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Power Wheelchair Batteries

by Carmen P. DiGiovine, BS
The power of a battery can vary just like the durability of a chair. What's right for you depends on your individual needs.

Selecting the correct battery for a power wheelchair means you need to take a serious look at how you use your chair. Then take a look at the characteristics of the different batteries now on the market. An incorrect selection can leave you stranded either because the battery power was depleted or because the battery was unable to provide the power required to navigate over rough terrain.

The battery is one of the major limiting factors in power wheelchair performance. Manufacturers must design much of the wheelchair around the battery package because of the package’s immense weight and volume.

Batteries will last longer, weigh less and take up less space when battery performance is improved through increased battery life (the number of times the battery can be recharged) and battery capacity (the amount of energy the battery can hold on a single charge). Only then will manufacturers have more options in designing wheelchairs. In turn, this will provide you with a more reliable wheelchair, which has lower maintenance requirements and an extended range.

Manufacturers design power wheelchairs for use with deep-cycle lead-acid batteries. They typically use two deep-cycle lead-acid batteries in series, each producing 12 volts (V), for a total of 24 volts (V). The voltage a battery is able to produce describes the electrical force available to drive the motors. Lead-acid batteries are used because they are readily available, inexpensive, rechargeable and available in sizes appropriate for the power requirements and range requirements of power wheelchairs.

Wheelchair batteries are grouped by size, which is indicated by a standard number. The group size defines the dimensions of the battery as shown in Table A. A typical deep-cycle lead-acid battery is shown in Figure A. The ampere-hour rating defines the battery’s capacity. That is, if the power wheelchair runs on 10 amperes and has an ampere-hour rating of 90 Ah, then the power wheelchair will be able to run for nine hours at room temperature. In practice, however, you almost never use your wheelchair continuously nor will you need to completely discharge the battery. The capacity of typical deep-cycle lead-acid batteries ranges from 30 to 90 ampere-hours which translates to 3 to 10 hours of continuous use for a power wheelchair that runs on 10 amperes.

Table A. Standard Power Wheelchair Battery Group Sizes. All dimensions are in inches.

Group Number

Length

Width

Height

U1

7-3/4

5-3/16

7-5/16

22NF

9-7/16

5-1/2

8-15/16

24

10-1/4

6-13/16

8-7/8

27

12-1/16

6-13/16

8-7/8

12 V Deep Cycle Battery
Figure A: Typical 12 V, deep-cycle gel-cell lead-acid battery used in power wheelchairs.

Even though automobiles also use lead-acid batteries, and the size of power wheelchair batteries may seem the same as automobile batteries, automobile batteries cannot be used in power wheelchairs.

  • Automobile batteries are known as starter batteries. They are designed to provide large bursts of energy over a short period of time, therefore they are almost never discharged to their full capacity, nor are they designed to be discharged to their full capacity, or to 50 percent of their full capacity for that matter.
  • Deep-cycle lead-acid batteries used in powered wheelchairs are designed to be discharged quite deeply, where each discharge cycle utilizes the majority of the battery’s available capacity.

There are two types of lead-acid batteries, wet-cell batteries and gel-cell batteries. Both may be used in power wheelchairs, however, manufacturers typically recommend gel-cell batteries over wet-cell batteries due to environmental and maintenance concerns with wet-cell batteries.

In general, deep-cycle wet-cell lead-acid batteries have a larger capacity (ability to travel a longer distance on a single charge), are able to provide more power, and typically cost less. In one study, a set of power wheelchairs with wet-cell batteries traveled approximately 1.25 times farther than the same set of power wheelchairs with gel-cell batteries. There are two types of wet-cell lead-acid batteries: vented and sealed (a.k.a. maintenance free). Because both have chemicals, neither are allowed on airlines.

  • Vented wet-cell batteries have maintenance requirements since one of the components that makes up the battery is water, which tends to evaporate over time. Therefore, water must be added to the battery on a regular basis. Another weakness of vented wet-cell batteries is that the consumer is exposed to the acidic materials whenever the casing is opened to add water. Acidic materials can cause chemical burns on the skin and corrode the wheelchair. Also, chemical spills can occur if vented wet-cell lead-acid batteries are overfilled with water, if boiling occurs due to incorrect charging (typically overcharging) of the battery, or if the battery case is damaged.
  • Sealed wet-cell lead-acid batteries are considered a closed system, so maintenance of the water level is not required. However they can still have chemical spill due to boiling or damaged battery cases.

Because of these drawbacks, gel-cell lead-acid batteries were developed. There is no potential for chemical spills and they are maintenance-free. These are the only lead-acid batteries allowed on airlines.

Because of the strengths and weaknesses of both wet-cell and gel-cell lead-acid batteries, it might be appropriate to obtain a set of wet-cell batteries for daily use and a set of gel-cell batteries for travel. Furthermore, if your range and power requirements are not extreme, a gel-cell lead-acid battery might be appropriate for everyday use as well. A comparison of the strengths of the deep-cycle wet-cell lead-acid battery versus the deep-cycle gel-cell lead-acid battery is listed in Table B.

Table B. Strengths of a deep-cycle wet-cell lead-acid battery versus a deep-cycle gel-cell lead-acid battery

Wet-Cell Lead-Acid

Gel-Cell Lead-Acid

Less expensive

Approved for transport on airlines

Larger capacity per charge/discharge cycle

Reduced potential for chemical spills

More power for traversing obstacles

Low maintenance

Chargers

There are two types of chargers: manual and automatic. Manual chargers require constant supervision. If not shut off after the battery is charged, damage may occur to the battery. Therefore, an automatic charger is recommended since the charger automatically monitors the charge. An automatic charger will decrease the rate of charge when it senses the battery becoming close to a full charge; it effectively shuts off when a full charge is reached. However, some charging does still continue so a charger should not be left attached to a battery for over 24 hours.

If a battery is overcharged, permanent damage to the battery may occur. 24V automatic chargers allow both batteries to be charged simultaneously; consequently they cannot be used with a single 12 V battery. Chargers are designed for either wet-cell lead-acid batteries or gel-cell lead-acid batteries. Therefore, the appropriate charger must be purchased to match the battery. If an incorrect charger is used, permanent damage to the battery may occur.

Multipurpose automatic chargers exist which allow charging of either a wet-cell lead-acid battery or a gel-cell lead-acid battery. However, care must be taken to select the appropriate settings on the charger.

A final consideration in the selection of a charger is the country in which the device will be used. This is important because of the different power transmission standards in different countries. For example, in the United States, the standard for power transmission is 110 V at 60 Hz, whereas in Europe the standard for power transmission is typically 220V at 50 Hz.

Recently, manufacturers have begun to integrate the charger into the design of the power wheelchair. This is advantageous, since the power wheelchair can be recharged anywhere there is an outlet. Since these chargers are single purpose chargers, they typically can be used on only one type of battery (gel-cell or wet-cell) and in only one country, which may be a drawback.

Safety

Safety issues should always be considered.

  • Sensors should be built into the charger and wheelchair to detect if the connections between the battery and wheelchair and between the battery and charger are correct. If incorrect connections are detected then the wheelchair and charger should automatically shut down in order to prevent damaging the wheelchair, charger or batteries.
  • The wheelchair, charger, and batteries can also be protected by using specially configured connectors that only allow proper connections, similar to a three-prong wall outlet, and by color coding the connectors.
  • If a single purpose charger is purchased, the charger should match the charging characteristics required of the battery;
  • The charger must meet the power requirements of the local country’s power supply; and
  • The power wheelchair should become inoperable when the batteries are being charged.

Improvements

People who use power wheelchairs have asked for improvements in battery technology, however, the technology remains mostly unchanged. This is partly because of the relatively low number of power wheelchairs purchased -- about 500,000 per year -- when compared to automotive applications -- about 6.6 million per year by a single manufacturer. Also, because of limitations in the health care market (e.g. Medicare, Medicaid, private insurance), there is not an incentive for manufacturers to develop new battery technologies since they may not be reimbursed for batteries other than lead-acid batteries. Currently, only lead-acid batteries are reimbursable by Medicare and Medicaid. If a battery has a greater capacity, a longer range, greater power potential, it may not be paid for because it is not necessary under the definition of "medical necessity," even if the increased benefits outweigh the increased costs.

Alternatives to the lead-acid battery do not presently exist in the traditional power wheelchair market, even though the potential for new technologies do exist. Yamaha is marketing a power assist system for people who use a manual wheelchair. The motors are built into the hub and assist the individual as he/she pushes on the pushrim. Nickel/Metal Hydride (Ni-MH) is used in place of lead-acid batteries. Unfortunately, this wheelchair system is only available in Japan. Other alternatives to lead-acid batteries may be seen as automobile manufactures try to find more environmentally friendly alternatives to gasoline and diesel fuel. Also, alternatives may rise from the personal computing industry where a premium on capacity and range are present for laptop computers. Some of the alternative batteries are Nickel/Cadmium (Ni-Cd), Nickel/Metal Hydride (Ni-MH) and Lithium Ion. Finally, the replacement of batteries with flywheels has been examined as a power source for both the automobile industry and the power wheelchair industry.

The main characteristics of the deep-cycle lead-acid battery are its capacity (expressed in ampere-hours or Ah), its current rating (expressed in amperes or A), the type of cell (wet or gel), and the type of charger. Generally, the larger the battery, the larger the capacity and the greater the range. The larger the current rating (current is the rate of flow of electrons), which is dependent on the speed of the chemical reactions within the battery, the more torque which can be generated by the motor. This translates into the ability to traverse rough terrain. The type of cell will depend on the primary use of the battery, as described previously. Finally, the charger will depend on the type of battery selected and the country in which the wheelchair will be used.

Bibliography

Cooper, R. A., Rehabilitation Engineering: Applied to Mobility and manipulation, 1995, Institute of Physics Publishing, Ltd., Philadelphia, ch. 8, pp. 291 – 336.

Cooper, R. A., Wheelchair Selection and Configuration, 1998, Demos Medical Publishing, Inc., New York, ch. 8, pp. 227 – 251.

Cooper, R. A., "Wheeled Mobility: Wheelchairs and Personal Transportation" in The Biomedical Engineering Handbook, Bronzino, J. D., ed., 1995, CRC Press, Inc., Boca Raton, FL, ch. 137, pp. 2071 – 2085.

Cooper, R. A. and Tai, T., "Feasibility of Flywheel Batteries for Electric Powered Wheelchairs", Proceedings of the 20th Annual International Conference – IEEE / EMBS, Hong Kong, pp. 2261 – 2263, 1998.

Cooper, R. A., VanSickle, D. P., Albright, S. J., Stewart, K. J., Flannery, M., and Robertson, R. N., "Power Wheelchair Range Testing and Energy Consumption during Fatigue Testing", Journal of Rehabilitation Research and Development, v. 32, n. 3, pp. 255 – 263, 1995.

Karp, G., Choosing a Wheelchair: A Guide for Optimal Independence, 1998, O’Reilly and Associates, Inc., Sebastopol, CA, ch. 8, pp. 72 – 88.

Levy, S. C., Battery Hazards and Accident Prevention, 1994, Plenum Publishing, New York, ch. 1, pp. 3 – 21.

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