Electric Vehicle Conversion/Technologies
Note the presence of hazardous materials and conditions that must be approached with proper precautions and procedures to avoid damaging, injurious, or even fatal consequences.
- 1 Battery technologies
- 1.1 Lead acid
- 1.2 Nickel cadmium (NiCd or "Nicad")
- 1.3 Nickel metal hydride (NiMH)
- 1.4 Lithium-ion (LiIon)
- 1.5 Nanotechnology batteries
- 1.6 Pack sizing
- 2 Pack voltage
- 3 Balancing circuitry
- 4 Battery charging
- 5 Motor/controller technologies
- 6 Range Extenders
- Life cycle cost
- Initial cost range
- Energy density
- Peak power capacity
- Internal resistance
- Voltage depression
- Available form factors
- Expected degree of discharge
- Low temperature operation
Lead acid cells are sensitive to "break in", in that severe use early in life can shorten the lifespan. They are also sensitive to temperature, with low temperatures severely reducing the amount of energy available - sometimes battery heaters and insulated boxes are appropriate use in cold climates. If sufficiently discharged the electrolyte can freeze at extremely cold temperatures, which can break the case from expansion.
It is important that lead acid cells be kept charged. A deep discharge without an immediate recharge can significantly reduce the capacity of the batteries. Lead acid batteries also give the most total energy throughput over their useful life if the discharge cycles are relatively shallow, but such repeated shallow discharging can reduce the total energy available between recharges. The electrolyte is sulfuric acid in water and is not especially strong but is hazardous to skin, eyes, and clothing and corrosive to steel and aluminum. It does not affect most plastics and stainless steel. It is easily neutralized with common baking soda (sodium bicarbonate), which should be kept available near the batteries. (Note that a common "dry powder" fire extinguisher is charged with sodium bicarbonate and is appropriate for use on electrical equipment.) When combined with the acid it will evolve carbon dioxide, leaving a relatively harmless sodium sulfate residue. When working on the batteries some baking soda should be kept at hand in case of an electrolyte spill. Safety glasses should be worn when working near or upon the batteries. Sometimes acid will leak from the seal between the top and the case, or may be spilled when moving or testing the batteries. Such an acid leak can not only corrode the vehicle's structure, it can also cause a highA–resistance current leak, which can cause some types of battery chargers to fail with a ground fault. In the case of an acid spill, battery cases and tops should be wiped down with a solution of water and baking soda. This should be repeated until no bubbles of carbon dioxide are seen.
Discharge and failure to recharge will cause sulphanation, a chemical change on the surface of the plates that reduces capacity and increases internal resistance. Keeping the battery properly charged and well watered (where appropriate) is the best care to ensure long service life. Overcharge of some types of batteries can also be harmful.
If the battery is not likely to be used and quickly recharged it should be placed on a "float charge", where the charger applies a voltage that will not cause overcharging (which is harmless to wet cells but consumes the water in the electrolyte through electrolysis into hydrogen and oxygen and is damaging to other lead–acid battery types).
Batteries in general, and lead acid in particular, will sustain a greater lifetime energy throughput if only partially discharged, generally less than 50 percent being considered an economical design point. As lead acid batteries in particular have a high weight for a given capacity, and this weight must be hauled by the vehicle, it becomes uneconomical in energy to have much excess capacity, but since the greatest expense is not energy cost but rather battery replacement, the main effect of having too much battery weight will be in its effect upon acceleration performance and brake wear.
Batteries suitable for EV conversions are typically sold as "golf cart" batteries and offer the lowest initial cost per watt-hour of capacity, high watt–hour capacity and reasonable life when properly cared for. A typical configuration is a six volt battery containing three cells. They are also available in 4-cell, eight volt configurations but these are rarely used in EV conversions owing to a higher total life cycle cost. Flooded lead–acid batteries are conditioned periodically by overcharging to equalize the state of charge of individual cells in the pack. Care must be taken when charging batteries indoors due to the creation of potentially explosive hydrogen gas. If such cells are charged in an enclosed space the charging station (or other switched outlet) should also be connected to a vent fan of appropriate size and location. Being "wet" cells they require maintenance in the form of addition of distilled water, or the addition of a Hydrocap™ - a catalytic device that recombines evolved hydrogen and oxygen, reforming the water that would otherwise have to be replaced. When these caps are used the temperature may be monitored to good effect; when all cells are fully charged, all of the caps will be warm from the exothermic recombination of hydrogen and oxygen to water. Devices can be purchased that allow fast and easy central-point watering of the batteries, if catalytic recombiners are not used.
Wet lead–acid cells will also shed material from their plates, which accumulates at the bottom of the cell. Cells intended for long service life and deep discharge have extra space to accumulate these materials. When the accumulation reaches the plates it will cause a (non-catastrophic) short circuit, eventually rendering the individual cell useless.
This type of cell is particularly resistant to overcharging, since the dissociated water vented as gas may be replaced, or recovered using a catalytic cap. This type is less resistant to vibration than other types and the entire pack will become useless if the electrolyte in a single cell is lost due to case damage, which by not allowing currrent to flow through the cell will bring the entire vehicle to a halt.
The flooded lead acid cells used in EV conversions are readily available at low cost compared to other batteries owing to their high volume use in other vehicles such as golf carts. These batteries tend to have high weight in proportion to their peak capacity and a higher internal resistance than other lead-acid types. Owing to the lower voltage in a battery unit (three cells with a nominal six volts) it is not practical to build a high voltage light weight battery pack. Thus the advantages of the wet lead-acid cell are best exploited in a light truck conversion and in lower voltage automotive applications (96 volts or less using six volt batteries, or 120 volts using eight volt batteries) in lighter vehicles. Note that for certain battery voltages and sizes (e.g. the six volt Trojan T-105) the actual purchase cost can be significantly lower that for other related batteries (e.g., the 16% heavier Trojan T-145), due to the highly competitive golf cart market.
Batteries using tapered post connections are generally considered better than batteries with other connection arrangements such as vertical stud, owing to the greater contact area and less tendency to loosen in service as a consequence of plastic flow of the lead. The more expensive cable to post connections will add to the total cost, however. If the stud type is used a small amount anti-seize paste (typically used on exhaust manifold bolts) consisting of aluminum flakes in a grease carrier, should be applied to only the stud threads before assembling the connection, keeping the electrical contact area clean.
Batteries intended for automobile starting are not suitable for use in EVs as they are optimized for a different charge-discharge profile and their short service life in this application will greatly reduce their cost effectiveness.
Wet lead acid cells offer a high ampere-hour capacity compared to other types, 225A·h being common in golf cart types. The 225A·h refers to a twenty hour discharge rate and the actual energy available under typical electric vehicle conditions will greatly reduce the total energy available from this type.
Periodically the battery pack should be given a balancing charge. For such a charge the charging voltage is increased to a value that will cause fully charged cells to electrolyze the water in the electrolyte. Any cells not fully charged will continue to charge. Note that this can only be done with the wet cell type as this procedure will damage or destroy other battery types.
A well cared for flooded cell pack in normal light truck service is generally expected to last about three years. It may be possible to extend this life through electrolyte treatments, an option not available with other lead–acid battery types. Buildup of shed plate material below the cell will limit the ultimate life of the battery.
The few major brands available for this type of battery use different connection locations and so the vehicle may require new cables if the type of batteries installed are significantly changed. Many house brands of batteries will use one of the few major manufacture's form factors.
Absorbed glass mat (AGM) lead acid
Compared to wet lead acid these cost far more, have shorter life, far less ampere–hour capacity, but much more maximum power output per unit weight, greater durability, and lower maintenance requirements. Owing to their high output and lower weight per cell this type is particularly suitable for high performance vehicles, being commonly used in electric drag racers and low cost sports cars. Some configurations have been optimized for light weight electric vehicles, offering 12 volts from six cells in a 45 pound (20 kg) package, but only 55 A·h at a slow discharge rate. This type of cell offers low internal resistance and so batteries made from these cells can deliver high power outputs for their size and weight. While substantially more expensive both per ampere-hour and in lifetime energy throughput than wet lead-acid cells they are still considerably less expensive than other types of batteries.
This type is suitable as a replacement for wet cells but must be carefully protected from overcharging as there is no means for replacing lost electrolyte water due to outgassing. The ability to mount these batteries at an angle may be an advantage in some smaller vehicles. The leading developer of this type of battery, Optima, was purchased by Johnson Controls and folded into their Interstate battery group. Subsequent problems with new dead batteries (palette load shipments) have been reported on some electric vehicle forums.
Lead acid gel-cell
Gel cells are sealed and the electrolyte is non-liquid. This requires that they be operated and charged within carefully controlled ranges to avoid overcharging. They are also unsuitable for the high-current requirements of a typical direct current application. They are suitable for low current high voltage applications such as is typical in an alternating current system. Gel cells are commonly used in electric bicycles and scooters, and in computer uninterruptible power supplies. This type of cell has been used in CalCars' plug-in hybrid conversion of a Toyota Prius, but is intended only as proof of concept for that application owing to its weight compared to other technologies. While infrequently used in EV conversions this type of cell is available in a wide variety of battery shapes and sizes and this is advantageous if battery space is limited. As many of these batteries have limited total power and current capacity they are sometimes used in parallel groups (called "buddy batteries") to construct a pack. This type is more sensitive to damage by incorrect charging profiles than is the AGM type. As the gel immobilizes the electrolyte it is not subject to stratification as is the wet cell type. The gel makes overcharging particularly destructive to this type of battery since any evolved gas bubbles (hydrogen or oxygen) will remain at the surface of the respective plate and so prevent electrolyte contact and hence can severely reduce the storage capacity of the battery. Gel-cells are used in the Solectra Force, a commercial conversion of the Geo Metro that is no longer in production.
Valve regulated lead acid (VRLA)
A version of this battery by Panasonic was used in General Motors' EV1 electric vehicle, and a version by Delphi in Ford Motor Company's Ranger EV. As with most other batteries, careful charging patterns are required to avoid overheating or loss of electrolyte. VRLA batteries contain catalytic materials to recombine evolved hydrogen and oxygen back into water before they escape to the atmosphere.
Foam plate and anti-sulphination coatings
A recent developent the carbon foam grid was initially developed by Caterpillar Inc. in an attempt to create a more robust battery for use in grading and construction equipment. Such equipment is often inactive during a portion of the year and is subject to strong vibrations when in use. This environment will require the frequent replacement of lead-acid service batteries. Carbon foam was being investigated by Caterpiller [for use in radiators when it was noticed by an in-house battery investigator and subsequently applied for use in the cathode and anode plates of flooded lead acid wet cells. The carbon foam has a very good resistance to repeated bending (a high fatigue life) and a very large surface area, both beneficial to Caterpiller's battery applications. A spinoff company, Firefly Energy was created for the commercial development of this technology. The first commercial product of this effort, a Group 31 truck battery, is expected in 2008, will be sold under the Oasis brand. As trucks are needing considerable energy for "hotel" loads to operate sleeper cabs when the truck is not in use the application is more akin to that of a deep-cycle marine battery rather than that of a simple vehicle starting load. The expectations are that this new battery have better than 40 percent additional capacity and will allow up to 800 full recharge cycles compared to the typical 200 cycles for a conventional Type 31 truck starter battery, and this while replacing only the negative plate with carbon foam. A more advanced application will replace also the positive plate, with projections that at least a third of the weight may be eliminated for the same storage capacity. Special coatings have also been developed by Firefly that reduce the sulfination effects of long discharged times. It is unlikely that batteries specifically suitable for electric vehicles will be available before 2010, and the current Group 31 battery will initially be available only for fleet use. Given the expected performance and cost parameters of this technology there is considerable interest in these developments within the EV conversion community.
Nickel cadmium (NiCd or "Nicad")
The nickel cadmium cells suitable for EV use are "wet" or "flooded" cells. The cadmium in a nicad is particularly toxic, far more so than lead. The battery does not use an acid electrolyte but rather a base, potassium hydroxide also known commonly as lye, used in processing food, making soap from fat, cleaning drains and many other applications. The electrolyte is hazardous to the eyes and can burn skin but is easily neutralized with common vinegar, which should be kept available when performing battery maintenance. Unlike acids, bases do not attack steel but are extremely corrosive to aluminum.
For long term use the higher cost may be well repaid owing to longer life. NiCads will typically have about 50% more energy for the same weight than lead-acid, and can be safely discharged to very low levels. Most wet Nicads also are said to lack "punch", not having the ability to supply high currents seen from AGM lead-acid cells.
Nicads also exhibit "memory", requiring periodic deep discharge (usually a series of three full discharge-recharge cycles) in order to regain battery capacity
This type of battery was applied commercially in the Th!nk City EV.
Nickel metal hydride (NiMH)
These have been used in some production vehicles such as some versions of the Chrysler TEVan, Ford Ranger EV, GM EV1, Honda EV Plus, and the Toyota RAV4 EV. They are also used in most hybrid vehicles, Their use in hybrid vehicles may result in substantial reduction in cost over the next few years (written 2005). As is well known by cell phone and some digital camera users these batteries have a high self-discharge rate.
These offer the highest energy density commonly available, although at substantial expense. They were used in the Nissan Altra electric vehicle. They are currently not cost effective for most converters but the technology is developing rapidly and this type is being used in portable power tools. Previously available versions of this type of battery would degrade over time even if not frequently cycled - exhibiting a" calendar life". Improvements are expected in the near future as indicated by the recent development of enhanced versions of this type specifically for application in hybrid vehicles by Panasonic. This type will be used in a third party conversion of the Prius hybrid to create a plug-in hybrid, expected to be available in 2006.
A new development in lithium-ion technology may be promising to EV conversion. A123Systems claims their new battery technology delivers up to 10X longer life, 5X power gains and five minute recharge. This new product also boasts a significant cost and weight savings vs. NiMH or conventional Li-Ion technology. The price seems comparable to high end lead acid batteries with developer kits available. Homebuilders should be cautious of new technologies however until the product is in the marketplace for some time. Dewalt will be offering this technology beginning in 2006 so this should give real world testing of this battery
The Tesla Roadster will be powered by LiIon, with a range over 200 miles, 0-60mph in 3.9 seconds, and top speed of 130mph.
Several considerations apply to selecting pack capacity, particularly when lead-acid cells are used. Lead-acid cells tend to degrade in capacity in a predictable manner. If the pack is undersized for the application it may perform satisfactory for a time, but due to loss of capacity may not provide sufficient range for its intended use, even though the batteries have substantial useful life remaining. Excess initial capacity will thus increase the service life of the pack. On the other hand, larger packs are not only more expensive, but have to be hauled, so excess capacity will both reduce economy of operation, acceleration and increase brake wear.
Lightweight vehicles not suitable for freeway use will typically use 24, 36, 48, or 72 volt battery packs.
DC propulsion systems in EV conversions intended for freeway use will usually be 96, 108, 120, 144, or 192 volts. AC systems will usually be 192 volts or higher.
96 volt and 108
Using wet cell golf cart batteries
96 volts, obtained using sing sixteen 6 volt wet cells* (used in electric golf carts) and a nine inch DC motor in a light truck this will give sufficient performance for around town and for limited (but not hasty) freeway driving. The total pack battery, holddown, and cabling weight will be about 16×65 lb = 1040 lb (470 kg), representing a substantial reduction in the vehicle's useful load carrying capacity. The nominal watt-hour capacity will be 225×96 = 21.6 kW·h, with a practical yield considerably less than this value. Nominal specific capacity is 20.8 W·h/lb (45.8 W·h/kg). With sufficiently high (120VAC mains) it may be possible to use a simple non-isolated charger to charge 18 batteries for 108 volts nominal. An 18 cell pack using Trojan T-145 or equivalent will offer over 140 percent of the system capacity of 16 T-105 or equivalent, for a substantial increase in range with about 130 percent of the weight. The higher capacity batteries will be less cost effective (by a factor of about .92), due mostly to the larger volume demand for the popular T-105 type.
- Calculations based upon Trojan-T-105 with two pounds allowance for cable and hold-down.
Using AGM batteries
With a light vehicle and eight 12 volt AGM batteries the performance should be substantially improved and quite suitable for short range use. Note that AGM batteries have a substantially higher peak current capacity and lower internal resistance than wet cells. The total pack battery, holddown and cabling weight will be about 8×46 = 368 lb (167 kg). This lower weight is much more suited to a lightweight vehicle than are the wet cells described above.
- Calculations based upon Optima D34/78-950 (yellow top) with two pounds allowance for cable and hold-down. Use of blue tops would result in a pack weight of about 8×62 = 496 lb (225 kg).
120 and 128 volts
These are configured using twenty 6-volt, ten 12-volt, or fifteen 8-volt (wet cell golf cart) batteries. These configurations were rare since other equipment (the pack to 12-volt converter) was not easily available in these input voltages, a problem now solved by some vendors. 120 volts is especially attractive since a pack may be constructed with twenty six-volt batteries. As suitable wet cell traction batteries in quantity are packaged as pallet loads of 40, two vehicle owners may purchase and divide a pallet with no leftovers. Since common controllers will typically operate between 96 and 144 volts the controller is not operating at the upper limit of its voltage range.
Under similar circumstances this should enable a DC conversion to operate with the acceleration normally obtained from the ICE original. If the same battery types are used one could expect about 40 percent additional range compared to a 96 volt system. Note that if six volt wet cells are used the additional eight batteries will weigh an additional 560 pounds (250 kg), representing a substantial decrease in load carrying capacity and in economy of operation. If more advanced battery technologies are used then the performance penalties are substantially less.
Using the popular Trojan T-105 battery a pack of 24 batteries (exclusive of cabling and tie-downs) would weigh 1392 lb (672 kg).
Where this voltage is desired in a somewhat lighter and more compact wet cell pack it may be constructed using eighteen 8 volt batteries. Using the Trojan T-860 a pack would weigh 1008 lb (600 kg). As the T-105 and T-860 batteries are the same size, the total volume required by the T-860 would use considerably less volume, 75% of a pack built with the T-105 (but would have only 66% of the 5 hour rate ampere-hour capacity. The nominal capacity will be 21.6 kW·h, so this pack will have the same overall capacity as that of a 96 volt pack using 6 volt T-105s, but will require two more battery stations. The added voltage will give an improved performance, but if used often that extra performance will have less range. As there is far less demand for eight volt batteries as compared to the common six volt golf cart battery, the cost per watt-hour stored will probably also be higher.
144 volts is also a common upper limit for voltage applied to certain commonly available mid-priced controllers such as some models of the widely used Curtis. Owing to the current limitations of that controller the main effects compared to lower voltage systems will be to increase the range and to ensure effective performance at deep discharge levels. Effective exploitation of the higher voltage to obtain increased acceleration requires the use of a specialized high-current controller, such as the Zilla.
This is becoming the conversion industry standard for high performance street vehicles such as sports car conversions. This voltage is not typically used with large wet cells owning to the high weight of the total battery pack (32×70 = 2240 lb, about 1000 kg), Instead usually either AGM lead acid (740 pounds or 340 kg for Optima Yellow Top) or wet cell NiCads (908 lb or 410 kg for SAFT STM).
Higher voltages are used in specialized vehicles such as drag racers and for some AC motor conversions. Since high voltage systems use substantially less current it is possible to use gel-cell lead acid, unsuitable for higher current applications as they are subject to permanent damage at high current flows.
Most performance systems use other than the "flooded" wet-cell golf cart batteries. Unlike the robust golf cart batteries it is not practical to rebalance the pack by overcharging as this can damage the batteries. Instead, additional circuits and controllers are added to connect batteries to distribute the charge to cells that need it. These circuits are currently (2005) available only for 12 volt batteries. Note that individual cells within the batteries are not balanced by this circuitry.
A charger appropriate to the battery technology installed must be used to ensure the maximum battery life.
While a charger may be installed where the vehicle is parked, most conversions use an "on–board" charger. Some chargers are "transformerless" and so are of light weight, but more sensitive to ground faults ( some of these may be avoided by a specialized charging circuit shown in a later section). Where a higher voltage (144 volts or more) is to be charged it may be most practical to divide the pack into two banks. These may then be charged using two 110 volt chargers, each of which uses one leg of a 220 VAC circuit (North American 220 volt circuits provide a two-way neutral that allows this).
A means of providing AC power to the charger is required. By providing an Avcon paddle port it is feasible to use power provided at malls and transit stations. Alternative methods allow the use of a long extension cord, either for home use or for "opportunity charging". Such opportunity charging must be done using a ground fault protected circuit with a properly connected ground wire.
If only a single 110 volt charger is used and it is desirable to use a public charging station then the builder should include a 220 to 110 volt transformer of sufficient capacity and appropriate electrical connections. The use of properly polarized "pigtails" (short connectors) or disconnect methods such as switches or switching circuit breakers must be used to avoid shock hazards. Do not attempt to design a connection system with "hot" male plugs.
A charging station is a source of AC ("mains") electricity used to power the AC to DC converter (the charger), which is usually installed in the vehicle. Most home conversions use a high capacity extension cord (U.S. wire gauge 10 or 12) plugged into a ground fault circuit interrupter (GFCI). Some EV owners obtain a commercial grade charging station such as is used at malls and transit stations. Some transit stations also provide four wire "twist lock" sockets for use by EV users.
Series DC (direct current)
The most common motor and controller combinations are modifications of air cooled devices originally designed for use in electric fork lifts (Curtis). A system with air cooled components is simple to install and maintain but is not suitable for high power operations. Similar controller and charger designs with liquid cooling are available for street and also extreme high performance applications (up to 2000 amperes) ('Zilla). Liquid cooling of components is especially suitable where liquid cooling is to be provided for NiCad batteries during charging, as multiple systems can use a common circulating pump and radiator.
- Low cost
- High starting torque
- Does not require particularly high battery pack voltage, enabling use of low cost flooded batteries
- Electrical and acoustic noise from commutator.
- Relatively inefficient compared to AC technologies.
- Radio interference from controller, unless higher frequencies are used
- Regeneration (battery recharging when slowing down) requires special design of the electronics to prevent excessive arcing at the motor brushes and overcharging of the batteries (Otmar). Although Curtis has designed and produces a regenerative DC controller for use in Europe, this model is not marketed in the United States.
- Limited speed range requires the use of a multiple speed transmission, and so also (usually) a clutch, frequently eliminated in production AC motor designs.
AC (alternating current) Induction
The conventional alternative to DC.
- No wearout mechanisms (i.e.brushes). No regular maintenance is required.
- Efficient when cruising. Overall efficiency exceeds that of Series DC.
- Effectiveness over large speed range allows elimination of transmission shifting, but with an overall performance penalty.
- Does not have the unfortunate failure mode of DC motor controllers of failing fully ON.
- Regen is very simple, usually comes at no cost.
- Use in newer commercial EVs will make this technology more easily available.
- More expensive, largely due to increased controller costs since there are typically three sets of regulating components rather than one as is used in DC systems. Most AC systems have been built by or for large corporations for research projects with little regard for price.
- Lower startup torque due to controller limitations. Induction motors have a similar current vs torque curve to series DC motors, which would be available if the controller were built accordingly.
- If no gear shifting is available it is important that the motor be speed-limited, which will limit vehicle speed to a maximum determined by the overall gear ratio and tire size. Single speed gearing suitable for high speed will limit low speed acceleration.
- Higher voltage demands require smaller batteries which have a higher cost per Watt-hour stored, more battery box locations, and more cables and terminators
Permanent magnet motors are now being used for auxiliary motors, low-power motors for ultralight EV use, and as assist motor/generators in the newest hybrid vehicles such as the newest Lexus SUV hybrid vehicle. This type of motor is expected to be used in the recently announced 2006 Toyota Camry hybrid. The brushless type of this motor is controlled using controllers similar to those used with AC motors. With appropriate control circuitry this type of motor may be used as a generator for energy recovery during braking cycles. Conventional brushed motors use the magnets to create the stator field, with a commutator carrying current to the coils of the rotor. High performance motors for EV use will instead use the permanent magnets for the rotor, with multiple stator coils driven by the controller. This eliminates the use of brushes and the commutator, a great advantage in increasing reliability and reducing maintenance.
On Board or Towed Genset
A genset uses some kind of fuel to make DC or AC electricity which is wired to charge the battery pack and provide power for the car's motor. A genset trailer mounts the genset on a trailer towed behind the vehicle. Such a system is a Series Hybrid. There are several practical problems building a series hybrid using a small, cheap genset:
- It requires a lot of power to push a car at highway speeds, usually in excess of 10kW. A generator big enough to give unlimited range at highway speeds must have a continuous rating greater than that required to push the car. Such a generator is usually expensive, large and heavy. A smaller generator will require periods of slower driving in order to catch up.
- Most generators have no emission controls. The pollution caused by the generator over and above the pollution caused by an alternative vehicle on the same trip may exceed the pollution saved during non-assisted trips. This obviously depends on how bad the generator is and the ratio of assisted to non-assisted driving.
- It is likely that any generator would have to meet the emission control regulations that apply to the vehicle's original engine.
- Most generators are not very efficient. If trips requiring the generator are rare, running costs may not be a concern.
- The connection between the generator and the rest of the system must prevent overcharging of the batteries and overloading of the generator.
The space, weight & money used by the generator system may actually be better used adding more batteries or improving the efficiency of the vehicle in other ways. Alternatively the money spent installing a generator could be spent hiring an efficient car for long trips. There has been at least one attempt to produce a commercial generator trailer, the [Rav Long Ranger] and a related project for the [tzero]. It wasn't based on a cheap off the shelf generator. A photovoltaic cell can also be used to increase the range.
A trailer packed with extra batteries is called a baset trailer.
A pusher trailer is a trailer with a motor driving its wheels, which pushes the "towing" car. Such devices are usually made from the driving end of a 2wd vehicle, where the donor's original motor, emission controls and efficiency are retained.