22Battery performance existing systems

All battery technologies can offer some solution to the peak power problem but there is only one parameter which ultimately matters, and this is the internal resistance of the cell. This is much more related to cell geometry than cell chemistry, as we shall discover. When AA, C, D and F cells were originally designed, nobody was thinking of discharging them at hundreds of amps, so it is hardly surprising that they are not ideal for the purpose. This problem will become even more extreme as power density improves:

D cell characteristics

Lead Acid

NiCad

NiMh

Lithium

Aluminium

Amp Hr (20hr)

2.5

4.0

6.5

7.5

50

Cell voltage

2.0

1.2

1.2

3.6

3.0

Max C rate

40.0

25.0

11.0

40.0

Unknown

Ah/25°C

0.5

3.0

5.0

6.0

Unknown

The first volume hybrid electrics in the market came from Toyota (Prius) and Nissan, Fig. 2.1. Toyota uses a 288 V string of NiMh D cells to give a peak power of 21 kW. The battery is mounted in horizontal strings of 6 cells in a matrix weighing 75 kg and is made by Matsushita Panasonic EV Energy. Nissan has worked with Sony who have developed the lithium ion battery, of AEA Technology (UK), into a high performance D cell, of 3.6 V 18 Ah, with a peak power of 1700 W/ kg. (This cell is 250 mm long x 32 mm OD and includes an electronic regulator.) Both solutions use advanced thermal management and both deliver the performance but at high cost. The task in hand is to reduce costs by 50%.

2.2.1 BATTERY PERFORMANCE: ALTERNATIVE APPROACH

Question: Which application gets the best peak power battery performance at present? Answers: Portable power tools and model aeroplanes; chemistry: nickel-cadmium; cell size: RR/C 1-2 ampere hour; peak discharge: 30° C; maximum current: 32 amps limited by connections (tags),

Toyota Prius battery pack

Principal specifications

Characteristics of nickel metal-hydride batteries for HEVs

Item

Battery module

Battery pack (example)

Battery construction

Nominal voltage

Nominal capacity

Energy

Power

Regenerative power

Weight

6 cells in series 7.2 V

4G modules in series 288 V

75 kg

Battery pack (example)

Modules

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Example of battery system for HEVs (a)

Fig. 2.1 Production hybrid battery technologies compared: (a) Toyota Prius; (b) Sony/Nissan.

i Power |=J> Detected date

Example of battery system for HEVs (a)

Fig. 2.1 Production hybrid battery technologies compared: (a) Toyota Prius; (b) Sony/Nissan.

Example of battery pack system for HEVs (Toyota spec.)

Fig. 2.2. The performance issues of these cells are complex. Amazingly there are 14 different types of sealed Ni-Cad cells in four main product groups: (a) standard, (b) high discharge current, (c) fast charge and (d) high temperature.

It is the double-sintered fast charge cells that are used in model aeroplanes and in the world championships the Sanyo Cadnica is specified using a 1.7 Ah cell. A Tamiya connector can deliver up to 32 amps. Battery packs of 7.2 V six packs are usually fitted. In the power tool area, Black and Decker have the Versapack, a three cell string. Two or four strings are used in series with larger cordless power tools. One problem is that sealed Ni-Cad cells cannot be connected in parallel. At the end of the charge cycle, the cell voltage falls, causing rapid charge/discharge between parallel cells. On the face of it, the packaging problem is daunting, with more than 2000 cells in a pack.

Sony/Nissan battery pack

Specifications of prototype cell

Rated capacity (4.0 V) Cell shape Dimensions, mm Weight

22 Ah

Cylindrical D50x L250 1.2 kg

0 0

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