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Accumulators for mobile devices

December 13, 2000



NiCd, NiMH or Li-ion? Or wouldn't it better to get a Li-Pol accu? What accumulator works most of all? What makes less trouble?

Types of accumulators

Today for mobile devices, portable computers and equipment the following accumulators are used most of all: sealed lead acid (SLA), nickel-cadmium (NiCd), nickel-metalhydride (NiMH) and lithium-ion (Li-ion). Besides, lithium-polymer (Li-Pol) accumulators are becoming more and more popular in the sphere of cellular phones and portable computers. And, nowadays a lot of researching works concern fuel elements and some other perspective technologies of accumulator production.

The technical characteristics of the main types are shown in the table below by authority of Landata company.

Parameter Type of electrochemical system
SLA NiCd NiMH Li-ion Li-Polymer
Energy density (W*h/kg)
30
40 - 60
60 - 80
100
150 - 200
Number of working charge/discharge cycles (capacity reduction up to 80%)
200 - 500
1500
500
500 - 1000
100 - 150
Internal resistance at 3.6V (milliOhm)
No data
100-200
150-250
150-250
No data
Minimum charge time, hrs
8 - 16
1.5
2 - 4
3 - 4
8 - 15
Tolerance to overcharge
High
Average
Low
Very low
No data
Month self-discharge
5%
20%
30%
10%
No data
Voltage per element, V
2.0
1.25
1.25
3.6
2.7
Load current
0.2 C
>2 C
(0.5 - 1.0) C
<1 C
0.2 C
Working temperature range, Celsius degrees
-2060
-4060
-2060
-2060
No data
Service periodicity, days
90 - 180
30
60-90
Not required
No data
One cycle cost, USD
0.1
0.04
0.14
0.1 - 0.2
0.6

Table 1. Accumulator types according to electrochemical system

Notes:

  1. With regular service the number of working cycles can reach as high as 4000 instead of 1500 cycles guaranteed by manufacturer. Without service cycles being made, the number of working cycles can be reduced more than 3 times.
  2. The number of working cycles depends on the discharge depth. A small depth provides the large number of cycles.
  3. Self-discharge practically ceases on the second day after a complete charging of an accumulator. Self-discharge of NiCd accumulators constitutes 10% during the first 24 hours, then falls down to 10% a month. Self-discharge increases with temperature going up.
  4. An integrated protection circuit consumes approx 3% a month.
  5. 1.25 V is voltage value of one element, 1.2 V can be often seen in manuals. The both values concern one element type.
  6. 2.5 - 3.0 V depending on on the material of anode.
  7. Concerns only discharge; temperature range for the charge is more limited.
  8. Costs according to 1999.

Each type of accumulators features its merits and downsides. Let's consider both in detail.

SLA accumulators

They are used in Uninterruptible Power Supply (UPS), burglar alarm systems, devices of railway automatics and communication, wheelchairs, reserve lighting. Earlier such accumulators were included in complete sets of some mobile telephones and video cameras. In general, I should say that SLA accumulators are used usually if large power and low price are required, with size and weight being not so important. The capacity range for portable devices is 1- 30 ampere-hour. In modern mobile devices these accumulators are not used anymore.

NiCd accumulators

Range of application: cellular phones, usual and trunk radio stations, home radiotelephones, portable computers, video cameras, hand powerful electric tools, medical equipment, production equipment. Though, new models of cellular phones and portable computers are not equipped anymore with such accumulators. However, in trunk and usual radio stations they are still widely used.

Highlights

  • Fast and simple charge method. NiCd accumulator permits charging with the current higher or equal to the rated capacity. That's why you can charge the accu for an hour. And in case you need higher charge speed, you can set the current to be twice or three times more than the rated value. Though, do be careful. Besides, in case of fast charge you need special charging units which can define when charging is completed and stop fast charge.
  • It's preferable to use impulse charging method than that of direct current (distribution of discharging impulses between charging ones). This method (also called as reversing) supports high area of active surface of plates thus increasing efficiency and service life of an accumulator. Reversing charge improves fast charge since it stimulates recombination of gases evolved during charging. As a result, an accumulator heats less and charges more effectively as compared with the charging method of direct current.
  • Capability to give higher current.
  • Durable provided that operating conditions and periodical service are implemented.
  • Weak sensitivity to wrong operating, easy recovery in case of reduction in capacity and after long rest (from 60 to 70% of accumulators found unworkable can be restored). Recovery of an accumulator is implemented by discharging of it according to a definite algorithm down to 0.4 V per element. Recovery cycle is used in case of service cycles don't help.
  • Low price.

Lowlights

  • Necessity in periodical service in order to eliminate "memory effect" (enlarging of crystal grains of the working material and therefore decrease in area of active surface and real capacity of accumulator). Under service (service cycles) we understand periodical discharge of accumulators down to 1V per element which results in preventing of "memory effect".
  • High self-discharge (up to 10% during the first 24 hours and up to 20% in the first month after charge).
  • Large size and weight as compared with accumulators of other types.
  • The accumulator contains cadmium and therefore requires special utilization, that's why in some countries it has been restricted.

NiMH accumulators

Not so long ago NiMH accumulators replaced NiCd. However, widely advertised, they didn't manage to ensure 100% satisfaction of demands of users, mainly due to reduced service life.

NiMH features

  • Energy density is approximately 30-50% higher than that of NiCd accumulators, and correspondingly, the size and weight are less.
  • They "prefer" slight discharge (not deep one) and its service life is directly connected to the depth of discharge.
  • NiMH accumulators dissipate much more heat during the charge as compared with NiCd and require a more complicated algorithm of complete charge control. As a rule, they are equipped with an additional sensor for it.
  • NiMH accu can't charge so quickly as NiCd do. The charging time is usually two times more than that of NiCd. The recommended charging current is one fifth to one half of the rated capacity.
  • Less inclination to "memory effect".
  • Environmentally appropriate technology.
  • Small number of charge/discharge cycles.
  • High self-discharge (up to 30% in the first month).

Li-ion accumulators

Range of application: cellular phones and different portable computers. According to Varta company Li-ion accumulators will start forcing out soon not only NiCd but also NiMH accumulators. This type contains an internal control and protection circuit intended to restrain peak voltage of each element during the charge and to prevent going the voltage below an acceptable level during the discharge. Besides, the maximum charging and discharging current should be limited and element's temperature should be controlled.

Highlights

  • High energy density, small size and weight.
  • Low self-discharge (about 3-5% in the first month, then reduction to 1-3% per month, plus about 3% per month is consumed by the control circuit).
  • No service is required during the whole service life.

Lowlights

  • High price.
  • Necessity to keep the accumulator charged.
  • Aging proceeds, even if accumulator is out of use. Deterioration of capacity approximately a year after the release date. After two years the accumulator gets malfunctioning often. We don't recommend you to give it a rest for a long time.
  • Note that difference in voltage of a single element of Ni-based and Li-ion accumulators (1.2 and 3.6 V correspondingly) often doesn't allow replace one by the other. For example, in cellular phones with 4.8V. Besides, these accumulators require different charging units.

Li-Pol accumulators

Li-Pol is a next step of Li technology development. Potentially, they are less expensive than Li-ion accumulators. But as they are not widely adopted, the price is very high at this moment.

Range of application: cellular phones and different portable computers. The complete data on the features are still not known. However, we should note small number of charge/discharge cycles and small load current. As for highlights, we can say that the production technology allows different geometric forms, non-traditional for usual accumulators, including rather thin in width and capable to fill free space in a device.

So, we have considered basic features of the most widespread types of accumulators in the chronological order. And according to this order they replace one another in mobile devices and portable computers. However, do not reject the aged beforehand. In many devices they continue working flawlessly. Their production technology, design and service elements are still improving. For example, in video cameras.

To be continued...


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