Forum Proceedings

We seek technologies that will help people using powered wheelchairs and scooters to more accurately monitor the state of charge in lead-acid batteries.

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 primarily by people with the ability to walk short distances but who require assistance when shopping or interacting within the community - wishing 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. Power wheelchair and scooter users, need accurate knowledge of a battery's state of charge. Fear of being stranded can limit a user's independence and access to environments. Minimally, being stranded is an inconvenience to the user. At worst, being stranded is a serious threat to the user's safety. A gauge that routinely and accurately tracks power discharge and remaining power and then translates this into a measure of remaining capacity would substantially improve the user's independence and would be welcomed by manufacturers, service providers, and clinicians. The power monitor should also track a battery's rate of degradation and alert the user when battery replacement is necessary.

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. Most power indicators for wheelchair batteries are simple voltmeters. They are widely used because they are inexpensive, robust and compact. However, voltmeter based monitors are at best inaccurate and at worst misleading indicators of remaining battery capacity. A voltage monitor produces instantaneous variations in state-of-charge readings, in response to loads placed on the battery during typical daily use. No means exist to translate remaining power into estimates of remaining capacity or range. This inadequacy forces the user to estimate the time/distance remaining through trial and error.

Battery monitors used in other industrial applications have some of the capabilities being sought for lead-acid batteries. For example, the computer industry uses an SMBus for monitoring a battery's status. In this case, both the power-monitoring and charging systems are paired with the battery technology. The power monitor tracks the flow of current into and out of the battery. The battery's full charge level is also tracked, so the power monitor can assess battery degradation over time. This monitoring/charging system has the additional feature of being programmable, so that key parameters can be entered and the charging system accurately matched to battery performance specifications.

Technology Needs and Barriers

Information gathered from users, manufacturers, clinicians, and other stakeholders has identified characteristics of an ideal motor. 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 monitor...

1. Must be compatible with wet (open or closed) or gel electrolyte, lead-acid battery technology.
2. Must provide an accurate and instantaneous measure of residual power - analogous to a fuel gauge.
3. Should translate the battery power into a percent of capacity, time (hours/minutes) or distance (miles/feet) remaining, depending upon the user's preference.
4. Must monitor power use by the wheelchair and its accessory devices (e.g., augmentative communication systems, cellular telephones, laptop computers).
5. Must compile the charge/discharge/recharge history to develop an energy consumption profile for wheelchair user.
6. Must meet or exceed all relevant ANSI/RESNA standards for operating parameters. See http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfStandards/search.cfm.
7. Must protect the user from shock hazards and the manufacturer from liability.
8. Must not interfere with wheelchair controller functions.
9. Must have easily seen, accessible, yet unobtrusive display and control features. The display should be numeric or have other user-friendly characters, rather than using lights or stacks of LED bars.
10. Should analyze the history of power consumption to track the battery's condition (e.g., degrading charge capacity, accumulated watt-hours).
11. Could integrate monitor functions into an intelligent battery charger or wheelchair controller.

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