Lithium-Ion (Li-Ion) batteries have become popular for portable electronics and microcontroller projects because they boast the highest energy density of any commercial battery technology. Other benefits include thousands of recharges and no occurrence of the “memory effect” that provoked early rechargeable cells. Further, Lithium Ion cells tend to be either rectangular or cylindrial. They are hardshelled with a strong casing. They often weigh a little more and come in larger capacity but they are also more sturdy and are hard to puncture.
Depending on the design and chemistry of your lithium cell, you may see them sold under different nominal “voltages”. For example, almost all lithium-ion batteries are 3.7V or 4.2V batteries. What this means is that the “maximum” voltage of the cell is 4.2V and that the “nominal” (average) voltage is 3.7V. As the battery is used, the voltage will drop lower and lower until the minimum which is around 3.0V. You should see the number 3.7V written on the battery itself somewhere. So, it is very easy to energize today’s 3.3V microcontroller circuits using 3.7V lithium-ion battery packs just by adding Schottky diodes in series with the power supply rail as indicated in the example shown below.
Aboveboard, I am sure that a dc-dc buck converter is not needed here. You may use a diode in series with the power rail as described. The voltage drop may be adjusted by selecting regular diode (1N4007) or a schotky diode (1N5817). Get the diodes datasheet to see the current versus voltage drop. This will also avoid damage if the battery polarity is accidently reversed by user misoperation.
The above mentioned idea seems good for any 3.3V microcontrollers, but if you want to operate some 5V devices within the 3.3V circuit (or you are using a 5V microcontroller), powering the system from the 3.7V lithium-ion battery pack demands a suitable dc-dc voltage booster circuit to convert the existing 3.3V to 5V dc level. Described here is an economical yet efficient solution for this problem. The circuit in picture is nothing but an ultra-simple boost converter realized using the popular 8-pin chip MC34063A.
The MC34063 is a monolithic control circuit containing the primary functions required for DC to DC converters, consists of an internal temperature compensated reference, comparator, controlled duty cycle oscillator with an active current limit circuit, driver and high current output switch. This IC was specifically designed to be incorporated in Step-Down (buck) and Step-Up (boost) voltage converting applications with a minimum number of external components. MC34063 is quite flexible, but picking the right component values can be tricky, especially if the output current can vary across a wide range. However, we want a cheap switching regulator for a circuit draws roughly a fixed amount of current well below 100mA, the MC34063 is a great choice to consider here. Author’s prototype was tested with a 100µH/1A inductor (L1), and 1% tolerance resistors as R3-R4.
Due to small number of components used, it’s unlikely that the construction of the circuit will give you a boredom evening. The use of a small veroboard makes things easier. Carefully check the finished circuit board before applying power, and try to mount the assembled circuit in a small plastic box, including the lithium-ion battery pack (better, rests on a double-sided glue-tape stuck on the bottom). Once power supply has been switched on, the uC Power Bank is ready for use. And that’s it!
Li-Ion Battery Charging
Lithium-Ion operates safely within the designated operating voltages, however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Lithium-Ion batteries are extremely power dense, makes them great for reducing size and weight of projects. However, they are not “safe” batteries and require extreme care. Charging or using the batteries incorrectly cause cause explosion or fire. So, always use a proper Li-Ion battery charger for recharging your Li-Ion battery. Fortunately, dedicated chips for this task are widely available now, and we already published a diy circuit based on the renowned smd chip MCP73831.