The Smart Power Selector technology splits the charge and load currents according to the demand.įor the charger circuit in this application note (Figure 4), we use the DC input, which gives us greater flexibility in setting the various limits. The IC can also be powered from an adapter via pins 2 and 3 (DC), where the current limit can be increased up to a maximum of 2A.įigure 3. In this case, the current drawn is limited to 500mA (the maximum for a USB 2.0 port). The MAX8677A can be powered from a USB port via pins 15 and 16 (USB). When there is no external power source available, the load is powered solely by the battery. If the load requires more power than the charger can deliver, the MAX8677A can make the battery provide the extra current. With an external power source, the MAX8677A can use the available power to both charge the battery and supply the load. The MAX8677A is very flexible and features a Smart Power Selector™ topology (see Figure 3), which consists of three electronic switches that direct the charge and load currents according to the situation. Block diagram of the internal circuit of the MAX8677A. This IC works completely autonomously so there is no need for a microcontroller (and hence no software)! A number of LEDs are used by the MAX8677A to indicate the state of the charging process.įigure 2. The charger circuit described in this article is built around a lithium-ion charger IC made by Maxim Integrated, the MAX8677A (see Figure 2). In that case, we will just have to build our own lithium-ion charger! The Circuit It is difficult to figure out which components are part of the charger circuit since no circuit diagram is normally made available for portable equipment. The original equipment usually contains a special charger circuit for the battery, most likely on a small part of the PCB. However, this still leaves the need to charge the battery. Board with three penlight cells replaced by a single lithium-ion battery for use in a Lego train. The author, for example, replaced three penlight cells in a Lego ® train with a lithium-ion battery (see Figure 1).įigure 1. Since electronics hobbyists tend to be an inventive lot, they will usually find a way to incorporate and recycle a battery. What can you do with that old MP3 player or cell phone that's been replaced by a newer, better version? Usually the electronics cannot be used for any other purpose, but the battery can still be used, even for toys. This type of battery has been used in most portable equipment produced in the last few years because it can be easily manufactured in various sizes and shapes, and it has a relatively large capacity (compared to NiMH and NiCd batteries). Like most people, you probably have old devices lying around that use lithium-ion batteries for the supply. Fortunately, it turns out to be fairly easy to build a charger for used (or new) lithium-ion cells. In other words, there is no separate charger that can be reused. It can be quite tricky to reuse lithium-ion batteries from discarded equipment since these cells are often charged inside the equipment. The designer needs to create their own charger circuit, which this application note explains how to do in detail.Ī similar version of this application note was published in Elektor magazine in June, 2013. One challenge is that the battery charger in these older devices cannot usually be reused. This can all be done without the need for a microcontroller (or the required software). This application note describes how to recycle lithium-ion (Li+) batteries from older devices for use in other electronic devices, such as toys.
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