Lithium-Polymer Peak Charger

This circuit was developed to charge the Lithium-Polymer cells used in a model aircraft. Lithium-Polymer cells are incredibly lightweight compared to Ni-cad battery packs of the same voltage and amp-hour rating. Their only drawback is that they require a rigid charge and discharge regime to achieve maximum life. The most important points of note are as follows:

Circuit diagram:

1. They should be charged using a constant-current, constant-voltage method, which stops the charge once the current has dropped to about the C/10 rate. For example, for an 800mAh pack, charging should be terminated once the current falls to approximately 80mA.
2. They should never be discharged below 3V per cell otherwise they will be permanently damaged.
3. should not be charged or discharged above their rated current otherwise an explosion and fire can result!

To initiate charging, the momentary "Start" button (switch S1) is pressed, closing the relay contacts and connecting the battery pack to the output of REG1. The circuit will then charge two 800mAh cells in series at a constant current of 600mAh until they reach a peak terminal voltage of 4.2V per cell (nominal terminal voltage for these cells is 3.7V).

REG1 and transistor Q2 form a current-limited voltage regulator. When the charge current exceeds about 600mA, the voltage developed across R7 turns on Q2, which in turn pulls the adjust terminal of REG1 towards ground. This shunts the voltage adjustment resistance chain formed by VR2 and R3, thereby limiting the output to 600mA.

When the battery voltage reaches about 8.4V, the regulator limits any further voltage increase, as set by VR2. The charge current will then slowly decrease as the cells reach full capacity. As a result, the voltage across R7 also falls, until the bias voltage on the base of Q1 is too small to keep it in conduction. When Q1 turns off, the relay also turns off, isolating the fully charged battery.

The charger is set up as follows:

1. It to 12V DC and place a digital voltmeter between the output of REG1 and the negative output for the battery pack. Adjust VR2 for a reading of 8.4V.
2. VR1 so that the voltage on the base of Q1 is at maximum.
3. Place an ammeter in series with the battery to be charged and press the "Start" button. The output current will shoot up to around 600mA, then slowly decrease over the next one to two hours.

Once it falls to around 80mA (or whatever the C/10 rate is for your cells), slowly turn VR1 until the relay switches off and indicator LED goes out.

The circuit should now charge your battery packs to within 97% of their rated capacity.

Finally, note that in most cases, REG1 will need to be fitted with a heatsink.

Source: Silicon Chip January 2005