In the past the practice of not putting the charging circuit directly into the end product has been largely driven by the desire to avoid very large components in the end product. In the days of an external AC transformer adapter you would have needed a rectifier, filter caps plus the voltage regulator. DC adapters will also require a voltage regulator but for both the AC and DC transformer designs the head voltage variation is so large that the regulator design must withstand a lot of voltage drop and corresponding temperature rise. Neither of these was very welcome inside a target product.

Several factors today now make the inclusion of the charger circuit directly inside an end product very feasible. One of these is the availability of very low cost wall wart technology that uses switcher technology instead of a transformer. These devices are efficient, run cool, work nicely on world-wide voltage mains, and can deliver a regulated DC voltage.

A second factor is that the switcher power adapter unit can be designed to provide the necessary voltage for the in-built charger such that the voltage drop across the charger circuit is minimized. This significantly reduces power and heat in the end product. Look around...you can see examples of adapters with strange voltages such as I have at hand here, 5.9 volts, 7.2 volts and 19.1 volts.

A third consideration to take into account is the battery technology in use today really should be re-charged with a proper type circuit. NiMH batteries for example, which are commonly available in standard battery cell package sizes, are best if used in an application that actually meters the charge into and out of the cells. This can greatly increase cell life and SAFETY. These cell types also benefit greatly if the charger can include a temperature sensor in nect to the cells. (I need to make note here that there a large percentage of the low cost chargers in the market for NiMH cells are total CRAP, intrinsically unsafe and will severly overheat your batteries).

A final factor to consider for an in-built charger design is that the IC manufactures now offer many many components that make a charger design feasible in terms of performance, cost, and PC board space. This starts with even the simplest components...it is now possible to find medium power Schottkey diodes that have very low forward voltage drop (see the new Zetex ZHCS350 for example). The advent of USB distributed power peripherals has also expanded the availability of small power management components including switches and LDO regulators, which can see direct application in an efficient charging circuit design. Some vendors are also offering rather nice integrated charger control chips too. Check out the offerings from TI/Benchmarq.

Michael Karas