The instrument is intended to overcome some limitations of the basic charger and offer more flexibility to the users.
The advanced charger is based on a hardware similar to the basic charger one. The main differences are in the charge/discharge profiles management that now can be entered in "local" and without the PC support.
Differences from the basic charger
1. Possibility to have a costant voltage charge with current limiting (Pb and Li-Poly batteries)
2. Possibility to function as wattmeter
The schematics for Advanced 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 20MHz in order to have a true calculation power of 5 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 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 buzzer is wired directly to the PIC open collector output pin and the key switches uses an input port with internal pull-ups.
The "BREAK" key is directly connected to the reset pin of the PIC microcontroller for "break and restart" fast operation.
The charge/discharge block is built with a double P-channel mosfet (in parallel) for the charge and the double N-channel (in parallel) for the discharge. The selected components mounted on a Pentium-style cooler (with fan) reaches 30A for discharge and 10A 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.
Differences with respect to the BASIC CHARGER
The hardware modifications between the BASIC and ADVANCED charger are very minimal. The schematic baseline and the concepts are always the same, it changes only a few particulars.
PIC firmware description
The PIC microcontroller program is written in assembler using the Parallax-like macro (included in the package). The program structure is quite simple because it is based on a periodic interrupt with 5msec period. This basic tic allow the processor to sample the two analog inputs (voltage and current) that are read in a 16-bit format at each tic.
Charge section: The charge section of the interrupt program controls at 200Hz the charge current (via pwm signal) with a reference feedback of the signal coming from the current to voltage converter. The program passes to the routine only the target current and the zero current reading.
Discharge section: The discharge section of the interrupt program controls at 200Hz the discharge current (via pwm signal) with a reference feedback of the signal coming from the current to voltage converter. The program passes to the routine only the target current and the zero current reading.
Power supply: The power supply section of the interrupt program controls at 200Hz the source current (via pwm signal, in practice is a cherge current) with a reference feedback of the voltage reading on the load. The pwm signal is also decreased if the maximum current (5A) threshold has been reached. The program passes to the routine the target load voltage and the zero current reading .
The processor, with substantial difference with respect to the basic charge, works on a asynchronous cycle for charge/discharge processes: the actions performed in the single routine are below detailed:
The charge routine starts sampling the zero current reading and initializing all the charge-related parameters recalling the correct profile from eeprom. During the charge process the PIC display the charge current, the battery pack voltage, the mAh charged, the time and the delta peak. The charge end is checked at each iteration and when the delta peak is reached or the timeout has expired or the user remove the battery, the charge ends.
The charge routine starts sampling the zero current reading and initializing all the discharge-related parameters recalling the correct profile from eeprom. During the discharge process the PIC display the discharge current, the battery pack voltage, the mAh discharged, the time and the cut off. The discharge end is checked at each iteration and when the cut off is reached or the user remove the battery, the discharge ends.
Power supply routine
The power supply routine performs the checks on pressed keys for voltage adjust. The loop stabilization is checked in the interrupt routine.
The parameters settings allows the user to enter, change and re-store all the parameters for a battery pack usage.
The PC serial management is useful to get the data coming from the charger in real time (archiving, waveform displaying) or to change easily all the parameters.
Power supply fail
If the power supply fail or the user press the break key, the charger restarts with the hello message and waits 10 seconds or a key strike. If the 10 seconds expires and the user has pressed no key the charger will restart the interrupted action, if one.
The choice of battery profile is very simple and is performed via two keys.
The battery cycle is a simple discharge-then-charge cycle for battery safe and easy charging.
The Advanced charger building has been simplyfied because the user display and keys are mounted on a separated PCB linked to the main via a standard flat cable for very easy wiring. Please refer to the total charger building diagram for correct operations.
For wiring we recommend a silicon wire of at least 6sqmm. The preferred panel connectors are the 4 mm red/black plugs.
The polarized component orientation is the following:
BUGS in previous PCB version
The known bugs for the previous versions are the following:
All these problems are fixed in the files currently on the web site.
Firmware release 1.1
The improvements introduced in the new firmware release, directly imported from the HV version, are the following:
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