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Design News, May 2013

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Electronics & Test of six cells in series in two parallel sets, column 1 (6S-2P); Li-ion based on the widely used cobalt-oxide chemistry in 4S-4P (column 2) and 3S-6P (column 3) configurations; the series connection sets the output voltage and the paralleling of the pair determines capacity. Despite the roughly similar run times, the Li-ion battery packs have about one-seventh the weight and one-fifth of the volume, a truly significant improvement. Making the Change If migrating from SLA to Li-ion was just a matter of getting close enough to the original nominal cell voltage, and matching the battery pack's physical size, upgrades would be fairly straightforward in many cases. However, a complete Li-ion battery pack is much more than just the cells alone: it is a highly engineered electrical, mechanical, and thermal design, which must include performance and safety features (Figure 2). These include the cells, of course, plus a small printed circuit board (PCB) for the electronics, which provides pack management via various algorithms, an enclosure, contacts, and insulation.The management circuitry may also include a sophisticated encryption algorithm (such as the SHA1), which works with the OEM product to verify that the pack is genuine and not a counterfeit with substandard performance or even dangerous design shortcomings and manufacturing defects. Even the virtue of flat output during A complete Li-ion battery pack is much more than just the cells alone. discharge of Li-ion adds to the upgrade challenge. SLA cells have a fairly linear downward slope linking discharge time and cell output voltage, and this voltage is relatively easy to measure. In contrast, the Li-ion cell output stays close to nominal during most of the discharge cycle, Figure 3, and has to be measured with precision to accurately determine remaining charge and cell status.As a result, most new designs implement the so-called "fuel gauge" function (sometimes referred to as the "gas gauge" though there is no gas or gasoline involved). Using coulomb counting, they measure the number of coulombs of charge being transferred out of the battery (recall that an ampere is the flow of one coulomb of electrons per second). Design News | MAY 2013 | w w w. d e s i g n n e w s . c o m –40– magenta cyan yellow black ES244621_DN1305_040.pgs 05.02.2013 07:03 UBM

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