Batteries

Vehicles entered in the eV Challenge must only be powered by a set of batteries. The batteries must not be changed or charged during the course of the event. However, spare batteries may be brought along for use during pre-event and post-event testing of the eV.

The main rules regarding the batteries:

• must be of sealed construction with a maximum (nominal) battery bank voltage of 36V.
• the maximum battery storage allowed is 432Watt-hours. This equates to 6V at 72Ah, 12V at 36Ah, 18V at 24Ah, 24V at 18Ah, or 36V at 12Ah.

Battery Capacity, Discharge Rates and Energy Storage

Batteries are usually rated by their capacity or how much energy they store. This is given with a voltage rating and an 'amp-hour' rating and is printed on the side of most batteries. Sometimes it is also given by a 'watt-hour' rating and this is simply equal to the voltage rating x the amp-hour rating: Wh = V x Ah. For example, a battery might be rated at 12 volts (nominal) and 18 Ah capacity and this is equal to a total storage of 216 Wh (12 V x 18Ah).

Now the capacity of a battery is a funny thing and will vary depending on how fast the energy is taken from a battery. If all the energy is sucked out of the battery very quickly, the battery will heat up and lose efficiency. If the battery is discharged slowly, then it will stay cooler and the chemical reactions will happen more efficiently, giving out more energy as it does so. For example, if we discharge a battery in 1 hour and we measure how much energy we get out, then we say that the battery's rating at the 'C1' rate (discharged in 1 hour) is such-and-such. At a rate in which the battery will be completely flat in 10 hours, we will get another rating called the 'C10' rating and this will be a bit more than the C1 rating. For example, a 12 volt battery might be tested at the C1 rating and be found to have a capacity of 30 Ah or a total energy storage capacity of 360 Wh. At a C5 rating it's capacity might be measured to be 40 Ah (480 Wh) and at a C10 rate it will be 50 Ah (600 Wh). Going even further, at the C100 rate - a very slow rate where the battery is completely flattened over about 4 days - the amount of energy that we can get out might be 70 Ah, giving a total amount of energy of 840 Wh.

What does the 'nominal' voltage mean? With a battery, the voltage will vary depending on whether it is fully charged, discharged, charging, discharging and how fast these are all happening. A '12 volt' battery will be fully 'flat' but still show 10 volts on a meter. A fully charged battery with nothing connected to it will show 13.8 volts or maybe even a bit higher. With a small light-bulb connected, it might show 13.5 volts but with a big load connected - say a car starter motor drawing a lot of power, the voltage may show 11 volts. With a medium load, the battery will show about 12 volts, so that's why we call it a 12 volt battery.

What types of Batteries can be used in the eV Challenge?

There are a number of different types of batteries available and these include LEAD-ACID, NICKEL-CADMIUM, NICKEL-METAL HYDRIDE and Lithium-ion types.

Lead-Acid types are the most common type of large storage battery. They have sheets of lead and lead compounds in an electrolyte bath of sulphuric acid (H2SO4). The lead and lead compounds is very poisonous and the sulphuric acid is poisonous and can cause serious chemical burns. This is why only sealed batteries may be used in the eV Challenge.

A 12 Volt 25 Ah Sealed Lead Acid Battery

There are three main types of Lead-Acid batteries:

• Car batteries - designed to provide very high currents for a few seconds to start a car engine.
• Float batteries - designed to occasionally supply a small current to power equipment that has only small, infrequent power demands such as alarms and emergency lights.
• Deep Cycle batteries - designed to supply small to medium currents for frequent use such as for electric vehicles, renewable energy systems and back-up and uniterruptable power supplies.

Car batteries and float batteries can be used in the eV competition but are not recommended as they have only a very limited life - often only 20 charge/discharge cycles when used in electric vehicles. Deep-cycle batteries may last for hundreds of cycles. For an example of what is available, have a look at Siomar Battery Industries or Batteries Plus.

Nickel-Cadmium (NiCd) batteries have plates of nickel and cadmium in an electrolyte bath of Potassium Hydroxide (KOH) and Lithium Hydroxide (LiOH). Cadmium is very poisonous and potassium and lithium hydroxide are poisonous and also cause chemical burns. Nickel-Cadmium batteries are available as either small sealed cells - AAA, AA, C, D and other sizes and also as larger cells and batteries. The larger ones are used in electric vehicles, in renewable energy systems and uninteruptible power supplies.

Compared to lead-acid batteries, NiCds store more energy for the same size and weight - alternatively, the same amount of energy can be stored in smaller and lighter batteries compared to lead-acid batteries. NiCds are also more expensive than Lead-acid batteries and cannot be charged using lead-acid battery chargers.

Nickel-Metal Hydride are similar to Nickel-Cadmium but store even more energy for their size and weight.

There are many other types of batteries such as Lithium-Ion, Nickel-Iron and so on, however due to the cost of these they probably can't be used in the Murdoch University eV Challenge.