46 Part I — Hardware Hacks

battery. If this is battery is run down, then the device may not work properly and will need repairing. Therefore, when storing a GPS unit with this kind of backup battery, always be sure to fit new (or recently recharged) batteries and check them regularly.

Never put a GPS receiver that contains old batteries into long-term storage, as this increases the risk of damage from leaking batteries.

Battery Do’s and Don’ts

The following guidelines will help you get the most from your batteries:

Recharge NiCd and NiMH batteries as soon as possible after discharge to maintain peak performance.

Never store batteries loose in a bag or pocket, which increases the risk of a short circuit that can result in fire or injury. Store batteries in a proper battery box or holder.

Store batteries at room temperature. There is no need to store batteries in a freezer or refrigerator to maintain peak performance.

Never use batteries after their expiration date.

Never dispose of batteries in a fire, as this can result in an explosion and cause serious injury.

Extreme temperatures reduce battery life. Keep battery-powered devices away from extreme heat or cold.

Take care that you insert batteries properly into your device. Some devices that use three or more batteries might still function with one battery inserted incorrectly, but this can cause battery damage that can potentially damage your device.

Dispose of used batteries responsibly. For online guidelines visit www.duracell.com/care_disposal/disposal.asp.

Power Hacks

There is no doubt that having auxiliary power (in addition to the internal batteries) is handy when out and about with your GPS. The easiest way to accomplish this is to carry a stash of batteries and replace them as they run out. This method, while effective, is very expensive and not very eco-friendly.

When in a vehicle, another option is to use a power cord that draws power from the automotive 12-volt system via a cigarette lighter socket. Chapter 2 describes these kinds of cables and how to make them.

Refer to Chapter 2 for details about how to build your own power cord cables.

Chapter 3 — Power Hacks 47

But what about when you are on the move? How can you supply power to your GPS receiver when walking, hiking, or geocaching?

Carrying Your Own 12-Volt Power Supply

One thing that you can do is carry your own 12-volt power supply around with you. No, this doesn’t mean having to lug a heavy car battery that can spill sulphuric acid all over the place! Other options are available to you.

One thing you can do is buy a portable lead-acid battery designed for large flashlights. They contain a cigarette lighter socket and often come with a case and belt loops. These batteries usually have fuses, as shown in Figure 3-5. They come in a variety of sizes; generally, the larger the battery the longer it lasts. The downside, however, is that the larger the battery, the heavier the battery.

These packs will give you hours of additional battery time and the batteries can be recharged when you get back to your vehicle by plugging them into the cigarette socket. Charging back at base is via a charger that plugs into a mains power outlet.

However, you might have handy batteries suitable for this job lying about. If you have an old PC uninterruptible power supply (UPS) unit lying around (the kind of thing that any selfrespecting geek would have!), these typically contain a battery similar to the battery packs that you can buy for flashlights, as shown in Figure 3-6.

FIGURE 3-5: A fused 12-volt lead-acid battery

48 Part I — Hardware Hacks

FIGURE 3-6: A lead-acid battery from a UPS device

Usually, you remove the battery through a hatch at the back of the UPS unit (remember to disconnect it from the mains power supply first). Once you have the battery out, you will need a few things to complete the build:

Wire (automotive wire like the type used to wire in spot lamps is ideal)

A cigarette lighter socket

A fuse and fuse holder (such as those used in an automotive spotlight or in car stereo parts; a 2A fuse will do just fine)

Connectors to fit the connectors on the battery (sizes will vary and depend on the battery)

A small car charger (to charge the battery when not in use)

Some sort of case for the battery (heavy-duty nylon or cordura( works just fine)

Assembly is easy. Wire the cigarette lighter socket to the battery, negative (–) to the frame of the socket and positive (+) to the center post, remembering to add a fuse holder with the fuse in the wire. Test the whole circuit with a multimeter before plugging a GPS power cord into the socket.

Chapter 3 — Power Hacks 49

These small lead-acid batteries are extremely handy auxiliary power packs and keep their charge for a long time in storage (recharge them every three months when not in use). A sticker on the side of the battery (see Figure 3-7) is ideal for keeping track of charge dates!

Lead-Acid Battery

Charge history

Last charge . . . / . . . / . . .

Next charge . . . / . . . / . . .

FIGURE 3-7: Suggested information to keep on the battery

When you are outdoors, take great care when carrying a lead-acid battery. Any damage or crack in the battery will leak dangerous, corrosive sulphuric acid.

If you want an alternative to lead-acid batteries, the following section describes how you can make simple battery packs from ordinary batteries (or rechargeable batteries).

Battery Packs

Making a battery pack isn’t hard. All you need is a holder for the batteries and some way to wire that to the GPS you use. The easiest way to wire it up is through the GPS connector.

Before continuing, however, first a little battery theory. If you take two AA batteries and put them in a circuit end to end (see Figure 3-8), the voltage will equal 1.5 volts plus 1.5 volts, resulting in 3 volts.

	3V
+	− +	−

FIGURE 3-8: Two AA batteries end to end provide 3 volts

This is called putting the batteries in series. However, if you place the batteries in parallel, that is, take a wire and connect them positive to positive and take another wire and connect that negative to negative (see Figure 3-9), and then measure the voltage across the two wires, you will get a voltage measurement of 1.5 volts.

50 Part I — Hardware Hacks

	1.5V
+	−
+	−

FIGURE 3-9: Two AA batteries in parallel provide 1.5 volts

However, there is a side effect. The capacity of the batteries is increased. Therefore, if you take four AAs and put them into two pairs of two in series and join the two pairs in parallel, as shown in Figure 3-10, the output voltage would still be 3 volts, but the capacity (or how long the batteries would last) would be doubled.

3V

+	−	+	−
+	−	+	−

FIGURE 3-10: Four AA batteries arranged to provide 3 volts

As long as you have no more than two batteries in series, you can add a few sets in parallel to get additional power from the system. This is true no matter what kind of battery you are dealing with (although rechargeable batteries generally run at a lower voltage — about 1.2 volts — than alkaline batteries do).

Remember that you aren’t limited to using AA batteries. The bigger the battery, the more power you’ll get from it!

The power order of typical alkaline batteries is as follows:

Battery Size	Nominal Voltage (V)	Approximate Capacity (mAh)
AAA	1.5	1100
AA	1.5	2800
C	1.5	7500
D	1.5	15000
9V	9	580

Chapter 3 — Power Hacks 51

As you can see, a typical alkaline D-cell battery has approximately six to eight times the capacity of a AA NiMH battery. The disadvantage is one of weight — twelve AAA batteries weigh about the same as one D-cell, but the D-cell has 14 times the capacity.

Now that NiMH AA batteries are capable of capacities of 2500 mAh, they are getting close to the capacities of their alkaline rivals.

What You Need

This section describes what you need to build a battery pack. Remember that there are many ways to build one and the final pack can be of any shape. The only important caveat is that you don’t subject the GPS unit to too much voltage; if you stick to the normal battery voltages you’ll be fine.

For example, a Garmin eTrex has two AA batteries and therefore runs at 3 volts, while a Garmin GPS III+ runs on four AA batteries, providing the operating voltage of 6 volts.

Feeding more voltage through the GPS than it needs serves no purpose. If you feed it only what it needs, you avoid wasting batteries (and carrying extra weight), and supplying wasted additional voltage.

Batteries

A good battery to use is the AA battery because it is quite light and easily available. If you want greater capacity, then you might want to consider using D-cells, which have a far greater lifespan but are heavier to carry.

Battery Holder

Battery holders can be almost anything that can hold a battery. You can use a specific battery box (available from electronic outlets), a box you have lying around your home or office that you wire up, or something that you may already own that holds batteries.

A good example of something that already has a battery inside is a flashlight. Moreover, a flashlight has the added advantage of having a ready-made switch (although you may want to wire in a different switch because you might not want the flashlight on when you are running your GPS from the batteries).

Wiring and Connections

Figuring out the wiring and connections isn’t hard if you follow the instructions in Chapter 2. All you need to do is hook up a connector suited to your GPS to a length of wire (covered in Chapter 2) and solder that wire to the connection in your battery pack, paying special attention to ensuring that you connect the positive and negative terminals correctly (see Figure 3-11). Load the holder with batteries and away you go!