Battery Selection Criteria

Here's a look at the key characteristics to consider before you buy batteries.

Size & Weight

The more voltage, the more speed. Though the minimum for a highway-capable vehicle is 96 V, some systems go to 300 V or more. The physical size of the battery—volume per volt—will determine your top speed, so you'll need to figure out how many volts can fit into the space available and whether that voltage will be enough to reach your desired speed.

Look at volume per volt—how bulky should the pack be to give me the speed I want? Then examine watt-hour (Wh) capacity per pound (a.k.a. "energy density")—how heavy is the pack I need to give me the range I want?

For example, a 96 V pack of 6 V FLA batteries will take up approximately 7.35 cubic feet. A pack of the same voltage (i.e., yielding the same top speed) using 12 V SLA batteries would only take up 2.45 cubic feet. The flooded 6 V pack weighs about 1,072 pounds, while the sealed pack of 12 V units tops out at 350 pounds.

Saving space and shedding pounds may be necessary with smaller vehicles, but consider your range requirements first. The flooded pack has a 23.2 kilowatt-hour capacity compared to only 5.3 kWh for the sealed pack. So while the flooded pack weighs about three times as much and takes up three times as much space, it has almost four and a half times as much range capacity. The sealed pack will require more care in charging and will be almost three times the price for an equivalent amount of kWh, not counting the cost of the battery management system (BMS).


A battery pack is made up of many batteries interconnected to form one power source. The number of interconnects needed to achieve the series voltage will determine the ease of the job and the pack's overall reliability. Every time you double the number of connections, you quadruple the number of possible failure points.

The type of interconnect used is dictated by the battery terminal style. Clamps are used on round automotive-style terminals, while flat lugs are bolted to flat "L-style" terminals or sometimes directly into the top of the battery. Avoid using lugs over threaded studs sticking up from lead posts—lead is soft and allows the stud to gradually creep upward to relieve the tension of the nut holding the lug. If neglected for too long, this loose connection can melt off the battery terminal or start a fire.

To obtain the desired voltage and amp-hours, batteries can be configured in parallel or in series, or a combination of both.

• In a parallel circuit, positive posts are connected to positive, and negative posts are connected to negative. In this configuration, amp-hour rating is multiplied by the number of batteries and the voltage stays the same as in a single battery.

• In a series circuit (which is most common in an EV), batteries are connected positive to negative. The voltage is multiplied by the number of batteries, and the amp-hour rating stays the same as a single battery.

These two points initially won over some EV enthusiasts, but the technology has proved to be less than ideal for EV conversions, since the batteries are more expensive, harder to find in large formats, have higher self-discharge rates, and, ultimately, are more dangerous to use in traction applications.

If too deeply discharged and then charged too quickly, a reaction can occur in NiCds that generates heat inside the battery until it melts down or catches fire. Because of this risk, many NiCd batteries have been removed from service and replaced with lead-acid batteries.

NiCds also suffer from "memory" problems. If the battery is repeatedly discharged partially and then recharged, it will "remember" that partial level of discharge and act as if that level is its capacity. These batteries need to be fully discharged periodically to prevent this from happening, and require a BMS to properly charge and equalize the pack.

A lithium-ion battery pack in CalCars' EnergyCS/EDrive-converted plug-in Prius.

Battery Maintenance

All batteries should be checked periodically for clean and tight connections. In particular, FLAs require regular fluid checks and, when necessary, their electrolyte levels must be topped off with distilled water. FLA packs also need to be "equalized" to keep all the batteries in balance at the same state of charge.

Rate & Depth of Discharge

Battery life is related to two primary factors: (1) the rate of discharge—how quickly energy is drawn out of the battery (measured in amps); and (2) the depth of discharge—how low the voltage can drop before recharging (measured as a percentage). The less severe the rate or depth of discharge, the longer the battery's life cycle.

Some batteries can release large amounts of energy more quickly than others. There are also differences in how deeply they can be discharged without causing damage. If either issue is a high priority for your use, be sure to consider rate or depth of discharge when battery shopping.

Balancing & Management Systems

Imbalances among batteries are typical within battery packs—the same make and model can have small differences in voltages. Over time, these imbalances tend to grow, shortening the life of the pack, so regularly rebalancing or "equalizing" the pack is necessary.

When a battery pack is charging, some batteries reach full charge sooner than others. With equalization, the charging continues until the "weaker" ones catch up with the other batteries. The key is not to overcharge the "stronger" batteries, since overcharging can irreversibly damage or destroy a battery.

Some batteries—typically older, "low-tech" FLA batteries— can be balanced by a standard charger. To ensure safe and effective charging, most batteries require a BMS with a sensor/control for each battery that monitors the voltage and controls the charge going into the battery. More sophisticated systems may have circuits for each battery, while simpler versions have a voltage tap running from each battery to a voltmeter. Some require additional wiring and parts, while others are already built into the battery.

Typically, a BMS adds to the cost of the vehicle and the complexity of the wiring. Expect to spend half the cost of the battery pack, and be sure to allow extra time—and patience—for installation. Follow your battery manufacturer's recommendations for the BMS.

Physical Containment & Ventilation

All batteries must be physically contained, but some require additional measures. Some need to be tightly packed, while others need space between them. Ventilation may be required for carrying away fumes or cooling purposes. Conversely, some batteries may need to be kept warm in cold climates. These packaging considerations affect how many batteries will fit into the vehicle and where they can be placed, as well as the overall cost of the installation.

Toyota Prius Batteries

The nickel metal hydride (NiMH) batteries used in the Toyota Prius are designed to work in tandem with the factory enclosures and battery management system. While these batteries work wonders in a Prius, they should not be salvaged or adapted for use in EV conversions. These batteries were not designed to be the sole power supply for a vehicle nor to handle deep discharges; they are designed to be discharged a small amount and then sip a little recharge. If packaged improperly or charged at even slightly too high a rate, they could catch fire and possibly explode.

Unlike lead-acid batteries, which need only a little space between them, NiCds need to be firmly compressed to keep the electrolyte covering the plates. Like lead-acids, they need to have their fluid levels checked and topped off as needed, and tops wiped clean of electrolyte mist.

Because NiCds come in 1.2 V cells, they require more interconnects—five to 10 times as many for the same voltage of a 2 V lead-acid bank. This means more work when assembling the pack and less reliability in the long run, since doubling the number of connections quadruples the number of possible failure points.

NiCds do not need the insulation that lead-acid batteries need in cold climates, but they may need cooling ventilation in hot climates. Although NiCds store more energy and perform better in colder climates than lead-acid batteries, they are not recommended for use in conversions today because of the chance of thermal runaway.

Plastic Battery Mold A123
Exotic batteries often come in a variety of physical packages. Pictured here: individual lithium cells from Foxx.

Nickel Metal Hydride

In the 1990s, nickel metal hydride (NiMH) batteries were the "next big thing"—all the manufacturers used them in their EVs and hybrids. Not only can they pack the same voltage into a quarter of the volume of FLA batteries, and half to three-quarters of the volume of SLA or NiCd batteries, they also have much greater energy density, at 35 to 40 Wh per pound.

NiMH batteries do not have the memory problems of NiCd batteries nor do they require watering. They do, however, require a BMS for charging. The main drawbacks are that they are not sold at retail level and cost several times as much as lead-acid batteries.


Lithium-ion (Li) batteries are the "next big thing." Their claim to fame is much the same as NiMH technology, only more so—more capacity in a lighter package, with energy densities ranging from 30 to 95-plus Wh per pound.

Lithium batteries, which have lithium metal or lithium compounds as an anode, are available in different chemistries, but lithium phosphate is the chemistry most widely used in EVs. (See "Options" sidebar.)

Lithium batteries come in two basic shapes: cylindrical or rectangular (a.k.a. "prismatic"). Each shape presents different challenges for physical arrangement, interconnects, and thermal control. For example, the more uniform internal distribution of temperature in prismatic cells increases performance. Cylindrical cells may fit better in a shallow space, whereas prismatic cells stack better into uniform blocks, with less wasted space.

Lithium cells are relatively maintenance free, but they do require a BMS and ventilation for cooling, since high temperatures will degrade the batteries' performance and cycle life. Forced cooling with a fan is the minimum, but liquid cooling (with coolant flowing through tubing or jackets around and through the battery pack) is often recommended.

Despite their higher energy density and a cycle life that's about 2.5 times that of a lead-acid battery, Li batteries

Options in the Lithium Market

The first lithium batteries to appear on the EV scene were small, sealed cells much like flashlight batteries. At approximately 3.3 V per cell and a very low amp-hour capacity, it took about 4,000 of these batteries to power a car. That meant hours of welding connections and too many places for potential connection failure.

To simplify the process, A123 Systems offers lithium nanophosphate batteries in a "developer's kit" of six cells with connecting tabs attached. The drawback: Intended for "prototyping," the kits do not come with any technical support.

Other manufacturers are reducing the number of connections by building lithium cells into preconnected batteries. Valence Technologies—the best-known manufacturer of this type of lithium battery—made its batteries available to consumers starting in late 2008. Expect to pay $2,000 for a 12 V, 100 Ah (a 2-hour rate) module with a built-in BMS.

A rising newcomer is the Foxx battery, manufactured by Aten Energy, which can be built up into various-sized units. Expect to pay about $1,250 for a 12 V module.

have their challenges. Primary is their high cost—about 10 times the price of a lead-acid battery. Even with projected price drops, Li technology is still out of reach for most EV conversions.

Availability for retail sales is very limited and will likely remain so for the foreseeable future—most manufacturers are selling exclusively to vehicle manufacturers and continue to overlook the retail conversion market.

Lithium-based batteries are more sensitive to overcharging or overdischarging than any other chemistry. Under certain conditions, the batteries can catch fire. Manufacturers are,

A well-designed battery enclosure keeps batteries safe and secure.

A well-designed battery enclosure keeps batteries safe and secure.

An amp-hour meter is the EV equivalent of a gasoline tank gauge.

however, refining the design of the batteries, as well as the charging and battery management systems, to minimize the potential for catastrophic failures.

Because the technology is so new, data for cycle life, usable energy, and other performance specs are based largely on limited laboratory testing and extrapolation. Until these batteries have been on the road for a decade or more, manufacturers' specs are really just guesstimates.

Comparing Costs

Computing dollars per watt-hour is one way to compare different battery technologies. Take the basic unit in which the battery is sold, whether that's a single-cell or multicell unit, and multiply the voltage by the amp-hour rating to get watt-hours. Then divide the cost-per-unit by the watt-hours.

As a rule, avoid bargain or store-brand batteries. They may be supplied by multiple manufacturers and relabeled, making it impossible to get a matched, balanced pack. They're no bargain in the long run, since this imbalance will result in poor range and short cycle life.

Prices jump by an order of magnitude from one chemistry to another. Ultimately, you have to decide whether the advantages of a more expensive battery technology are enough to justify the added expense.


Shari Prange ([email protected]) is co-author with Michael Brown of the widely referenced book, Convert It: A Step-by-Step Manual for Converting an Internal Combustion Vehicle to Electric Power. She has been co-owner of Electro Automotive, a supplier of EV conversion kits, since 1983.

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