The Mobile Power Supply - an important Aspect of any Mobile System Design

Sven Bauer, General Manager BMZ and Neil Barrack, Marketing Manager Gleichmann Electronics UK Ltd.

Often we are unaware of how many batteries we actually use every day and the complexity of the battery system behind it, sometimes even using dedicated electronic components. Some examples: Rechargeable cells in our mobile phone, notebook, PDA and MP3 player, or primary cells in our calculator or alarm clock. One battery that commonly causes problems is the Lithium primary cell in our PC - when it suddenly dies and we lose vital information and are not informed that it should be replaced.

More and more Embedded Systems are going mobile, or use a battery backup in case the mains supply fails. Many options have to be considered early in the design cycle. High energy batteries are now designed into power tools, motorized golf caddies and bicycles. There are a wide range of cells available, each with different properties. As a leading battery pack assembler we have experience with many different cell suppliers and can help you to design an optimized solution for each application.

Many of the more advanced mobile applications would not exist without the low weight of high energy density battery technology. Table 1 shows an overview of the main rechargeable technologies.
One important aspect is often forgotten:- we are dealing with chemical reactions in the charge and discharge cycles, not with silicon. Experience with battery system design is mandatory for complex battery solutions.
Many battery aspects might apply, including:

- Voltage - Capacity - Battery system weight - Temperature range - Technology used - Lifetime expected - Number of charge/discharge cycles - Peak current - Average current - Charging time - Charging current - Charging Profile/ Behaviour over time - Minimum discharge voltage - Intelligence/ communication to the micro - Long term availability - Cost

For systems using primary cells the selection is easier. Average current, maximum current, voltage, capacity, weight and cost lead relatively quickly to a decision of which type to use.
The picture is more complex if the mobile embedded system requires a rechargeable custom battery solution, using the latest Li-Ion technology to reduce weight as much as possible (see table below).

Lithium technology
For new designs two main technologies are usually considered: Lithium Polymer and Lithium Manganese. Comparative advantages of these technologies over NiCd or NiMH, looking at a 12V / 3Ah Li-Ion solution are: about 40% less weight, 540g compared to 1kg in Nicad or NiMH, better performance at low temperatures, only 4 cells in series, up to 1000 cycles and a low self-discharge rate.

Li-Ion cells do have a very high energy density and must be treated well to achieve maximum lifetime and safety. One 3.6V Li-Ion cell is enough to run a mobile. A charging system is required to achieve maximum capacity and lifetime. However, if the capacity of one cell is not sufficient, more cells can be connected in parallel to achieve higher capacity and this is where the problems start.
Every cell is unique from a manufacturing point of view. Tolerances can vary depending on whether fully automatic manufacturing or more manual processes are involved. Different cell manufacturers will specify different tolerances in their data sheets.
Li-Ion battery packs are particularly sensitive when cells are used in parallel. The weakest link applies: The cell with the highest self-discharge rate will impact all of the other cells in parallel.

Cells connected in series
A similar issue applies for higher voltage requirements where some cells are connected in series. Four Li-Ion cells in series will give 3.6V x 4 = 14.4V. As each Lithium Polymer cell is a bit different in behaviour and capacity, the charging system has to measure each cell and adapt individual cell currents by shunting past the cell. Special ICs have been designed which are connected to all cells either directly or via transistors/MOSFETs to insure optimal charge. For more complex requirements or higher voltages, customised microprocessor-based control boards are attached directly to the cells.

Some battery systems have to indicate the actual voltage and charge left to allow for the embedded system to adapt. In such cases the charge and discharge energy has to be recorder by measuring charge/discharge current and voltage, with communication via a single wire bidirectional bus, resulting in, for example, the charge level displayed on our mobile phone.

Li Ion Manganese exhibits one unique advantage when designing custom packs, especially when higher capacity and voltage are required. Custom selection enables the direct connection of cells in series and parallel without individual cell charge balancing. Li-Ion Manganese battery packs using 4S10P (4 in series and 10 in parallel) are in production now, achieving 14.4V at a capacity of 16Ah for motor drive applications.

All of the considerations above should be reviewed at the beginning of the design cycle. Otherwise you could find that the energy requirements of the target system are high enough so that the planned battery pack no longer fits or is prohibitively expensive and the electronics have to be adapted.

Many battery pack solutions are like custom ASICs. They are designed to meet unique application requirements, often with a custom PCB in a custom casing purchased as a complete solution.
Make sure that all of the requirements are checked early in the design cycle to avoid "adaptations" later. Talk to the battery pack manufacturer as you often want supply of the complete battery pack in the right case, including the custom charge/discharge electronics. They can advise on technology and required charge/discharge electronics. If you have any questions please contact miltonkeynes@msc-ge.com.

Rechargeable Battery Technology Overview

	Lead Acid	Nicad	Nickel Metal Hydride	Lithium Polymer	Lithium Ion Manganese
Energy Density (Wh/kg)	30 - 50	45 - 80	60 - 120	100- 130	110 - 160
Life Cycles (to 80% Capacity)	<100	1500	300 - 500	300 - 500	up to 1000
Overcharge Tolerance	high	moderate	low	very low	very low
Cell Voltage	2V	1.25V	1.25V	3.6V	3.6V
Operating Temperature	-20 to +60ºC	-40 to +60ºC	-20 to +60ºC	0 to +60ºC	-20 to +70ºC

(Source: www.batteryuniversity.com) Values given are for guidance only. Application requirements will define battery pack behaviour over time.