17. Did you know . . . ? 2

GUIDE: Batteries in a portable world. 17. Did you know . . . ? 2

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17.1 The Cost of Mobile Power

Among the common power sources, energy from non-rechargeable batteries is the most expensive. Figure 17-1 reflects the cost per kWh using non-rechargeable batteries, also referred to as primary batteries. In addition, non-rechargeable batteries have a high internal cell resistance, which limits their use to light loads with low discharge currents.

In the last few decades, there has been a shift from non-rechargeable to rechargeable batteries, also known as secondary batteries. The convenience of recharging, low cost and reliable operation have contributed to this. Another reason for the increased popularity of the secondary battery is the larger energy densities available. Some of the newer rechargeable lithium systems now approach or exceed the energy density of a primary battery.

  AAA Cell AA Cell C Cell D Cell 9 Volt
Capacity (alkaline) 1100mAh 2500mAh 7100mAh 14,300mAh 600mAh
Energy (single cell) 1.4Wh 3Wh 9Wh 18Wh 4.2Wh
Cost per Cell (US$) $1.25 $1.00 $1.60 $1.60 $3.10
Cost per KWh (US$) $890 $330 $180 $90 $730

Figure 17-1:  Energy and cost comparison of primary alkaline cells.
Energy from primary batteries is most expensive. The cost increases with smaller battery sizes.

Figure 17-2 compares the cost of power when using rechargeable batteries. The analysis is based on the purchase cost of the battery and the number of discharge-charge cycles it can endure before replacement is necessary. The cost does not include the electricity needed for charging, nor does it account for the cost of purchasing and maintaining the charging equipment.

  NiCd
AA Cell
NiMH
AA Cell
Lead Acid
(typical pack)
Li-ion
18650 Cell
Reusable Alkaline AA Cell
Capacity 600mAh 1000mAh 2000mAh 1200mAh 1400mAh 1
Battery Voltage 7.5V 7.5V 12V 7.2V 7.5V
Energy per cycle 4.5Wh 7.5Wh 24Wh 8.6Wh 6.3Wh
Cycle life 1500 500 250 500 10
Cost per battery (ref. only) $50 $70 $50 $100 $6.00
Cost per kWh ($US) $7.50 $18.50 $8.50 $24.00 $95.00

Figure 17-2:  Energy and cost comparison using rechargeable cells.
Older battery technologies offer lower energy costs compared to new systems. In addition, larger cells are more cost-effective than small cells. The battery packs taken for comparison are for commercial applications at over-the-counter prices.

For this calculation, 840mA is used since subsequent capacities are rated at 840mA (60% of initial capacity). If the battery is discharged partially, a higher cycle life can be obtained.

Figure 17-3 evaluates the cost to generate 1kW of energy. We take into account the initial investment, add the fuel consumption and include the eventual replacement of each system.

Power obtained through the electrical utility grid is most cost effective. Consumers in industrialized countries pay between $0.05 and 0.15US per kWh. The typical daily energy consumption of a household is 25kWh.

  Investment
of equipment to
generate 1kW
Lifespan
of equipment before major overhaul or replacement
Cost of fuel
per kWh
Total Cost
 per kWh, incl. fuel, maintenance and equipment replacement
NiCd
for portable use
$7,000, based on 7.2V, 1000mAh at $50/pack 1500 h, based on 1C discharge $0.15 for electricity $7.50
Gasoline Engine for mobile use $30, based on $3,000/100kW (134hp) 4000 h $0.10 $0.14
Diesel Engine
for stationary use
$40, based on $4,000/100kW (134hp) 5000 h $0.07 $0.10
Fuel Cell $3,000 – 7,500   $0.35  
- for portable use   2000 h --> $1.85 – 4.10
- for mobile use   4000 h --> $1.10 – 2.25
- for stationary use   40,000 h --> $0.45 – 0.55
Electricity
from electric grid
All inclusive All inclusive $0.10 $0.10

Figure 17-3:  Cost of generating 1kW of energy.
This takes into account the initial investment, fuel consumption, maintenance and eventual replacement of the equipment. The most economical power source is by far the utility; the most expensive is portable batteries.

The fuel cell offers the most effective means of generating electricity, but is expensive in terms of cost per kWh. This high cost is made economical when comparing with portable rechargeable batteries. For mobile and stationary applications, the fuel cell is considerably more expensive than conventional methods.

Note: The costing information obtained on the fuel cell is based on current estimates and assumptions. It is anticipated that improvements and wider use of this technology will eventually lower the cost to be competitive with conventional methods.

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