powered from rechargeable batteries and use direct current (DC). One of the selling points is that a DC stair lift will continue to function during a power outage, provided the batteries are sufficiently charged. Most stair lifts have a 'charge point' where the unit will 'park' to charge its batteries. Some straight stair lifts have the ability to charge continuously no matter where they are left along the track. With most DC models the batteries are accommodated within the carriage and travel with it. Some models, however, were designed with three phase motors and the batteries (three in total) were housed in a cabinet mounted near the top or bottom of the rail. An inverter system was used to convert the DC energy to three phase AC. The power rating of drive motors for domestic straight rail stair lifts may be around 250 watts. The power requirement will be greater for heavy loads, very steep inclines, and wheelchair platform stair lifts. Controls Stair lifts are largely operated using a control on the arm of the lift. This is either a switch or a toggle type lever. This larger toggle switch enables users even with limited mobility or painful condition to use stair lifts easily and safely. Electronic controls are used extensively. Many stair lifts have radio frequency or infrared remote controllers. It is known that radiation from devices such as fluorescent lights can interfere with infrared stair lift controls. Also, heat and incandescent lights can, in some circumstances, have an adverse effect. Control circuit design varies greatly among the different manufacturers and models. Curved rail stair lifts have more complex controls than those with straight rails. The seat of a curved rail stair lift may have to be tilted so it remains horizontal whilst going around curves and negotiates different angles of incline. This requires an additional motor and link system. Also, the carriage is slowed down on bends but travels faster on straight runs. This means a more complex control system. Modern controls have small microprocessors which "learn" the characteristics of the journeys and keep the data in memory. They also record the number of journey and direction. This assists service engineers on maintenance calls. Some development of self-diagnostic controls began at the onset of the 21st Century. The idea was that stair lifts would predict when components were starting to deteriorate and automatically pass the information to the