Batteries – What kind and how many?

Battery Comparison

By: Ian Hillier

Lead acid

Pros:

• Cheap
• Readily available. Because lead acid batteries have been around for so long, they have become widely used, mass produced, and easy to use.
• Proven technology. Lead acid batteries have been around since the beginning… they work.
• Larger selection of chargers (inexpensive). There are chargers being mass produced for forklifts, boats, RVs, and many other applications which would work great for an electric car. Because there is such a selection, we could get closer to what we want for a relatively low cost.
• Lead acid batteries are recyclable and therefore are not as bad for the environment as most people believe.

Cons:

• Heavy – lead acid batteries are about 3 times the weight of lithium.
• Large and bulky. This causes mounting problems. If we were converting a pickup truck it would not matter but since we are using a small sports car we have limited space. We would like to mount as many batteries under the hood and in the place of the gas tank but this will be like playing tetras with batteries.
• Limited life. Depending on the amount of use and how you treat them, lead acid batteries will become useless in approximately 0-7 years. If we are going to be paying $1000+ on a battery pack then we would expect at least a life of a few years. An expensive charger along with regular full charge will significantly extend the life of the battery. However, an electric car is very hard on its batteries so we could only expect a life between six months to three years.
• The oxidation reduction reaction occurring within a lead acid battery is strongly dependent (just like all reactions) on temperature. At low temperatures, the reaction does not happen fast enough to produce the needed current. This means the car will not function as well in the winter as the summer.
• Charge time. A lead acid battery can be discharged approximately five times as fast as it can be charged. To properly maintain a lead acid battery, it should be charged using a staged intelligent charger and should be fully charged regularly. 70 percent of the charge occurs in the first half of the charge time. Because of this, it is easy to neglect them by not allowing a full charge which is essential to the life of the battery.

Nickel metal hydrate

Pros:

• Lighter then lead acid batteries.
• Much faster charge time but require intelligent chargers which would be expensive.
• Are made from environmentally friendly materials.
• Higher capacity.

Cons:

• High self discharge rate (over 30% per month).
• High internal resistance along with a large voltage drop when supplying high currents.
• Lower energy density then lithium ion batteries
• Expensive. For the price, lithium ion would be better.

Lithium ion

Pros:

• About a third of the weight of lead acid batteries.
• Offers high current output capabilities with a low voltage drop.
• Can be formed into a variety of shapes to fill the available space. This applies more to small electronic devices where only one cell is used. Since each cell is significantly smaller then a lead acid battery it would be easier to place effectively.
• Low self discharge rate (approximately 5% per month)
• Are low maintenance and do not suffer from any kind of memory effect.
• High energy density.
• Much faster charge times then lead acid batteries.
• Almost 100% charging efficiency because they do not heat up while charging.

Cons:

• Limited shelf life. A lithium ion battery will age by increased internal resistance and therefore lower current outputs and lower capacity. For our purposes we can expect a life of between 1-3 years.
• Require built-in safety devices which take up valuable space within the battery:
  • Shut-down separator (for over temperature),
  • Tear-away tab (for internal pressure),
  • Vent (pressure relief), and
  • Thermal interrupt (over current/overcharging).

• Require specific charging techniques. The manufacturer’s guidelines should be followed.
• Expensive to manufacture, expensive to buy.
• Fairly new technology still

Conclusion:

We have decided to use lead acid batteries. This is because they are inexpensive and the best choice for a first conversion. Lithium batteries would have significantly higher performance but as we are students we don’t have $10,000 to spend on batteries. Lead acid batteries will also give us the opportunity to upgrade in the future as they are a relatively low-cost investment (compared to lithium ion).

 

~ by James on March 18, 2009.