’, gave 0.0128 kwh/USD, or $77.8/kwh.

A typical rechargeable D-cell rated at 10,000mAH capacity costs (online in the U.S.) roughly $56 (per 8-pack), while a solar 4-in-1 charger is $30. Normalizing to one D-cell that has a cycle life of around 500 (typical of an NiMH battery), it can be shown that the energy delivered (until batteries need replacement) is approximately 344,800 mAH per USD, roughly equivalent to 0.379 kwh/USD or $2.6/kwh. Although this is a rough estimation that only takes into consideration manufacturer’s ideal specs, prices for rechargeable batteries and chargers may still have room to drop. This means that, for those with or without grid electricity, it makes economic sense to invest in rechargeable batteries.  For those not connected to the grid (or those who must purchase expensive electricity), further investment in a small solar charger would alleviate extra charging costs.

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