9. Internal Battery Resistance
With the move from analog to digital devices, new demands
are being placed on the battery. Unlike analog equipment that
draws a steady current, the digital mobile phone, for example,
loads the battery with short, high current bursts.
Increasingly, mobile communication devices are moving
from voice only to multimedia which allows sending and receiving
data, still pictures and even video. Such transmissions add
to the bandwidth, which require several times the battery
power compared to voice only.
One of the urgent requirements of a battery for
digital applications is low internal resistance. Measured
in milliohms (mW), the internal resistance is the gatekeeper
that, to a large extent, determines the runtime. The lower
the resistance, the less restriction the battery encounters
in delivering the needed power bursts. A high mW reading can
trigger an early ‘low battery’ indication on a seemingly good
battery because the available energy cannot be delivered in
an appropriate manner.
Figure 9-1 examines the major global mobile
phone systems and compares peak power and peak current requirements.
The systems are the AMP, GSM, TDMA and CDMA.
|
|
| |
AMP |
GSM |
TDMA1 |
CDMA |
|
|
| Type |
Analog |
Digital |
Digital |
Digital |
| Used
in |
USA, Canada |
Globally |
USA, Canada |
USA, Canada |
| Peak
Power |
0.6W |
1-2W |
0.6-1W |
0.2W |
| Peak
current2 |
0.3A DC |
1-2.5A |
0.8-1.5A |
0.7A |
| In
service since |
1985 |
1986 |
1992 |
1995 |
|
|
Figure 9-1: Peak power requirements
of popular global mobile phone systems.
Moving from voice to multi-media
requires several times the battery power.
- Some TDMA handsets feature dual mode
(analog 800mA DC load; digital 1500mA pulsed load).
- Current varies with battery voltage;
a 3.6V battery requires higher current than a 7.2V battery.
9.1 Why do seemingly good batteries fail on digital equipment?
Service technicians have been puzzled by the
seemingly unpredictable battery behavior when powering digital
equipment. With the switch from analog to digital wireless
communications devices, particularly mobile phones, a battery
that performs well on an analog device may show irrational
behavior when used on a digital device. Testing these batteries
with a battery analyzer produces normal capacity readings.
Why then do some batteries fail prematurely on digital devices
but not on analog?
The overall energy requirement of a digital mobile
phone is less than that of the analog equivalent, however,
the battery must be capable of delivering high current pulses
that are often several times that of the battery’s rating.
Let’s look at the battery rating as expressed in C-rates.
A 1C discharge of a battery rated at 500mAh is
500mA. In comparison, a 2C discharge of the same battery is
1000mA. A GSM phone powered by a 500mA battery that draws
1.5A pulses loads the battery with a whopping 3C discharge.
A 3C rate discharge is fine for a battery with
very low internal resistance. However, aging batteries, especially
Li-ion and NiMH chemistries, pose a challenge because the
mW readings of these batteries increase with use.
Improved performance can be achieved by using
a larger battery, also known as an extended pack. Somewhat
bulkier and heavier, an extended pack offers a typical rating
of about 1000mAh or roughly double that of the slim-line.
In terms of C-rate, the 3C discharge is reduced to 1.5C when
using a 1000mAh instead of a 500mAh battery.
As part of ongoing research to find the best
battery system for wireless devices, Cadex has performed life
cycle tests on various battery systems. In Figure 9-2,
Figure 9-3, and Figure 9-4, we examine NiCd, NiMH
and Li-ion batteries, each of which generates a good capacity
reading when tested with a battery analyzer but produce stunning
differences on a pulsed discharge of 1C, 2C and 3C. These
pulses simulate a GSM phone.
Figure 9-2: Talk-time of a NiCd battery
under the GSM load schedule.
This battery has 113% capacity and
155mΩ internal resistance.
A closer look reveals vast discrepancies in the
mΩ measurements of the test batteries. In fact, these
readings are typical of batteries that have been in use for
a while. The NiCd shows 155mW, the NiMH 778mW and the Li-ion
320mW, although the capacities checked in at 113, 107 and
94 percent respectively when tested with the DC load
of a battery analyzer. It should be noted that the internal
resistance was low when the batteries were new.
Figure 9-3: Talk-time
of a NiMH battery under the GSM load schedule.
This battery has 107% capacity and
778mΩ internal resistance.
Figure 9-4:
Talk-time of a Li-ion battery under the GSM load schedule.
This battery has 94% capacity and
320mΩ internal resistance.
From these charts we can see that the talk-time
is in direct relationship with the battery’s internal resistance.
The NiCd performs best and produces a talk time of 140 minutes
at 1C and a long 120 minutes at 3C. In comparison, the
NiMH is good for 140 minutes at 1C but fails at 3C. The
Li-ion provides 105 minutes at 1C and 50 minutes
at 3C discharge.
Batteries in a Portable World 2nd Ed.
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