Power Pack for Bicycle
I never did find out what the exact power consumption is for the ONE 3rd Ed. I couldn't even find the power consumption specs for any TomTom.
NOTE: I am a degreed Electrical Engineer. I'll keep this discussion as simple as possible, yet the electrical terminology and descriptions will be accurate.
For now, I'll use numbers from a Magellan Maestro 3100 as an
approximate starting point.
With a 1200 mAH capacity battery and an approximate 3 hour battery life, we can assume the Magellan consumes approximately 400 mA of current at any given time.
*********************************
Voltage: 5 Volts (USB port)
Current in milli-Amps: 400 mA (rough guess only)
Power in Watts: 2 Watts (calculated)
If we want to just use the Car Cigarette Lighter Cord/Adapter, we'll need a 12 Volt battery.
Now we'll need to figure out the Current draw at this new higher voltage...
(400 mA)*(5 volts) / (12 volts) = 167 mA
So at 12 volts we'll be drawing 167 mA from the battery.
*********************************
Let's look at one specific 12 Volt battery. This one is small and only weighs 3 pounds so you should be able to find a good place to attach to your bike. (5.3 x 2.6 x 2.4 inches)
http://www.batteriesplus.com
It's rated at 3.3 AH (Amp-Hours)
This simply means that it can supply 3.3 Amps continuously for one hour before total discharge.
3.3 Ah = 3300 mAh (milliAmp-Hours)
Let's figure out how long it takes to discharge this battery if we draw 167 mA continuously.
3300 mAh / 167 mA =
19.75 hours
*********************************
The great thing about using small 12 volt batteries is that you can charge many of them with an automobile trickle charger.
Here are two chargers:
500 mAh (6.6 hour recharge for battery above)
http://www.batteriesplus.com/p-34554-werker-12v-500mah-automatic-agm-charger.aspx
1000 mAh (3.3 hour recharge for battery above)
http://www.batteriesplus.com/p-34553-werker-12v-1000mah-automatic-agm-charger.aspx
*********************************
To use a 12 volt battery with your TomTom's Car Cigarette Lighter Cord/Adapter, you'll need a standard socket.
You can find these at most automobile parts stores. Here's an example:
http://www.tessco.com
*********************************
Wire it up using almost any wire. Since you're only drawing 167 mA, any copper multi-stranded insulated automotive wire should do. 16 or 18 gauge will be way more than heavy enough. Since it's many times larger than electrically required, now make sure it's heavy enough to withstand getting easily pinched or cut by accident.
Wire up the 12 Volt Lighter Socket with the positive (+) terminal to the center pin of the socket and the negative (-) terminal to the shell.
IMPORTANT: Make sure your polarity is correct. Check and double-check the positive and negative wires. Reversing these wires might blow your device. Sometimes safety circuits are built-in but the only way to find out is the hard way.
So make sure the wires are correct!
*********************************
You can use any rechargable 12 Volt battery & recharger you like. If you know the Amp-Hour (AH) rating, you can use the formula above to calculate how long a single charge will last.
*********************************
For safety, consider installing an inline fuse assembly. You can find these at the auto parts store too. Three Amps would be good. Don't be too concerned about the rating as 16 gauge wire can handle a lot more current than a few Amps. The fuse is in place in case you accidently pinch the wires together causing a short. Install it as close to the battery as possible.
A note about Fuse Size: It needs to be larger than the what you'll be drawing from the socket or you'll blow it every time you plug in a device. It needs to be smaller than the capacity of your wire if you still want to protect the wiring. 18 gauge wire Ampacity is approximately 8 Amps. So a three Amp fuse is what I recommend. Five Amps will work too.
*********************************
Alternate installation. You could also install the cigarette lighter socket right at the battery negating the need to fuse any wiring. The TomTom cord itself is protected with its own 2 Amp fuse inserted into the side of the plug assembly. With your new socket wired right next to the battery, the chances of a wiring short between the battery and socket is greatly minimized.
*********************************
Now you have a bicycle with a 12-volt rechargeable power source and 12-volt cigarette lighter style socket. Fully charged, you'll be able to power your TomTom for nearly 20 hours without any power saving features enabled. Bring your cell phone cord too just in case. If you have a Motorola Razr, you won't need another cord because it's identical to the TomTom's cord.
This project may require crimping connectors and soldering. Use your best judgement and don't tackle this project if you lack basic wiring skills.
