April 30, 2010
April 30, 2010
THIS PROJECT CAN CAUSE FIER!
The Li-Ion/Li-PO is the most generic rechargable battery for mobile gadgets. However it is very sensitive to the conditions on charge and discharge and inproper use can result fire and explosion. Therefore it is embedded into the equipments or battery packs with safety circuit and not being sold for consumers as bare-cell like Ni-MH.
Recently, I have used the Li-Ion/Li-PO for my projects by reason of its high voltage and high energy-weight ratio. The high voltage enables to eliminate SMPS at the 3V system with single-cell, so I needed to build a generic Li-Ion/Li-PO charger.
The charge operation of Li-Ion/Li-PO is done in CVCC method that also applied for SLA battery. The temperature compensation done on SLA is not needed for Li-Ion/Li-PO but thermal shutdown is needed. The constant voltage must be controlled to 4.2V within ±50mV. The Li-Ion/Li-PO charger can be easily built with a charger IC.
This is the schematic for the built Li-Ion/Li-PO charger. It uses a charge control IC BQ2057 from TI. The charge current can be selected with a rotary switch to charge various Li-Ion/Li-PO cells. The thermal control function of the BQ2057 is not used. The control transister Q5 dissipates approx. 1.5W maximum. The built circuit board is mounted on a heat sink and the Q5 is pressed against the heat sink by the circuit board that shown in the above photo.
This charger is for only single-cell Li-Ion/Li-PO batteries. The monitor LED on the circuit borad indicates the charge status. The green LED indicates that no battery is attached or end of charge. When a discharged battery is attached, a red LED is lit and charge operation is started. The current setting can be selected with a rotary switch. The number 1 to 15 corresponds to 100mAh to 1500mAh at 0.5C.
