This page covers some of the issues that I took into account when deciding on what kind of batteries to use in my Toyota MR2 EV conversion. Note that there are a lot of promising new battery technologies, including nano-structure lithium batteries and radical new Lead acid designs, but until you can buy them, I will be ignoring them for the purpose of this discussion.
Properties of Batteries
There are a variety of issues to be considered when looking at battery options for an EV. More reading available by following links on the EV Reference Material page. A brief summary follows:
- Specific Energy: This is the amount of energy (usually measured in watt-hours or kilowatt-hours depending on the size of the battery) per unit weight. For example, lead acid batteries have a nominal specific energy around one kilowatt-hour per 20 kilograms (45 pounds) of weight, or around 50 watt-hours per kilogram. Note: this is the same kilowatt-hour that is the unit of energy that your electric utility bills you for, and the meter on the side of your house reads out in. All else being equal, higher specific energy (like most other battery technologies have) is better. Unfortunately, it comes at a cost. More on that to follow. Lead acid batteries have an annoying habit of losing effective specific energy at an exponential rate with a linear increase in load. This effect can be modelled with a formula called Peukert's law. This effect is very real in an EV application and can halve (or worse) the effective capacity of the battery. Lithium and other battery types don't suffer from this significantly.
- Specific Power: This is how many watts (or kilowatts) of instantaneous power that the battery can deliver. It would be useless in an EV to have a battery with massive specific energy if that energy were only available at a trickle. (Analogy: you have a 5000 gallon water tank, but does the water come out as a drip, or a torrent?) Lead acid batteries usually have a specific power (in watts) of a couple times their specific energy (in watt-hours). Some types of lead acid are better than others in this regard, but most are sufficient for a typical EV application so long as the battery is appropriately sized. Other battery types can deliver much higher specific power, so these (such as Lithium-ion batteries) are used in EV dragsters and such.
- Cost: This is a no-brainer. Often given in dollars per kilowatt-hour. at $50 to $200 retail per kWh depending on the type, Lead acid is by far the cheapest option. The most expensive lead acid batteries are still 1/2 to 1/3 the cost of the next cheapest battery type that is even available, and will require less peripherial stuff, which also adds to the cost (and weight). Note that overall lifetime (cycle life) affects cost in the long run. If battery A cost twice as much, but lasts 3 times as long as battery B, then (in the long run) battery A is cheaper. Some battery types are not available in EV-appropriate form at any price for various reasons.
- Form Factor: Will the battery you want to use even fit in the car? Lead acid traction batteries are about twice the size of your typical car battery, and you will need many of them. Sodium batteries come in a large prepackaged module. Lithium and Ni-Mh batteries that are avaliable are individually small, but many will be required, and the interconnects between them take up space too.
- Lifetime: How many times can the battery be charged and discharged? This will of course vary with how it is used and maintained. For lead acid, the number is usually quoted at 500 to 1000 cycles. Lithium and Ni-MH are in the 1000 to 2000 range. Of course, the battery won't just die at its cycle limit, what will happen is a gradual degradation until the battery capacity is not sufficient for adequate range and/or performance. Note that lifetime affects cost in the long run.
- Environmental/Application issues:Will the battery be able to operate in the environment of an EV? Can it handle the temperature variations, vibration, humidity, etc? Lead acid batteries lose effective energy capacity at lower temperatures. Other battery technologies (Lithium-ion and NiMH) don't suffer from that problem, but can be damaged or destroyed by heat buildup. Sodium batteries must be kept at extreme temperatures to operate.
Types of Batteries
There are a wide variety of battery technologies out there that could be used in an EV, some of which I summarize here. Unfortunately, most suffer from Cost, Availability, and/or Care-and-Feeding issues. See the EV Reference Material page for links to more information on these battery types.
- Lead Acid Batteries: Relatively low specific energy, Cheap, Ubiquitous, Forgiving, Fairly durable. Annoying habit: effective capacity is reduced by extreme temperatures and extreme current draw, both conditions can occur in an EV. Some types should be used with basic charge management for best results.
- Nickel-Cadmium Batteries: Moderate specific energy, Fairly durable, good cycle life. Expensive and hard to find in sizes appropriate for an EV.
- Lithium-ion: Powerful and Relatively lightweight. Expensive as hell. Good cycle life. Requires Sophisticated battery management to control charge, discharge, and temperature. Difficult to find cells big enough to use in a car without resorting to massive parallelism. Thermal runaway is a frightening possibility if abused.
- Lithium-polymer: Like Lithium-ion, but even more powerful, more expensive, and harder to find.
- Nickel-Metal-Hydride: More energy dense than Lead acid, but less so than Lithium cells. Requires sophisticated charge and temperature management. Used in production electric vehicles from the 1990's like the GM EV1. Good durability, some of those 10-year-old EV1 packs are still going today in surviving EV1 S-10 pickups. Patent holders are currently preventing large NiMh batteries from being manufactured or sold in any quantity in the US, so appropriately sized cells are basically unobtainium for now.
- Sodium-Sulphur: High specific energy, moderate cost. Used in some european EV's. But must be kept at extreme temperatures (around 300 degrees farenheit) to operate. They "freeze" solid at normal temperatures. Sold as the "zebra" battery with integrated battery management. Due to temperature management there is significant self-discharge (from the heaters). Must be able to find a place in the car to install a large battery module.
Lead Acid Batteries
Given all of the above, Lead Acid batteries are really the only options for me because:
- They are the cheapest. My eventual pack of 17 Trojan T-875's will probably cost someplace between $2000 and $3000. An equivalent Lithium-ion pack would cost tens of thousands, not counting all the extra stuff you would need to manage them. An equivalent sodium, NiCd or NiMH battery might cost $10000 if available. I might be willing to consider if the lifetime were sufficient and other issues with that type of battery were sufficiently well mitigated, but for getting started, lead acid is far and away the winner in this category.
- The form factor is good. I can cram 17 of these into the car, and still keep the trunk free of batteries. The interconnects won't be too complicated, unlike with a lithium or Ni-MH pack.
- Lead acid (flooded) batteries do not require a battery managment system. This reduces complexity considerably. They will need regular maintenance in the form of cleaning and watering, but that is relatively easy and very cheap to do. (costs incurred: baking soda and distilled water)
- While lead acid batteries don't compete with more modern battery types in the specific energy department, the other categories are clear winners for Lead acid.
Types and Applications
However, just deciding on lead acid not the end of the story. There are many types of lead acid batteries, made for various purposes:
- Starting Battery: Designed for high specific power for cold-cranking an engine. Sacrifices cycle life in the process. Repeated deep discharges will destroy the battery (deep discharge is not incurred by a few seconds of cranking, and will be recharged by the alternator before the next start). Due to cycle life, not appropriate for EV's, except maybe a dragster.
- Deep Cycle Battery: Designed for deep and repeated discharge cycles, such as RV and energy storage applications. They are used in some EV conversions, but since they are not really designed for large current draw their life can be cut short in an EV application.
- Marine Battery: A compromise between starting batteries and deep cycle batteries. Possibly appropriate for an EV but homework must be done.
- Standby Battery: Used for backup power supplies and emergency power applications. Designed for long lifetime, and possibly large capacity, but repeated discharges and high current drain are not the intended applications.
- Traction Battery: Designed for high current drain and repeated, deep discharge cycles. That can mean it is bigger and heavier than other types of lead acid for the same voltage and energy capacity, but that is the cost of durability. Intended for golf carts, forklifts, industrial machinery. The best option (in my opinion) for a roadgoing EV.
There are several different lead acid battery technologies. Some technologies are more common than others for a given applications. For example, Most automotive starting batteries are sealed units. Most traction batteries are the flooded type. Performance-oriented automotive batteries (deep cycle and starting) are typically AGM technology. Standby batteries are almost always gel cells.
- Flooded Battery: Highest maintenance of the lead acid battery types. Also the most forgiving. There will be a cap that can be removed, through which water can (and must) periodically be added to top off the liquid electrolyte. Can withstand some overcharging without damage as long as proper maintenance is done. Most common type in large batteries like traction batteries.
- Sealed Battery: Basically the same as Flooded, but is sealed to prevent water loss under normal circumstances. Overcharging can cause gas to escape, permanently reducing the battery's capacity. Too much of that will ruin it, whereas with the flooded one you would have just needed to add some water to be back in business. These can be used in an EV, and there are sealed traction batteries available, such as the Trojan TE-35, but more careful charge management must be considered for optimal results.
- Gel Battery: Instead of liquid electrolyte, these have a gelled electrolyte. This makes them cleaner and less apt to make a mess. It also reduces their instantatenous energy capacity to a point where they aren't a good choice for an EV. Overcharging can also damage or destroy them.
- AGM (Absorbed-Glass-Mat) Battery: A newer type, often confused with Gel batteries. The best specific power for lead acid, and an OK cycle life makes them a common choice for performance-oriented EV's and lightweight cars. Thse are robust physically. They are the most expensive lead acid type, and also they tend to have a lower specific energy than flooded and sealed batteries. The most well-known brand here is Optima.
Flooded Lead Acid: My Choice
So, given all that, I have chosen to stick with the old standby: Flooded Lead Acid. There are a multitude of traction batteries available in this technology. It is the most forgiving, requiring the least complex battery management (checking the water now and then). It is the cheapest of all the Lead Acid technologies as well.
First Batch of batteries: 16 Trojan T-105s
|First batch of batteries: Sixteen Trojan T-105's|