This circuit was developed to charge Lithium-Polymer cells utilized in a model aircraft. Lithium-Polymer cells offer significantly lighter weight compared to Nickel-Cadmium battery packs of similar voltage and amp-hour ratings. However, they necessitate a precise charge and discharge pattern to ensure optimal longevity. The key considerations regarding these cells are as follows:
To initiate the charging sequence, the momentary “Start” button (switch S1) is pressed, activating the relay contacts and connecting the battery pack to the output of REG1. Subsequently, the circuit will charge two 800mAh cells in series at a constant current of 600mA until they reach a peak terminal voltage of 4.2V per cell (the typical terminal voltage for these cells is 3.7V).
REG1 and transistor Q2 form a current-limited voltage regulator. When the charge current exceeds approximately 600mA, the voltage across R7 activates Q2, which, in turn, pulls the adjustment terminal of REG1 towards ground. This effectively short-circuits the voltage adjustment resistance chain formed by VR2 and R3, thus restricting the output to 600mA.
When the battery voltage reaches approximately 8.4V, the regulator limits any further voltage increase, as determined by VR2. The charge current will then slowly decrease as the cells approach full capacity. Consequently, the voltage across R7 also diminishes, until the bias voltage on the base of Q1 becomes insufficient to maintain its conductive state. Upon Q1 turning off, the relay similarly disengages, isolating the fully charged battery.
Once the current falls to approximately 80mA (or the C/10 rate for your cells), slowly turn VR1 until the relay switches off, and the indicator LED ceases to illuminate.
The circuit is designed to charge your battery packs with a precision of approximately 97% of their rated capacity.
Furthermore, it is advisable to fit REG1 with a heatsink in most cases.
REG1 is a common DC-DC boost regulator, typically utilizing a MOSFET and a synchronous rectifier. Its primary function is to step up the input voltage to the required level for charging the Lithium-Polymer cells. These regulators are designed to efficiently convert DC voltage to a higher DC voltage while minimizing losses. They are often employed in low-power circuits, offering good efficiency and relatively compact size. They're commonly found in portable electronic devices and power supply applications where precise voltage regulation is crucial. The selection of REG1 ensures a stable and controlled output current for optimal Lithium-Polymer cell charging.