The schematics for Basic Charger is completely based on the PIC 16F876 device. All the other components are for basic I/O functions that the charger performs during normal operations. The PIC microprocessor has a crystal of 4MHz in order to have a true calculation power of 1 MIPS. The PIC interfaces the PC serial port using a MAX232 that translate the TTL levels to true RS232 levels. The version of the MAX232 used allow the usage of 100N capacitors for charge pumps.

The power supply for PIC is a linear regulator LM7805, capable to source 1A at 5V.
The fan cooler is commanded directly from the PIC using a small power npn transistor, in order to switch on the fan only during the charge or discharge operations.
The led is wired directly to the PIC open collector output pin and the key switches uses an input port with internal pull-ups.

The "Start" key is directly connected to the reset pin of the PIC microcontroller for "break and restart" fast operation.

The wires for power supply are doubled in order to have less resistance and better performances during the charge process. This resistance is critical because using a power supply of 12V or a bit less (computer power supply) there is about 1V of difference between the maximum of the pack voltage (10.7-10.8V) and the power supply. This difference divided by the mosfet P-channel on resistance plus the cables gives the maximum charge current.
For the same reasons there is no diodes between the power supply and the charger, with the disadvantage that a battery pack connected to a not powered charger turn on the charger itself because the current flows on the body diode of the mosfet P-channel, giving a not well specified functioning. Users that have a power supply of 12.5V or higher can introduce a 10A schottky diode with the anode connected to the power supply and the catode to the charger in order to avoid this problem. A fast work around is, in any case, to connect the battery pack to a powered charger.

The charge/discharge block is built with a P-channel mosfet for the charge and the double N-channel for the discharge. The selected components mounted on a Pentium-style cooler (with fan) reaches 30A for discharge and 5-6A for charge without problems. The mosfets are directly driven by a low pass filtered PWM signal coming from the PIC. This signal is referenced to GND for N-channel driving and to power supply for P-channels driving. The voltage sense wires are differents from the power wires in order to avoid the problem of the variable offset depending from the charge/discharge current.
The feedback for current and voltage are directly taken from a current to voltage (hall sensor based) converter and from a simple resistive partitor. This partitor has a relatively high output resistance and the value must be taken into account in the settling time of battery signal conversion inside the PIC processor. The current to voltage converter has a zero-current output of about 2.5V and swings 0.6V around this point for currents flowing inside the sensor in the two opposite directions.