Using the MCP1631 Family to Develop Low-Cost Battery Chargers

AN1137 Using the MCP1631 Family to Develop Low-Cost Battery Chargers COMMON CHARGE PROFILES Author: Terry Cleveland Microchip Technology Inc. NiMH Charge Profile INTRODUCTION Figure 1 shows a typical charge profile for NiMH batteries. The charge cycle begins once a battery is As portable rechargable applications continue to grow, detected by regulating a small current or conditioning there is an increase in demand for unique or custom current into the battery pack. If the cell voltage is above battery charger designs. In addition to the increase in 0.9V per cell, it is safe to charge the pack with a fast portable rechargable applications, battery chemistry charge or high current (for NiMH or NiCd, this current continues to improve and with that new charge can range from 50% to over 100% of the batteries methods and profiles are emerging. This all leads to the capacity). When the battery reaches capacity, cell increase in demand for new or custom charge profile manufactures recommend a top-off charge to complete designs. In this application note, a mixed signal multi- the charge cycle. It is typically not recommended to chemistry battery charger design technique will be trickle charge NiMH batteries, this can lead to discussed that can accommodate the changing overheating and reduced battery life. Fast charge portable power management world. termination for NiMH batteries can be tricky. As the battery reaches capacity, it no longer can accept aThe reliability and safety concerns with charging batter- charge. The energy from the charger that was stored inies can also benefit from programmable mixed signal the battery, now turns into heat causing the batterydesigns. Charge rates and constant voltage levels can temperature to rise. There are two primary methods tobe updated in the field with a change in firmware. This determine when the battery has reached full charge,allows the user to adapt to new smart battery packs and one is a sudden increase in temperature, the otherselect desired runtime versus cycle life. By charging being a subtle drop in battery voltage or -dV/dt. Withthe battery to a lower constant voltage, the run time is NiMH batteries, the -dV/dt can be difficult to detect,shortened but the number of charge cycles will since the change can be very small, especially withincrease. lower charge rate designs. The +dT/dt or temperature Another programmable battery charger feature is its rise is typically easier to detect. For a robust design, ability to charge multi-chemistry battery packs. By both methods should be used so either can terminate detecting the number of cells and cell chemistry, a pro- the fast charge portion of the charge cycle. Once the grammable charger can adapt to a new battery pack. fast charge is terminated, a timed top off charge is This enables customers to choose between portability, recommended, a continuous constant charge is not runtime and cost when purchasing a portable system. recommended for NiMH batteries.