The Designer's Choice of Battery

•  Introduction

At first glance the designer's choice of battery may seem a rather overwhelming decision. In practice it is not that complicated, although choosing the correct size of battery may be. Firstly the designer needs to decide whether he/she is designing a vehicle which will use batteries that are currently available either commercially, or by arrangement with battery manufacturers for prototype use. Alternatively the designer may be designing a futuristic vehicle for a client or as an exercise, possibly as part of an undergraduate course. The designer will also need to decide on the specification and essential requirements of the vehicle. For example, designing the vehicle for speed, range, capital cost, running costs, overall costs, style, good handling, good aerodynamics, environmentally friendliness, etc.

Use of Batteries in Hybrid Vehicles

Introduction

There are many combinations of batteries, engines and mechanical flywheels which allow optimization of electric vehicles. The best known is the combination of IC engine and rechargeable battery, but more than one type of battery can be used in combination, and the use of batteries and flywheels can have advantages.

Internal combustion/battery electric hybrids

IC engine efficiency is to be optimized by charging and supplying energy from the battery, clearly a battery which can be rapidly charged is desirable. This tends to emphasize batteries such as the nickel metal hydride, which is efficient and readily charged and discharged.

Battery/battery electric hybrids

Different batteries have different characteristics and they can sometimes be combined to give optimum results. For example, an aluminium air battery has a low specific power and cannot be recharged, but could be used in combination with a battery which recharges and discharges quickly and efficiently, such as the nickel metal hydride battery. The aluminium air battery could supply a base load that sends surplus electricity to the NiMH battery when the power is not required. The energy from the NiMH battery could then be supplied for accelerating in traffic or overtaking; it could also be used for accepting and resupplying electricity for regenerative braking.

Combinations using flywheels

Flywheels that drive a vehicle through a suitable gearbox can be engineered to store small amounts of energy quickly and efficiently and resupply it soon afterwards. They can be used with mechanisms such as a cone/ball gearbox. They can be usefully employed with batteries that could not do this. For example the zinc air battery cannot be recharged in location in the vehicle, and hence cannot be used for regenerative braking, but by combining this with a suitable flywheel a vehicle using a zinc air battery with regenerative braking could be designed.

 

Battery Modeling

The purpose of battery modeling

Modeling (or simulating) of engineering systems is always important and useful. It is done for different reasons. Sometimes models are constructed to understand the effect of changing the way something is made. For example, we could construct a battery model that would allow us to predict the effect of changing the thickness of the lead oxide layer of the negative electrodes of a sealed lead acid battery. Such models make extensive use of fundamental physics and chemistry, and the power of modern computers allows such models to be made with very good predictive powers.

Battery equivalent circuit

The first task in simulating the performance of a battery is to construct an equivalent circuit. This is a circuit made up of elements, and each element has precisely predictable behavior. The equivalent circuit is shown in Figure 1 . A limitation of this type of circuit is that it does not explain the dynamic behavior of the battery at all. For example, if a load is connected to the battery the voltage will immediately change to a new (lower) value. In fact this is not true; rather, the voltage takes time to settle down to a new value.

In these simulations the speed of the vehicles changes fairly slowly, and the dynamic behavior of the battery makes a difference that is small compared to the other approximations we have to make along the way. Therefore, in this introduction to battery simulation we will use the basic equivalent circuit of Figure 1 .

Although the equivalent circuit of Figure 1 is simple, we do need to understand that the values of the circuit parameters (E and R ) are not constant. The open circuit voltage of the battery E is the most important to establish first.

In the case of the sealed lead acid battery we have already seen that the open circuit voltage E is approximately proportional to the state of charge of the battery. This shows the voltage of one cell of a battery. If we propose a battery variable DoD (depth-of-discharge) , meaning the depth of discharge of a battery, which is zero when fully charged and 1.0 when empty, then the simple formula for the open circuit voltage is:

..................................................(1)

where n is the number of cells in the battery. This formula gives reasonably good results for this type of battery, though a first improvement would be to include a term for the temperature, because this has a strong impact.