Electric drive train consultant Tom Dougherty of Monolith engines explains why EV developers need to seriously consider the addition of untracapacitors to augment battery performace
Over the past few years Ultra Capacitors, sometimes known as UCaps, have realized substantial increases in power and life while attaining a reduction in cost to the consumer. This technology is now being used in hybrid buses, electric power conditioning and megawatt generating windmills. The HEV (Hybrid Electrical Vehicle) automobile, however, has not yet taken advantage of what could help with this vehicle’s reputation for poor cold weather performance. The following article will show both the advantages and value of using UCaps in HEV applications to help solve this performance issue.
A Little about Batteries
Lead-acid batteries have been used in motorised vehicles for over 100 years and they are continually being improved. New advanced nickel metal hydride (NiMH) and lithium Ion (Li-ion) batteries are now being used where lead acid batteries have shown to be too large and too heavy. In spite of all the improvements with the three batteries types, they still have two major inherent problems: kinetic losses at high currents and cold temperature electrolyte resistance.
The first of these major losses kinetics, simply stated, is something like friction. It is a loss of potential (voltage) when trying to get ions to react and form new chemical structures. For example, in many driving conditions, NiMH, excluding IR, may take a voltage swing of 20%-30% to go from discharge to charge. This is wasted power and the vehicle needs to supply this voltage difference to get the battery to accept a charge current while braking. A Li-ion battery has the least of these losses, but has the worst cold weather performance. Lead acid batteries are somewhere in-between both of these battery types but lack energy density.
The second major issue with batteries is electrolyte resistance. At temperatures around 0ºC, batteries begin to lose power and efficiency, especially at high rates. At temperatures below -30ºC, many batteries are rendered almost useless. In addition, these batteries cannot operate at full charge because, at that SOC, they do not have the ability to store the vehicle’s braking power.
Cold electrolyte, around -20ºC, can cause a Li-ion battery to lose over 70% of its ability to accept braking energy. This is evident based on the electrolyte resistance versus temperature for both batteries and ultracapacitors as shown in Fig 1. At temperatures below -20ºC, Li-ion battery systems may shut off any recharge during braking to avoid what is called Li-ion plating. Many Li-ion batteries being manufactured today use more expensive electrolytes to perform adequately in cold weather, but this reduces their life expectancy in warm temperature conditions. Also, to compensate for these problems, Li-ion electrodes are thinner and longer than what is necessary for the amount of energy they supply. By using a capacitor in combination with a Li-ion battery in a HEV, the electrode could be made 30% smaller (shorter and thicker), thus reducing the cost of separator, copper and aluminum used in the product. In addition, the shorter electrode takes less time to manufacture. It is also now possible to make these thicker electrodes using a new dry powder process. It is estimated that a cost reduction of over 15% is attainable by making battery cells in this manner.
In any value proposition, the understanding of the product’s application and its effective use in its environment is important. Today’s vehicles only use 50-60% of the battery’s capacity, so as not to overcharge or undercharge it, and the battery has to be made 30-40% larger than necessary due to the fact that it will lose capacity and power during its life. Along with that, there are energy storage losses, especially with NiMH, during braking due to battery kinetics, which could cost 30% of the braking energy and gas mileage improvements.
All of this also affects the vehicle’s performance, and with many HEVs, performance changes substantially with the seasons.
Ultra Capacitors Performance
The main reasons for using a UCap in HEVs is its fast time constant (complete charge-to-discharge times) and its cold weather efficiency. Unlike batteries, they love to cycle in the 2-10 second time frame. This is because a capacitor does not store its energy like a battery and, therefore, reactions are fast and the losses due to kinetics are almost nil. The only measureable loss in a UCap is due to Joule losses proportional to its resistance, which is much lower than a Li-ion battery of comparable size.
When comparing UCap’s input and output power in cold weather to Li-ion, UCaps are unmatched. Figure 2 shows a Li-ion cell’s ability in delivery power versus the UCap’s. This power delivery holds for the charge and discharge of both devices. Notice the output at -10ºC, which is a temperature often found in the northern part of the United States during the months of December through March. Already at this temperature the vehicle performance is dropping off, and at -20ºC, it degrades to the point where any performance advantage becomes questionable.
The National Renewable Energy Laboratory/GM Report on Ultracapacitor Testing in HEVs
Recently a report (the Gonder Report) was written by the National Renewable Energy Labs (NREL) and General Motors on the testing of a Saturn Vue – 48 V Belt Starter Alternator and was summarised as follows:
“ – Found Ucap HEV performance comparable to stock NiMH
– HEV achieved same fuel economy
(generally only using 18-25 Wh)
– Matched driving performance “
NREL completely replaced the vehicle’s NiMH battery with a Maxwell 48 volt UCap weighing 40% less. Again, no difference in drivability or gas mileage was seen, and this is with no change to the electrical system or optimizing the system for use with a capacitor!!! If this test was run at -20ºC and the voltage window was modified, the Ultra Cap would have had a significant advantage over the NiMH battery in both drivability and gas mileage.
Something Special about Ultra Caps
UCaps can actually store more energy in cold weather then in warm weather. If a UCap cell is charged up in warm weather, it can be safely charged to 2.7 volts. The energy stored is E = ½ C*V2. This means, for example, that a 3,000 F capacitor can store 10.93kJ at room temperature. At – 20ºC, the capacitor can be charged safely, with the same life expectancy, to a voltage of 3.2 volts, storing 15.36 kJ. That is an increase of 40% in stored braking energy. Using the formula for power P= V*I, indicates that the capacitor at 3.2 volts needs to supply 20% less current to deliver the same power to the HEV motor than when it was discharged at 2.7 volts. This lower current required from the capacitor in cold weather almost perfectly offsets the increased resistance: this means the vehicle will see no difference in drive ability in cold weather. The vehicle may even see an improvement due to the loss of resistance in the electronics and wiring in the vehicle’s inverter and battery/capacitor system and in the increased energy stored by the Ucap at low temperatures.
Although the focus of this article has been on Mild Hybrids, capacitors can work well with plug-ins and true EV’s with the same capability in storing braking energy. The battery in these vehicle types can then be designed as a true energy product, which does not need to store high braking power.
So why are we using only batteries and not batteries coupled with capacitors? There are actually two major reasons: the perception of system cost being too high and not understanding the true value of today’s UCaps in attaining good vehicle performance. The one caveat is that the battery and capacitor system may be larger than just the battery itself and this may be a deterrent to some vehicle manufacturers.
Calculations to Determine Payback for Capacitor/Battery System – “The Value Proposition”
(Micro or Mild Hybrids)
The following calculations were developed to show the size, cost and payback of a capacitor system when used with a lead acid battery for mild hybrids. Any battery can be selected, but for this analysis, a sealed AGM-type lead acid product was used. The analysis is comprised of four parts:
1 Calculating the vehicle’s stored energy during braking
2 Determining the total capacitance and current flow
needed to brake the car
3 Sizing the capacitor cell
4 Computing the cost of the total system and its payback
(All values mentioned can be changed in the computer model to fit the needs of the customer. This is only an example of one vehicle and one driving pattern.)
Vehicle Stored Braking Energy see figure 3 right
To start the value proposition, it must first be determined what energy the vehicle can deliver, in kJ, when braking from a given speed. The above spread sheet chart is extracted from the model which shows a method to calculate the kJ the vehicle can generate from its starting braking speed to about 10 miles per hour (both English and Metric units are shown). Taking ½ the mass times the differences of the square of the starting and ending braking speeds shows that this vehicle has about 202 kJ of energy it can deliver to the system electronics. (The 27 MPG fuel economy for this vehicle will be used later.)
Therefore: Energy = ½ *1.363 * (17.82-4.42) = 202 kilo Joules
Determination of Capacitance and Current Handling of the System Based on System Voltage
The second set of calculations is for sizing the system capacitance and current carrying requirements. The maximum voltage used for this calculation was 48 volts, fully charged, and 24 volts at end of discharge. Also needed for the calculations are efficiencies of the systems. The first number in the chart below is for the electrical system efficiency (85 %) which represents both the starter/alternator and the control electronics. Next, is the % of energy that needs to be applied to the mechanical brakes to ensure they are engaged during the stopping of the vehicle. Also, braking time is important to determine the current. The voltages chosen for the full charge and discharge of the UCap is what the system can handle and is also determines the number of 2.7 volt capacitor cells that will be needed. See figure 4.
From the model, the capacitance needed is equal to the kJ from the vehicle times 2, divided by the square of the starting and ending voltages times the efficiencies. From these equations it is determined that a 149 Farad, 48 volt capacitor that can handle an average of 409 amps is required. Remember, this is the average amps coming from the system, so the current will start higher and end lower. The model also factors the amount of current the battery will absorb, and in this, case it is 20%.
Capacitor Cell Sizing
With the capacitance and current calculations for the entire cell pack determined, the individual cell size must now be calculated. The next part of the model calculates this cell size by using the maximum cell voltage recommended by the manufacturer. This value is divided into the maximum system voltage and gives a value of just under 18 cells. With 18 cells chosen, 18 is multiplied times the system capacitance to get the cell size. The number of 2,683 Farads is close to the 3,000 F cells made by a number of manufacturers. Using the 3,000 F Maxwell cell as a reference, an acceptable power loss of 5% is selected to determine heat dissipation in the cell. The Maxwell 3,000 F cell also meets the system requirements for internal resistance and current carrying ability. See figure 5
Final Calculation – Payback
See figure 6
The final model spreadsheet calculates the payback for a Mild HEV using the above-mentioned capacitor. The first part of this model determines the gas savings. Simply stated it is miles driven in the city, divided by miles per gallon, times the percent of improvement the system is able to attain, and then times the cost of gas. The model shows that for a vehicle driven 15,000 miles driven per year, at 27 MPG, with $4.00 per gallon gas, 25% of miles driven on the highway and 75% in the city, and a vehicle gas mileage improvement of just 12%, the expected savings will be $201.
Next the cost of the system is calculated based on the cell cost. If 18 cells are needed at 2,683 F, (3,000F, if an off-the-shelf cell is chosen), and the cost per Farad is .8 cents, then the total capacitor cost would be $405. The added electronics and packaging would add about 33% to the system, giving a total cost of $541. The lead acid battery is in the vehicle today, so it does not have to be part of the payback, although it is also shown in the total system payback.
With a system costing $541 and a savings of $201, the simple payback is 2.7 years for the capacitor package. If the new starter/alternator and controller costs are known, and the reduction of costs for the elimination of the existing starter and alternator are available, then a total system payback can be attained.
A Short Term 12 Volt Opportunity
The value proposition of going to higher voltages may not work for all vehicles, but there is an opportunity to change present 12V systems to allow start/stop applications to be developed with little vehicle modification.
Over the past 10 years there has been a lot accomplished on 12 volt start/stop systems. The big problem, however, has been the battery performance, especially in cold weather. It was also imperative to keep the 12 volt battery nearly fully charged to get the necessary starting power.
If a Ucap is added to the system, it will operate with almost no power delivery loss in cold weather. With the capacitor independent of the battery circuit, it can also store a significant amount of the breaking energy. This will allow the capacitor to charge fully upon slowing down and even giving energy back after it starts the vehicle. This system, sometimes called a Micro hybrid, allows the battery to be designed for cycling, and it could be positioned outside the engine compartment. This gives battery engineers the ability to develop longer lived batteries with the potential to last the life of the vehicle.
It should be noted that this product could also directly replace existing car batteries, for cold weather applications, improving cold starting with no modification to the vehicle. The author is putting such a product in his SUV with the battery/capacitor system actually being smaller, lighter and with greater starting power then the battery presently designed for the vehicle.
With the dramatic reduction in cost per Farad, the improvement in life and the added feature of working significantly better than batteries in cold weather, the value proposition for the use of Ultra Capacitors in vehicle electrical systems is showing potential. Cold weather vehicle gas mileage will be enhanced, with this improved source of power, due to the efficiency of the capacitor to store breaking energy, even in 12 V systems. UCaps have also shown to be able to completely replace batteries in some systems with no reduction in gas mileage or vehicle performance. It is believed that most battery types can also be down-sized, made to last longer and for less money when coupled with capacitors.
Many vehicles are now being hybridized with engines and motors to enhance gas mileage. It may also be time to hybridize the electrical power source with ultra capacitors to improve upon these gains.
This model is available through E-mail request to firstname.lastname@example.org.
The author would like to thank Andy Burke of UC Davis, Jeff Gonder and his team from NREL and GM, along with John Miller, Mike Everett and Porter Mitchell of Maxwell for the help, reports, and information used in this report on ultra capacitors.
About the Author
Tom is President/CEO of Monolith Engines Inc. Waukesha WI, USA and is now consulting to the energy field with respect to battery and capacitor systems. Prior to Monolith, Tom was at Johnson Controls Inc. for 37 years working with lead acid, NiMH and Li-ion batteries and their electrical systems. When he retired, he was director of advanced battery and hybrid systems manufacturing. Tom has 46 patents in battery design and testing, system electronics and software, alternators, and internal combustion engine design. Monolith Engines Inc. is now working on a new patented (7,739,998) “One-Cycle” engine which is being prototyped for Series-HEV applications. email@example.com