Hardware Hacking Projects for Geeks

16

Part I: Basic Hacks, Tools, and Techniques

Figure 1-2: Laptop power jack

Project Overview

This project uses a set of ordinary alkaline D-size batteries to power a lap-

top computer.

You will assemble a battery holder and cable and test out the project. The

battery pack you assemble will be customized for your laptop model. Before

connecting it to another model, you will also need to check for voltage

compatibility. When you’ve completed this project, the battery pack should

power your laptop for two to six hours.

Hardware Assembly Instructions

The power extender you build will be customized for your laptop. You will

need to check the voltage requirement for your laptop and assemble a suit-

able pack.

1. Determine your laptop’s voltage input

Look on the bottom of your laptop computer and locate

the input voltage specification. Figure 1-1 shows the loca-

tion on an IBM model T20 laptop. The T20 specifies 15V

DC input at 3.39 amps. (This may seem like a large num-

ber, but on average your laptop will not draw this much

current.) If this information is not listed on the laptop

case, look in the owner’s manual or on the AC wall power

adapter that came with the laptop.

If the input specification is in AC volts, this hack will not

work. AC volts are specified with either the letters “AC”

or a wavy line after the voltage specification.

2. Locate a suitable power connector for your

laptop

Look at the back or side of your laptop and locate the

external power jack.

The IBM T20 external power jack is a yellow plastic jack

on the right side. You can see it circled in Figure 1-2. On

other laptops, simply look for the jack into which you

plug the AC wall adapter.

Figure 1-1: Underside of laptop

Project Overview

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17

Chapter 1, How to Build a Portable Laptop Power Supply

Look closely next to the power jack and you will also see its polarity. This

will tell you how to wire up the power plug from your battery pack. There

will be a small inset diagram similar to Figure 1-3.

Radio Shack has a large selection of power jacks from which to choose.

Either purchase a few and use the one that fits, or take your laptop into the

store and try out several until you find one that fits snugly.

3. Determine the number of required batteries

In Step 1 you determined the required voltage for your laptop. Divide this

number by 1.5 to determine the number of alkaline batteries required to

power your laptop. Odd-numbered cells should be rounded up to the next

even number. Even- and fractional-numbered cells should be rounded up to

the closest even number. For example, if you have a 16 volt rating on your

laptop, the number of calculated batteries would be determined using the

following equation: 16/1.5 = 10.66. Rounding this number (10.66) up to the

closest even number gives you 12. This means you will need 12 batteries

to power a 16V laptop. Note that a laptop input power plug can typically

withstand a few volts over its maximum rated input voltage. The voltage

on the alkaline cells will also drop once the laptop begins to draw current

from it.

Rechargeable batteries can also be used. If you want your battery pack

to work with both alkaline and rechargeable batteries, you will have to

install a switch that selects between the two types. The pack will have to

accommodate a larger number of rechargeable batteries since they have a

lower voltage per battery. Calculate the number of rechargeable batteries by

dividing the required voltage by 1.25 for NiCad or NiMH cells. Round the

number up to the closest even number as you did for alkaline batteries.

Center is positive

Center is negative

Figure 1-3: Power jack polarity

Hardware Assembly Instructions

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