Future Battery Technologies

Adding to the power budget means we need to add in additional battery capacity, in
turn adding size, weight, and cost to our 3G handset. We have mentioned battery technologies
twice already in this chapter—once in the context of uplink bandwidth and
once in the context of needing to meet the peak energy requirements implicit in bursty
bandwidth.
Table 4.12 shows a comparison of battery technologies in terms of Wh/kg and
Wh/liter. The best-performing batteries at present are based on lithium. Lithium polymer
batteries provide a reasonable energy density (70 to 100 Wh/kg) but have relatively
high self-discharge rates (they go flat without being used). They also lose about
20 percent of their rated capacity after about 1000 cycles.
Lithium ion batteries using a liquid electrolyte to deliver better energy density (120
Wh/kg) but also have a relatively high self-discharge rate. Lithium metal batteries,
using a manganese compound, deliver about 140 Wh/kg and a low self-discharge rate:
about 2 percent per month compared to 8 percent per month for lithium ion and 20 percent
per month for lithium polymer.

Lithium thin film promises very high energy density by volume (1800 Wh/liter).
However, delivering good-through-life performance remains a nontrivial design task.
Also, these very high density batteries have high internal resistance; which means they
like to hold on to their power. Given that we are trying to design adaptive bandwidthon-
demand handsets that may be delivering 15 kbps in one 10-ms frame and 960 kbps in
the next frame, then obviously we need a battery that can support bursty energy needs.
Methanol cells may be a future alternative. These are miniature fuel cells that use
methanol and oxygen with (usually) platinum as a catalyst. Fuel cells can potentially
deliver better than 100 percent efficiency, since they pull heat from the atmosphere (an
answer to global warming!). Even the best diesel engines struggle to get to 30 percent
efficiency, so methanol cells with an energy density of 3000 Wh/kg would seem to be
a promising way forward.
Motorola has a prototype direct methanol fuel cell (DMFC) that has a membrane
electrode assembly, a fuel and air processing and delivery system, a methanol concentration
sensor, and a liquid gas separator to manage the release of carbon dioxide. The
prototype measures 5 × 10 × 1 cm excluding the control electronics and fuel reservoir.
At the moment, the device can produce 100 mW of continuous net power, so there is
some way to go before we have a methanol-powered multimedia mobile.
Potentially, however, energy densities of over 900 Watt-hours per kilogram are
achievable. A20-gram battery would be capable of producing 18 Watt-hours of power.
An example is a microfuel cell from Manhattan Scientifics. The device can be produced
in kilometer long, thin printed sheets rather like a printed circuit—the main
challenge is in the microminiaturization of the air distribution system and the internal
plumbing to mix the hydrogen and air sufficiently well to make the device efficient.
Manhattan Scientifics claim an energy density of 80 mW/cm2, equivalent to 940
Wh/kg for the device. NEC has similar products presently in development.
Whether we are talking about conventional batteries or fuel cells, there are essentially
two considerations: capacity and the ability to provide power on demand. 3G
handsets are either specified for a maximum power output of 250 mW (Class 3) or 125
mW (Class 4), and this determines the instantaneous uplink bandwidth available. The
battery has to be capable of meeting this instantaneous demand for power, given the
voltage being used in the handset—typically 3 Volts or, in the longer-term, 1 Volt.
Second, the overall capacity of the battery determines overall uplink offered traffic
bandwidth from each individual user. A600 milliamp/hour battery will not be sufficient
for uploading video content and also determines downlink processor capacity.
Either lithium or, in the longer term, fuel cell batteries will remain as a key enabling
technology in 3G handset and network implementation; battery bandwidth intrinsically
determines uplink and downlink network bandwidth and bandwidth value. 136