RESNA 27th International Annual Confence
Autonomy of gel cell and wet cell lead acid batteries for motorized wheelchairs: Preliminary results
Energy storage is a major problem for motorized wheelchair users. As wheelchairs consume an increasing amount of energy and thus require more amperage, the ability of batteries to hold a charge and be recharged the maximum number of times is reduced. Since gel cell batteries now appear to be performing better following improvements in the technology, we decided to compare the autonomy of wet cell and gel cell batteries, both of which are used in motorized wheelchairs. Tests conducted in the field indicate that gel cell batteries provide over 3 times more autonomy than wet cell batteries. Our results support the recent commonly held belief that gel cell batteries provide much more autonomy for wheelchair users and are less expensive if cost per time of use is taken into account.
Motorized wheelchair, battery, gel cell, wet cell, lead acid, autonomy
The energy storage system is one of the greatest limiting factors on the performance of motorized wheelchairs (1,2). In the last few years, the mechanics, engineers and occupational therapists at the Institut de réadaptation en déficience physique de Québec (Quebec City Rehabilitation Institute) noticed that motorized wheelchair users are coming to the workshop more and more often to have their batteries changed. The reason they give is that their batteries no longer provide enough energy for their activities. Over the years, wheelchairs have become increasingly energy-consuming, which has resulted in a growing need for the motor energy generated by the batteries.
Users look for two characteristics in a battery: the ability hold a charge (single cycle) and the ability to be recharged the greatest number of cycles. We call these two capacities "instant autonomy" and "autonomy" respectively. In this study we are only concerned with the latter. In relation to batteries, wheelchairs or users, ideally autonomy should be defined in terms of the distance travelled, and also take into account the context of wheelchair use (presence of inclines, type of terrain/surface, user's weight, driving behavior, etc.) and hence the power required. However, since it is difficult to control all these parameters in an experimental situation, the level of autonomy is defined here as the time required between battery changes, i.e., the wheelchair battery's life.
There is little documentation in the scientific literature about the autonomy provided by wheelchair batteries. Using tests done in both exterior operating conditions and in the home, Kauzlarich et al. (2) showed that wet cell batteries fulfil the operating requirements for motorized wheelchairs (discharge time versus distance travelled) and are a better choice economically. Cooper (1) presented the results of standardized tests indicating that wet cell batteries provide a longer range of operation than gel cell batteries for different wheelchairs. In these tests, the amp-hour rating of a wet cell battery was higher than that of a gel cell battery. However, gel cell batteries that are available today have amp-hour ratings equivalent to those of wet cell batteries, which makes manufacturers claim that gel cell batteries now have greater autonomy.
The objective of this study was to compare the autonomy of gel cell and wet cell batteries in a usual motorized wheelchair operating context. Since gel cell batteries cost approximately three times as much as wet cell batteries, we wanted to verify the hypothesis that the ratio between gel cell and acid cell battery autonomy is greater than 3:1.
Gel cell batteries (manufactured by MK Powered, model 8G24, 12 Volts, capacity at C/100: 84 A-hr) were installed on sixteen motorized wheelchairs. The variable measured was how long the batteries were kept by the wheelchair users, i.e., the time between battery changes. For each client, we compared the average usage time of the wet cell batteries (manufactured by MK Powered, model DC24, 12 Volts, capacity at C/100: 75 A-hr) formerly used by the client (T av, wet ) and the usage time of the gel cell batteries (T gel ) in order to determine the autonomy ratio: R autonomy = T gel / T av, wet . All the data came from client files. The batteries were treated as pairs, i.e., if one of the batteries in a pair could not provide the energy needed to hold a charge long enough for the wheelchair user to do his/her activities satisfactorily, both batteries were considered to have reached the end of their service life.
Sixteen wheelchair users were recruited between June 2000 and July 2002 from the clients of the Institut de réadaptation en déficience physique de Québec (Quebec City Rehabilitation Institute). They were chosen because of their heavy wheelchair use and thus the high demand on their batteries. They had to have had a motorized wheelchair for at least a year and not have any cognitive problems.
Six of the sixteen participants still have functional gel cell batteries, hence some data are censored (right side). Thus, to do a survival analysis, we used a parametric regression analysis to estimate R autonomy , its standard deviation and 95% confidence interval (3). Significance threshold was set at p=0.05 to verify the hypothesis.
Table 1 shows the raw data for T gel , T av, wet and R autonomy for each participant. Hypothesis regarding normality are respected. According to the parametric regression analysis, estimates of R autonomy and its standard deviation are respectively 4.5 and 0.52. The 95% confidence interval varies between 3.4 and 5.5. Finally, the null hypothesis is rejected (p<0.05), which mean that R autonomy should be grater than 3.
Participant |
T av, wet |
T gel |
R autonomy |
|---|---|---|---|
1 |
130 |
392 |
3.02 |
2* |
143 |
1,136 + |
7.94 + |
3 |
250 |
764 |
3.06 |
4 |
152 |
566 |
3.72 |
5* |
152 |
489 + |
3.22 + |
6 |
126 |
236 |
1.87 |
7 |
126 |
689 |
5.47 |
8 |
138 |
871 |
6.31 |
9 |
208 |
1,001 |
4.81 |
10 |
152 |
158 |
1.04 |
11 |
115 |
409 |
3.56 |
12* |
275 |
928 + |
3.37 + |
13* |
274 |
888 + |
3.24 + |
14 |
101 |
594 |
5.88 |
15* |
271 |
755 + |
2.79 + |
16* |
160 |
495 + |
3.09 + |
| “ *” indicates participants with censored data. “+” indicates censored data. | |||
Based on these preliminary results it appears that the autonomy ratio, R autonomy , between commercial gel cell and wet cell lead acid batteries used for motorized wheelchair applications is significantly greater than 3. Results support the recent commonly held belief that gel cell batteries provide much more autonomy for wheelchair users and are less expensive if the cost per time of use is taken into account. For the model used in this study (MK Powered, model 8G24), T gel appears to be more than four times greater than T av, wet . These results cannot be generalized to all models or all battery manufacturers but they do provide an interesting indicator for payor organizations and policymakers when they decide what technology they will pay for.
The results of this study do not contradict other studies which indicated that wet cell batteries gave a better performance (1,2). In the 1990s, the autonomy of gel cell batteries was inferior to that of wet cell batteries (4). However, developments in recent years in order to market different electric vehicles have resulted in an improvement in sealed gel cell technology (5). In fact, our results support the previously unsubstantiated belief that gel cell batteries perform better in terms of autonomy.
Future work should include following a larger cohort or examining the databases of payor organizations. This should make it possible not only to validate our results but also determine if there are any performance differences in terms of autonomy between various battery models, battery manufacturers and even wheelchair manufacturers. Also, "instant autonomy" should be assessed to better document the autonomy of batteries in general.
This study was supported by the Division of Prosthetic, Orthotic and Assistive Devices Programs, Institut de réadaptation en déficience physique de Québec (Québec, Canada). Special thanks to Sophie Baillargean of Statistical Support Services, Mathematics and Statistics Department, Université Laval, for her expertise in the analysis phase.
François Routhier,
Centre interdisciplinaire de recherche en réadaptation
et intégration sociale (CIRRIS),
Institut de réadaptation en
déficience physique de Québec,
525 boul. Hamel est,
Québec,
Canada, G1M 2S8.
Phone: (418) 529-9141 ext. 6256.
Fax: (418) 529-3548.
Email: Francois.Routhier@rea.ulaval.ca .