Are Nickel-Metal Hydride Batteries Superior to Sealed Lead-Acid in Light Electric Vehicle Applications?
By Brad Duncan
School of Science and Technology
Table: Comparison of electrical characteristics
1. Question being addressed
Are Nickel-metal Hydride batteries superior to Sealed Lead-acid in light electric vehicle applications?
2. Hypothesis/Background
I intend to determine if Nickel-metal Hydride (NiMH) is a superior battery chemistry to the older Sealed Lead-acid (SLA) type for light electric vehicles used for personal transportation. I will determine this through research, measurement and observation. My experiment will involve comparing both battery types in a test fixture and in an electric bicycle application.
In contrast to conventional cars, Light Electric Vehicles (LEVs) offer dramatically reduced energy consumption. Many of our errands and trips are less than 10 miles, which is within the range of most LEVs. LEVs range in size from electric scooters up to one-person cars. For our comparison we will be using an electric bicycle. Electric bikes are everyday bicycles with an added battery-powered electric motor. They allow for human input, which can extend the range provided by motor assist,
3. Prevalence of LEVs
Sales of LEVs have increased anywhere from 40 to 200% annually over the last three or four years in the United States. They can now be purchased at most mass retail chains, like Target or Costco
4. Types of LEVs
Electric Scooterswere once thought of strictly as a mobility aid, but now are being embraced by those who appreciate and enjoy the riding experience and the efficiency of them. Electric Bicycles are perhaps the most practical LEV because they allow sustained human input and thus extended range.
Some people build them from scratch, but it is much more common that hobbyists and experimenters buy them and modify the motors, drive systems and batteries for additional speed or range.
5. Battery chemistries used in LEVs
Electric bikes and scooters are usually powered by Sealed Lead Acid (SLA) or Nickel Metal Hydride (NiMH) batteries in voltages ranging from 12-48 volts.
6. Why I selected NiMH and SLA to compare
SLA is an economical and traditional battery chemistry for electric bikes and vehicles. NiMH has emerged as a lighter weight alternative with proven reliability and charging systems. Lithium Ion batteries such as those used in laptops require more complex charging systems and require careful handling and use. Zinc composition batteries are showing great promise but are less commonly available in the amp-hour ratings and price points required for LEV applications.
7. Comparing SLA and NiMH in LEVs
In this section I will research and compare various characteristics that contribute to the usability of SLA and NiMH batteries in LEVs.
7a. Mechanical construction
The Mechanical construction of a battery contributes directly to the reliability and integrity of the pack. This is important because the pack is subject to vibration and shock that can cause early failures.
The SLA battery is fully self-contained in a plastic case. The individual cells are held in place inside the case and connected internally. Internal connection failures are almost unheard of. A 36 volt pack will require three 12 volt SLAs. Only two external connections are required to form a 36 volt pack.
In contrast, a 36 volt NiMH pack is composed of thirty individual cylindrical cells. Excluding the primary positive and negative terminals, there are 28 separate connections between batteries, each requiring two solder joints. There are also 30 cells compared to 18 in an SLA pack, so there is more potential for electrical failure as well.
Winner: The SLA is mechanically far less complex and thus less subject to mechanical failure. The NiMH pack employs more individual electrical cells, each of which is subject to early failure and faulty connections.
7b. Charging
Battery capacity (C) is the amount of current in amps that the battery can supply for one hour. In our tests, we are using an 8 Amp Hour (Ah) SLA, and a 9 Ah NiMH pack. Batteries can only be safely charged at a fraction of C.
SLA batteries can be safely charged at .2C
(8Ah*.2=1.6A) for 5-8 hours
NiMH batteries can be safely charged at .5C
(9Ah*.5=4.5A) for 2-4 hours
However, it’s not that simple. The NiMH pack cannot be charged when it is hot from being used. It must be cooled before charging since temperature is one factor used by the battery charger to determine when to shut off. Also, due to the complexity of the NiMH charging algorithm, NiMH chargers are significantly more expensive.
Winner: No clear advantage
7c. Discharging (use)
SLA batteries cannot be left in a discharged state or sulfation will begin to occur within a few days. NiMH batteries can be left in various states of charge with no detrimental effect. However, NiMH batteries are subject to an effect known as cell reversal, where small differences in voltages of individual cells can result in negative polarity being applied to the weaker cell, causing permanent damage.
The maximum depth of discharge for NiMH batteries is 1V per cell, or 30V total, compared to the SLA at 1.75V per cell or 31.5V total. Therefore, the NiMH pack can be used for slightly longer. Both battery chemistries are rated at a maximum discharge current of 5C, or 40A for the SLA pack and 45A for the NiMH pack.
Winner: NiMH. Both batteries have mostly equal characteristics, however the NiMH can be discharged slightly further.
7d. Thermal (during use)
Both battery chemistries can operate over a range of -20°C to 60°C with limitations.
At low temperatures, the performance of both battery chemistries drops drastically, with -20°C (-4°F) being the threshold at which the NiMH and SLA batteries cease to function.
A lead-acid battery will actually deliver the highest capacity at temperatures above 30°C (86°F), but prolonged use under such conditions decreases the life of the battery.
NiMH also degrades rapidly if cycled at higher ambient temperatures. For example, if operated at 30°C (86°F), the number of charge and discharge cycles is reduced by 20%. At 40°C (104°F), the loss jumps to 40%. If charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at room temperature.
7e. Thermal (during charging)
Charging a hot NiMH battery decreases the charge time, but the battery may not fully charge.
For an SLA, warm temperatures lower the battery voltage, and serious overcharge may occur if the cut-off voltage is not reached and charging current continues to flow.
7f. Thermal (duty cycle)
SLA batteries get only mildly warm during use or when charging for LEV applications.
NiMH batteries generate significant heat during both charge and discharge cycles. In fact, temperature is one method used to determine end of charge.
Winner: SLA battery. The NiMH battery must be allowed to cool between charging and use or vice versa. This requires a mandatory delay which could diminish usefulness in commuting situations.
7g. Weight considerations
Accurately calculating the effect of variations in weight on the top speed of a bicycle, either human-powered or motorized, involves some very detailed calculations. Modeling these factors is beyond the scope of this report, but an accurate and complete online calculator especially created for recumbent bicycles exists at this site: http://www.kreuzotter.de/english/eindex.htm.
The following input parameters were used to obtain the top speed:
Bicycle weight: 85 lbs
Rider weight: 175 lbs
SLA battery weight: 19 lbs
NiMH battery weight: 14 lbs
Input power: 750 watts
Grade: 5%
Top speed (NiMH): 18.1 mph
Top speed (SLA): 17.9 mph
Winner: No clear winner. Note: in many applications, SLA batteries outweigh NiMH by a greater margin, especially at higher amp-hour ratings. For lighter LEVs in the 35-50 lb range, the speed difference would be more dramatic.
7h. Storage
Self-discharge is voltage loss over time. SLAs have a very low self-discharge rate, about 5% per month. NiMH batteries loose far more, about 30% per month.
NiMH batteries can be stored at any state of charge without damage. However, SLAs are subject to sulfation, a corrosion of the battery plates if left in a discharged state. This is the single biggest cause of battery damage in LEVs.
Winner: NiMH. You can always top off a charge, but sulfation is usually irreversible.
7i. Economic
Our SLA pack cost $20 per 12V battery for a total cost of $60. The charger was an additional $40. Total cost for pack and charger: $100
The exact NiMH pack is no longer available, but a comparable pack can be purchased for $300. The charger is more sophisticated and costs $130. Total cost: $430.
Winner: SLA pack and charger costs 75% less to purchase than the NiMH.
7j. Longevity
The SLA battery can be recharged 200 to 300 times. The NiMH battery can be recharged 300 to 500 times, but will be at the lower end of the scale if fully discharged each time. Since electric vehicle applications demand frequent full discharge to obtain maximum range, the advantage is negated.
Winner: No clear winner.
7k. Environmental
The lead-acid battery is easily recycled. In the USA, 98% of all lead-acid batteries are recycled. They can be turned in at any automotive service center.
In comparison, only one in six households in North America recycles other battery chemistries. Unlike nickel-cadmium cells, nickel-metal-hydride is considered environmentally friendly. However, if ten or more batteries are accumulated, the user should consider disposing of these packs in a secure waste landfill.
Winner: SLA batteries for ease of recycling and reclamation of material.
8. Experiment: Discharge testing SLA and NiMH batteries
8a. Testing methodology
In order to observe the ability of each battery to provide sustained current over time, I used a resistive load and took voltage measurements at fixed intervals. This was repeated until each battery reached its maximum depth of discharge.
8b. Construction of an air-cooled load resistive load
Through the use of an ammeter temporarily fixed to the electric vehicle, I observed that current drain varied between 0 and 30 amperes under normal operating conditions. I chose a continuous 8 amp load because it was close to the battery’s 1C values and could be obtained using available wire-wound resistors.
8c. Safety concerns
Whenever working with batteries capable of delivering high currents, it is important to put a fuse in the circuit. I used a 15A fuse which would blow immediately if short circuited. Always verify all connections before applying power.
8d. Selecting the load resistance, power calculation and heat-sinking
For the load, I bought 6, 30 ohm, 50 watt wire-wound resistors. Each bank of two in parallel provides an equivalent resistance of 15 ohms. At a nominal voltage of 36 volts, 2.4 amps go through each branch (times 3 = ~7 amps). Power is 86 watts (P=I*E), which is at the high end of the operating range (100 watts) for the two resistors. When used this way, heat sinking is required. Each bank of two resistors was mounted on a heat sink with silicon grease used to help conduct heat.
8e. Use of forced air cooling
Even with heat sinks, the temperature could get hot enough to cause burns or melt the apparatus. To insure it operated within safe limits, a cooling fan was attached.
8f. Performing the discharge tests
Each battery was freshly charged and connected to the load device in series with an ammeter. A voltmeter was connected across the load terminals. At time zero, the load was switched on and an initial voltage reading was taken. At five minute intervals, the voltage was measured. This was repeated until the maximum depth of discharge voltage was reached.
8g. Test results
The NiMH battery had a higher initial voltage under load and throughout the measurements. It also maintained a usable voltage range for a greater duration than the SLA. The SLA battery began to decline more rapidly than the NiMH, showing a steeper decline, which would equate to less notice to the rider in actual operation.
Winner: NiMH battery had more staying power in bench testing.
9. Field Trial: SLA vs. NiMH in an electric bicycle application
9a. Real-world vs. bench testing
In actual use, current drain may vary between 0 and 30 amps. To see how each battery holds up in actual use, a discharge test based on a typical on-road course was designed.
9b. The test vehicle
The test vehicle was a Compact Long Wheel-Base recumbent bicycle adapted to use a brushless DC electric motor running on 36 volts. A space below the seat allowed interchanging the SLA and NiMH batteries.
9c. Designing the trial
A course of approximately 1.5 miles was selected that had a mixture of hills and level ground to simulate typical use. During the course, full throttle was applied on all uphill and level sections. Downhill, the motor and battery were allowed to rest, like in normal use. Since the motor is an assist to pedaling, constant, normal human input was applied whenever the bike was under power. An average speed of 18-22 MPH was maintained during the trials. In addition to motor assist, normal gear changes were used to obtain the target speed when climbing hills or on level terrain. My father, the owner of the bicycle performed the trials. His experience and stamina helped to obtain consistent results.
9d. Taking measurements
A digital voltmeter was attached to the bicycle frame. At the beginning of each “lap” was a short but steep hill that would put the battery under full load. Just before reaching the top of this hill, the voltage was noted. Upon cresting the hill, the motor was disengaged. Five seconds later, a second measurement was taken. Both measurements were then recorded into a digital voice recorder.
9e. Results
As in the bench test, the NiMH battery had a higher initial voltage. Under load was where the differences became visible. The NiMH battery could not sustain it’s voltage under the load typical of normal vehicle operation, which was closer to 30A rather than the 7-8A drawn during bench testing. At this point, more input would be required from the operator to maintain the same speed. Upon cresting the hill, the NiMH battery rebounded to a value closer to its initial voltage, and under more moderate loads provided a higher sustained speed.
The SLA battery, despite a lower starting voltage maintained higher potential under full load. It also did not drop as far under full load. Otherwise, its performance was similar to the NiMH until its characteristic steep decline in voltage as seen in bench testing occurred. Again, the NiMH outlasted the SLA in total usable time.
Winner: NiMH by a minor margin. It lasted longer overall, but did not hold up as well under load. The SLA also lost points for dropping off steeply at the end.
10. Conclusions
As outlined in this report, there are numerous factors to consider when comparing specific battery chemistries for use in LEVs. Either battery can be obtained in the size and amp-hour rating suitable to provide acceptable speed and range for practical operation. Bench testing and field trials demonstrate that although NiMH will provide slightly higher maximum speeds under nominal loads, the SLA battery yields acceptable performance and actually performs better on extreme grades.
One then needs to look at their own objectives and goals for owning and operating an LEV. If ultimate economy is the goal, the SLA is certainly less expensive to purchase and replace (which all batteries eventually need). If top speed and lightest weight is desired, the NiMH pack is the clear winner. In our tests, the batteries were similar in weight, but when higher capacity SLA batteries are used, the weight difference becomes a factor worth considering. For the operator who wants the highest degree of reliability, the SLA has the obvious advantage. It not only has substantially less electrical connections, the total number of individual cells is almost halved compared to the NiMH. NiMH batteries also have a disadvantage in thermal characteristics. They need to cool before being recharged and require a more sophisticated and expensive charger to avoid overcharging. SLA on the other hand, can be damaged if not charged after use.
Perhaps the most compelling argument for the SLA battery chemistry is in its environmental advantages. Lead recycling is commonly available, and a very large percentage of the battery by weight is reclaimed. Although NiMH batteries do not contain the toxic metal cadmium like their close cousins, access to recycling facilities for the average consumer is far less common and they are much more likely to end up in a landfill.
For the time being, lead is still a practical choice in Light Electric Vehicles.
Last fall, Watertown, MA-based startup A123 Systems announced that its advanced lithium-ion batteries would make rechargeable circular saws and drills more powerful than plug-in tools (see "More Powerful Batteries"). The company, having delivered on its promise (the tools will be available at The Home Depot this weekend), has now built a battery pack that Ric Fulop, one of the company's founders and its vice president of marketing and business development, says could make hybrid vehicles cheaper and more convenient, while maintaining or improving performance.
The new hybrid battery pack was unveiled this week at the Advanced Automotive Battery and Ultracapacitor Conference in Baltimore. It could be appearing in vehicles within three years, Fulop says. The pack weighs about as much as a small laptop computer, yet fits into a case smaller than a carton of cigarettes. Ten of them would replace the 45-kilogram battery in the Prius, Fulop says; and if one failed, the consumer could continue to drive the car using the remaining batteries, then replace the faulty one as easily as changing the battery on a rechargeable tool.
Such convenience could start to look more and more attractive as today's hybrid cars age and drivers face the need to replace worn-out batteries -- especially second owners who won't have warranty coverage. So far, however, battery replacement isn't a big issue in the industry. In Japan, where the Prius has been on the market much longer than in the United States, for instance, Toyota just got up to a few hundred batteries last year in its recycling program.
Probably more important than ease of replacement, though, is the potential for cost savings and increased safety. Because the advanced lithium-ion batteries put a lot of power into a small, light package, a much smaller battery is needed to power the car, which could reduce hybrid prices. As a result, a variety of cars in a fleet could come with a hybrid option that costs about as much as the option for an automatic transmission, Fulop says. Furthermore, lower-priced hybrid cars that have the acceleration and other performance features customers want could help hybrids capture more of the vehicle market, especially if a hybrid drive train can be offered on a wide variety of vehicles, according to analyst Hideo Takeshita of the Institute of Information Technology in Tokyo.
In the short term, however, Takeshita's seemingly logical assumption about lower-cost hybrid cars might not be right. Scott Miller, CEO of the market-trend analysis company Synovate Motoresearch, in Royal Oak, MI, says a major reason consumers buy hybrids today is to have a "badge of honor" that shows their commitment to the environment or to curbing gasoline use. And it's an opinion shared by Toyota's Hermance. Part of this distinction, as Miller sees it, comes from having to pay a price premium for the vehicle. Hence, in the short term, he says, it might actually be wise for carmakers to leave hybrid prices higher.
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