Lead Acid Battery Charger using the PIC14C000

The PIC14C000 comes with several peripherals specifically aimed at the battery market. The programmable reference and onboard comparators are useful for creating charge control circuits, while the analog-to-digital (A/D) converter can monitor the charge state to prevent overcharge. The control software is written in ?C? for maintainability and transportability. Where necessary, in line assembly is used. This application note is intended to demonstrate the use of the PIC14C000 in an intelligent battery charger. The charger is designed to charge a sealed lead-acid battery (YUASA NP7-12 12V, 7AH); however, the charge parameters are easily modified to work with different lead-acid batteries. The typical method of charging lead-acid batteries is with a constant voltage, current-limited source. That method allows a high initial charge current that tapers off until the battery reaches full charge. This design uses a constant current, allowing the voltage to rise until the battery voltage reaches a full charge. The charge current is then turned off to prevent overcharging. This allows a high initial charge to quickly bring the battery to a full charge and a low maintenance charge current as needed to maintain the full charge. The constant current design is also easily adaptable to NiCd batteries. As voltage rises during the charge cycle of the lead-acid battery, it quickly passes 2.1 V/cell. As charging progresses, oxygen begins to be liberated at the posi-tive plates at 2.2 V/cell. At 2.3 V/cell, hydrogen is liberated at the negative plates. This is considered a full charge, as any further current passed into the cell simply releases gasses rather than charging the battery. Hence, the upper voltage limit is set at 13.8V (2.3 V/cell), and the lower voltage is set at 12.6V (2.1 V/cell). As a practical consideration, the lower voltage limit is set slightly lower (12.5V) to lengthen the charge cycles. The battery voltage takes just minutes to decay from over 13.8V to 12.6V. It then takes several hours to decay from 12.6V to 12.5V.