02. Battery Chemistries 4

GUIDE: Batteries in a portable world. 2. Battery Chemistries 4

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2.3 The Nickel-Metal Hydride (NiMH) Battery

              </p><p>Research of the NiMH system started in the 1970s as a means 
                of discovering how to store hydrogen for the nickel hydrogen 
                battery. Today, nickel hydrogen batteries&nbsp;are mainly used 
                for satellite applications. They are bulky, contain high-pressure 
                steel canisters and cost thousands of dollars&nbsp;each.</p> <p>In the early experimental days of the NiMH battery, the metal 
                hydride alloys were unstable in the cell environment and the 
                desired performance characteristics could not be achieved. 
                As a result, the development of the NiMH slowed down. New 
                hydride alloys were developed in the 1980s that were stable 
                enough for use in a cell. Since the late 1980s, NiMH has steadily 
                improved, mainly in terms of energy density.</p> <p>The success of the NiMH has been driven by its high energy 
                density and the use of environmentally friendly metals. The 
                modern NiMH offers up to 40&nbsp;percent higher energy density 
                compared to NiCd. There is potential for yet higher capacities, 
                but not without some negative side effects.</p> <p>Both NiMH&nbsp;and NiCd&nbsp;are affected by high self-discharge. The 
                NiCd loses about 10&nbsp;percent of its capacity&nbsp;within the 
                first 24&nbsp;hours, after which the self-discharge&nbsp;settles 
                to about 10&nbsp;percent per month. The self-discharge of 
                the NiMH is about one-and-a-half to two times greater compared 
                to NiCd. Selection of hydride materials that improve hydrogen 
                bonding and reduce corrosion of the alloy constituents reduces 
                the rate of self-discharge, but at the cost of lower energy 
                density.</p> <p>The NiMH has been replacing the NiCd in markets such as wireless 
                communications and mobile computing. In many parts of the 
                world, the buyer is encouraged to use NiMH rather than NiCd 
                batteries. This is due to environmental concerns about careless 
                disposal of the spent battery.</p> <p>The question is often asked, “Has NiMH improved over the 
                last few years?” Experts agree that considerable improvements 
                have been achieved, but the limitations remain. Most of the 
                shortcomings are native to the nickel-based technology and 
                are shared with the NiCd battery. It is widely accepted that 
                NiMH is an interim step to lithium battery technology.</p> <p>Initially more expensive than the NiCd, the price of the 
                NiMH has dropped and is now almost at par value. This was 
                made possible with high volume production. With a lower demand 
                for NiCd, there will be a tendency for the price to increase.<br /> <br /> </p> <table border="0" cellspacing="0" cellpadding="3"> <tr> 
                  <td width="482" colspan="2" valign="top"> <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#000000"> <tr> 
                        <td></td> </tr> </table> </td> </tr> <tr> 
                  <td width="482" colspan="2" valign="top" bgcolor="#E6E6E6"> 
                    <p align="center"><b>Advantages 
                      and Limitations of NiMH Batteries</b></p> </td> </tr> <tr> 
                  <td width="482" colspan="2" valign="top"> <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#000000"> <tr> 
                        <td></td> </tr> </table> </td> </tr> <tr> 
                  <td width="65" valign="top"> 
                    <p><b>Advantages</b></p> </td> <td width="417" valign="top"> 
                    <p>30 – 40&nbsp;percent higher capacity over a standard 
                      NiCd. The NiMH has potential for yet higher energy densities.</p> <p>Less prone to memory than the NiCd. Periodic exercise 
                      cycles are required less often.</p> <p>Simple storage and transportation — transportation 
                      conditions are not subject to regulatory control.</p> <p>Environmentally friendly — contains only mild toxins; 
                      profitable for recycling.</p> </td> </tr> <tr> 
                  <td width="65" valign="top" bgcolor="#F3F3F3"> 
                    <p><b>Limitations</b></p> </td> <td width="417" valign="top" bgcolor="#F3F3F3"> 
                    <p>Limited service life — if repeatedly deep cycled, especially 
                      at high load currents, the performance starts to deteriorate 
                      after 200 to 300 cycles. Shallow rather than deep discharge 
                      cycles are preferred.</p> <p>Limited discharge current — although a NiMH battery 
                      is capable of delivering high discharge currents, repeated 
                      discharges with high load currents reduces the battery’s 
                      cycle life. Best results are achieved with load currents 
                      of 0.2C to 0.5C (one-fifth to one-half of the rated 
                      capacity).</p> <p>More complex charge algorithm needed — the NiMH generates 
                      more heat during charge and requires a longer charge 
                      time than the NiCd. The trickle charge is critical and 
                      must be controlled carefully.</p> <p>High self-discharge — the NiMH has about 50&nbsp;percent 
                      higher self-discharge compared to the NiCd. New chemical 
                      additives improve the self-discharge but at the expense 
                      of lower energy density.</p> <p>Performance degrades if stored at elevated temperatures 
                      — the NiMH should be stored in a cool place and at a 
                      state-of-charge of about 40&nbsp;percent.</p> <p>High maintenance — battery requires regular full discharge 
                      to prevent crystalline formation.</p> <p>About 20&nbsp;percent more expensive than NiCd — NiMH 
                      batteries designed for high current draw are more expensive 
                      than the regular version.</p> </td> </tr> <tr valign="middle"> 
                  <td colspan="2" height="18"> 
                    <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#000000"> <tr> 
                        <td></td> </tr> </table> </td> </tr> </table> <p><a name="_Toc507815236"><span class="caption">Figure&nbsp;2-</span></a><span class="caption">3:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
                Advantages and limitations of NiMH batteries</span></p></blockquote><p align="center"><font size="2"><a href="/guides/electronics/bpw/c02_03">&lt;&lt; Previous page</a>&nbsp;&nbsp;<a href="/guides/electronics/bpw/00_toc">INDEX</a>&nbsp;&nbsp;<a href="/guides/electronics/bpw/c02_05">Next page &gt;&gt;</a></font></p>

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