Sunday, May 16, 2010

Battery Article

Batteries, - a simple guide to practical usage (and a bit of supporting theory).

As a designer and manufacturer of battery belts and other associated power equipment, I have been asked to write an article about the benefits and disadvantages of the various battery technologies, with particular reference to the sort of applications used by readers of this journal. My notes here have been necessarily generalised, and are not necessarily related to any manufacturer's specific product, although I have used a few examples from manufacturer's product lists for comparison purposes. In particular the major manufacturers are continuously improving their products, so any cost, capacity or weight comparisons I make may not apply to the particular devices you are familiar with (SONY VAIO VGN-FZ Battery).

First of all, before we start, when dealing with batteries, please observe the following safety notes:

Do not dismantle any battery, most of the chemicals are poisonous and/or corrosive.

Do not dispose of batteries by trying to burn them, or otherwise get them too hot, since the chemicals will be forced out of the case, often violently.

Dispose of batteries (SONY VGP-BPS8 Battery) properly. Ni-Cads in particular are far from biodegradable or environmentally friendly, so you shouldn't just put them in the dustbin, and in fact the use of Ni-Cads are prohibited in some countries.

In my experience, most confusion with electrical matters stems from folk not understanding the basic concepts and terminology. If you appreciate the difference between an Amp (I don't mean an amplifier!) and a Volt, between series connections and parallel connections, own and know how to use a simple multimeter, then you will be able to understand the following notes and be able to avoid getting into all sorts of trouble with basic battery usage and maintenance(SONY VGP-BPS11 Battery), so read on....

Definitions

I will start with a few basic definitions, so that we can establish a common terminology so that we all know exactly what is being discussed. A battery consists of a number of cells. A cell may be either a primary cell or a secondary cell. Most common cells operate by converting a chemical reaction into electrical energy. A primary cell is a single use cell, i.e. it can not normally be recharged, it produces electrical energy until its active materials are consumed. A secondary cell can be recharged a number of times, to regenerate the active materials. In most cases, a battery consists of a number of identical cells connected in series. The nominal output voltage of a battery (SONY VGP-BPL9 Battery)consisting of cells connected in series will be the sum of the individual cell voltages. e.g. if you want 6 Volts from Ni-Cads (which have a cell voltage of 1.2V), then you will need 5 cells, if you want to use lead-acid cells, which have a 2V cell voltage, then you use 3 cells.

Batteries and cells are normally rated as, say, x V at a capacity of y mAh (mili-Ampere-hours). Sometimes the manufacturer will quote mWh (mili-Watt-hours) instead of mAh. This results in a larger number being quoted, and the naive customer will think he is getting a more powerful battery. All manufacturer's figures will be nominal, since for virtually every type of battery, the voltage will vary over time, even if no load is connected, and the voltage also depends on the exact state of charge of the battery(SONY VGP-BPS10 Battery), ambient temperature and other factors. However, to convert from mAh to mWh, then simply multiply the mAh rating by the battery Voltage (e.g. a lithium-ion battery with a nominal voltage 7.2V, quoted as 1500mAh could also be rated at 10800mWh or 10.8 Wh.)

There is another definition that needs to be understood, which is mainly used in discussions concerned with charging secondary cells - C.

1C is a number which indicates the amount of current when the rated capacity of a cell is discharged in one hour. In general, charging and discharging currents are expressed in multiples of C. C's unit of measurement is in Amps (Do not confuse this with the standard electronic abbreviation for a capacitor...., with units measured in Farads). For example, a safe charge current for many secondary cells is 0.1C, so for a battery (IBM Thinkpad T400 Battery) with a capacity of 2500mAh, a safe charge current would be 2500 x 0.1 = 0.25A . (It would be usual to charge at this rate for a period of 12 to 16 hours.)

To summarise the above - all batteries are not created equal. Reputable manufacturers will print the capacity on the battery case. They will also provide you with full specifications of their products. For comparison purposes convert capacity to mAh if given as mWh. The voltage normally stated will be a nominal voltage, the actual measured voltage will be different (HP PAVILION DV2000 Battery).

Capacity

As an example, we can consider a 6V battery with a capacity of, say, 1500mAh. Does this mean we can extract 1500mA over a period of one hour, or can we extract, say, 150A over a period of 36 seconds and still have a battery voltage of 6V at the end of the period? In fact it depends on the manufacturer's specification. Many manufacturers quote a five hour rate, which in the case of our six volt battery means it will give 300mA over a period of five hours. However, another factor to bear in mind, is that the final voltage will not be 6V, but will be 'the discharge end voltage' - defined as the limiting voltage when the battery (HP PAVILION DV2 Battery)is considered to have no residual capacity. The standard end voltage for most Ni-Cad batteries is 1.0V per cell, and it is this value on which the manufacturers base the capacity calculation, but the actual end voltage will be dependant on your particular application. If we substantially increase the load current, then the active materials are used less effectively, and the effective capacity will be reduced. For most batteries, a low ambient temperature causes the internal resistance of the battery(Dell INSPIRON 1525 Battery) to be higher, so the output voltage will be lower, also the active materials will be less reactive, so the capacity will be reduced at lower temperatures.

No comments:

Post a Comment