Forum Proceedings

We seek technologies that will help improve the battery recharging process for people using powered wheelchairs and scooters.

Description of the Problem

Power wheelchairs are used predominantly by people with both lower and upper extremity impairments resulting from cerebral palsy, high-level spinal cord injury, or muscular dystrophy. There are more than 93,000 power wheelchair users in the United States. The "standard power wheelchair" accounted for $166 million in Medicare expenditures in 1997. Scooters are used mostly by people with the ability to walk short distances but who require assistance when shopping or interacting within the community and wish to remain active despite growing physical limitations. Scooters are used commonly by elderly persons. It is estimated that 64,000 scooters are currently in use in the United States. In the near term, lead-acid battery and charger technology is expected to remain the standard for power wheelchairs and scooters.

Existing chargers compromise battery performance and integrity, in part, because they do not promote effective power management by the users. Improper battery charging reduces battery life and increases replacement costs. Two factors can significantly improve charging practices. First, chargers need to be more convenient. Most wheelchair battery chargers are too large to be portable or placed on-board, while smaller chargers that are available are not considered to be durable and reliable by some Forum participants. Wheelchair users' mobility depends upon their wheelchair. A charger that is readily available and reliable would have a significant impact on a user's independence and access to environments. The user may be seated in the wheelchair during the charging process. Any portable or on-board charger must protect the operator from hazards such as electric shock and explosion. Second, chargers should provide users with accurate, real-time information on battery charge status; both during the charging process and while the wheelchair is in use. Existing chargers do not provide accurate information on the level of charge, and some may not automatically charge the battery completely. Improper charging (either over or under charging) degrades expected battery life span.

Powered wheelchair manufacturers would enthusiastically welcome an improved battery charger. Existing chargers for various kinds of batteries contain some of the capabilities of the ideal charger as specified by the wheeled mobility stakeholders; however, no existing chargers adequately meet all of the user's needs.

Current State of the Technology

Deep discharge wet and gel electrolyte, lead-acid batteries are the standard power source for nearly 100,000 users of powered wheelchairs. They include Group 22NF (for standard chairs), Group 24 (for large/hi-performance chairs), and Group U1 (for children's chairs) batteries. A battery's life span strongly depends on its pattern of daily use and charging. Forum participants indicated that lead-acid battery life under ideal conditions is four or five years. Forum participants indicated that typical lead-acid battery life is perhaps two years.

Most battery chargers are based on linear charging technology, which converts line voltage (115 VAC to 24 VDC), and use relatively large and heavy transformers. A few chargers are based on switch-mode technology, which does not use a transformer and are relatively smaller and lighter. A typical charger uses a constant, tapered or pulsed current profile. Some chargers automatically stop charging when the battery is fully charged (typically sensing a predetermined voltage level) in order to prevent overcharging the battery.

An important problem for both wet and gel, lead-acid batteries is the buildup of lead sulfate deposits on the battery plates (plate sulfation). Plate sulfation reduces charging efficiency, decreases performance (capacity and peak power delivery), and shortens battery life. Operation in high temperatures, prolonged high current draw, and incomplete charging all increase the rate of plate sulfation. Due to internal power losses, plate sulfation also takes place during battery storage. Recently, a patented "pulse technology" (http://www.pulsetech.net) has been shown to clean away plate sulfation and dramatically improve the efficiency, performance, and life span of lead-acid batteries. Pulse (plate cleaning) technology has also been integrated into chargers.

Technology Needs and Barriers

Information gathered from users, manufacturers, clinicians, and other stakeholders has identified characteristics of an ideal charger. Minimum requirements are identified as items that "must" be addressed to make a design acceptable; other attributes that will benefit the design are identified as items that "should" be addressed. Specifically, an ideal charger...

• Must be small enough for on-board integration (approximately 6x4x2 inches) and sufficiently lightweight for pocket portability. On-board versions should be compatible with existing powered wheelchairs.
• Must charge both wet (sealed and open) and gel electrolyte, lead-acid batteries and be adjustable for different input voltages (i.e., 115, 220, 230). Charging protocols should maximize the expected life span of the battery.
• Must work under surge current conditions - i.e., meet current UL/CSA requirements for power-line operated equipment, including FCC Part 15 for radiated and conducted electromagnetic interference - and recharge rapidly for all load conditions.
• Must meet or exceed ANSI/RESNA standard (Part 14) of charging to 80% of capacity in eight hours.
• Must meet or exceed all ANSI/RESNA standards for operating parameters - e.g., storage and operating temperature range, vibration, dropsee http://www.fda.gov/cdrh/modact/recstand.html#phymed.
• Must protect the operator from all shock hazards while permitting the user to remain in their wheelchair during charging. Should include ground-fault-interrupter (GFI) for added safety.
• Should have a simple user interface - e.g., standardized plugs with formable bodies, barrel connectors, retractable cord, easy access from sitting position, and silent operation for overnight charging in bedroom.
• Must monitor battery status and alert the user when a new battery is needed or when the battery charge level decreases to 60% of full charge. Should notify the user to charge their battery on a daily basis, and should eliminate user-based charging errors.
• Must sense charge rate and charge level and automatically provide an optimal charge profile. Should collect/store daily charging history for use by support service providers and battery manufacturers.
• Should cost less than $50 to $70 to manufacture - existing chargers sell between $150 and $250 retail.

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