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Low cost charger

Project features

Project description and main features listing

Aim of this project is to build a battery charger and discharger with the minimum cost, but without compromises on the quality of the charge and discharge process.

Charger characteristics:
- Current setting and control fixed, in two independent profiles for NiMh and NiCd
- Delta peak voltage for end of charge detection (NiMh and NiCd)
- Two technology (NiMh and NiCd) peak threshold setting
- Timeout period (1h) embedded in the PIC firmware
- Number of cells selectable modifying a single resistor

Discharger characteristics:
- Current setting and control fixed in two independent profiles for NiMh and NiCd
- Cut-off cell voltage embedded in the PIC firmware (NiMh and NiCd)

The general instrument features are the following:
- Very compact construction for direct power dissipator mounting
- Single key for settings, double LED for operation status
- Automatic charge, discharge and discharge then charge operations
- Very low cost

The cost reduction is achieved sensing the charge or discharge current through a cheap ceramic 6W resistor. This resistor is not very critical because the current sensing can be trimmed in a second time. All the circuits are reduced to their minimums in order to have a very high parts reduction and consequent cost breaking. The only active parts, except the two mosfets is the PIC micro that manages all the activities and controls the charge and discharge process. The total cost is under 10€ and for this reason can be replicated for charge series of battery packs or for single cell charge/discharge. The current settings are calculated in order to complete a cycle (discharge the pack and recharge completely) within an hour for the common used battery packs (2400mAh for NiCd and 3300mAh for NiMh). In the file download section of this project you can find a collection of firmware programming files for most commonly used battery parameters.

Schematic description

Low cost charger schematic diagram description

The low cost charger schematic is very simple because the entire circuit must be fitted in a 5x5 cm printed circuit board. The charger/discharger is based on a variable current source/sink directly driven by a small PIC microprocessor (PIC12F675).

Charge Block

The current source is based on a very basic and simple circuit. The 5V PIC output is firstly reduced by a trimmer (that in fact sets the maximum charge current) and filtered low pass in order to allow the current reduction with a PWM modulation. This signal (that should be at the pin 12 of the operational at 0.5V maximum) enters in a block (U5D and Q5) that translate the 0.5V to the Power supply minus 0.5V. This referencing is useful for the current source that equalize the voltage drop caused by the charge current in 0.1Ohm resistor to this input signal. In practice this force the maximum charge current to 5A with a setting of 0.5V in input. The power mosfet is a very general purpose P channel capable of sourcing much more than the requested 5A. The selected operational amplifier is selected with two main features: the rail to rail input and output stage (necessary for current source driving) and the power supply (at least 12V if the charger must operate with 6 NiCd or NiMh cells).

Discharge Block

The discharge block is very simular to the charge block, and in practice is a variable current sink. The trimmer R18 sets the maximum discharge current, that has a conversion factor equal to the charger: 0.5V in input are converted in 5A current.

PIC and miscellaneous

The PIC processor is configured for a internal 4MHz operations and the key on a master reset act as "START" key. The other two keys are for operational mode change. The charge and discharge output also drives two leds for charge/discharge process identification. The power supply is a very basilar 5V, 100mA linear regulator for PIC purposes.