Biomedical Engineering Reference
In-Depth Information
Table 2.3 Ratings of Secondary Cells (Rechargeable Batteries)
Battery Type a . b , c
Specific Energy
(Wh/kg)
Energy Density
(Wh/L)
Nominal
Voltage (V)
Number of
Charging Cycles
Nickel cadmium
(Ni-Cd)
40 60
50 150
1.25
1500
Nickel metal hydride
(Ni MH)
60 120
140 300
1.25
500 1000
Lithium
ion (Li
ion)
100
265
250
730
3.6
400
1200
a Battery rating from Soykan (2002)
b http://www.panasonic.com/industrial/batteries-oem/oem/lithium-ion.aspx
c http://batteryuniversity.com
silver-oxide (button cell types or watch batteries). Implantable medical
devices now rely on the reliable lithium chemistries for long service life
(proven for decades on pacemakers).
From Tables 2.2 and 2.3 , it is evident that primary cells have higher
capacity (specific energy and energy density) than secondary cells. It is
not surprising that primary cells are typically employed for implanted
devices, while secondary cells are commonly used for inductive links
(externally). While the first pacemaker batteries were made from mer-
cury cells; nowadays the service life of lithium-based primary cells makes
them obvious candidates for long-term biomedical implants. Sadly, pri-
mary cells do not fare well when compared against gasoline. (Specific
energy of 13,000 Wh/kg or energy density of 9,700 Wh/L. Gasoline is at
least one order of magnitude higher than the highest primary cell.)
However, for some combustion engines, primary cells are about the
same level (Figure 1.2). Secondary cells are limited by the number of
charging cycles they undergo before significantly losing capacity. Most
importantly, they release heat when recharging which is not
acceptable for medical implants. Thus, primary cells are without doubt
the best powering alternative for implantable medical devices.
2.2 ENERGY GENERATION FROM THE HUMAN BODY
The human body is a machine that burns 2,000
2,500 food calories per
day in order to function appropriately. One food calorie or Calorie
(with upper case) is equal to 1,000 calories (with lower case), where
1 calorie is equivalent to 4.19 J of energy. Thus, 2,000
2,500 Calories
correspond to 8.4
2.9 kWh power consump-
tion, a relatively large number of energy (the same energy consumption
10.5 MJ in energy, or 2.3
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