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Old 04-22-10, 06:47 PM   #1
Tony N.
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Aux. power for car gps mounted on bike

How I can over come the battery longevity issue? I will be attempting my first 200k brevet to see if I like brevets and don't want to get lost and receive a dnf on my first brevet. I have an harman/kardan car gps that cost about $100 several years ago at Cosco but don't know how I would ad aux power to it. My wifey says if I get into it and like it that I could get the Garmin 705 with all the bells and whistles.
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Old 04-22-10, 07:29 PM   #2
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I have used the APC Mobile Power Pack in the past for powering usb devices.. This has standard and mini-usb connections.. I have had mine for a couple years now and it work great.. You just need to make sure you have a charging usb cable and not a data cable for it to work properly.
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Old 04-22-10, 08:21 PM   #3
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When I ride with my car GPS on the bike, if it's going to be a long ride I keep it turned off most of the time and only turn it on when I need to look at the map or get directions. The battery will last all day this way. We're so used to having the GPS constantly on while driving the car but it really doesn't have to be like that on the bike.
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Old 04-22-10, 08:37 PM   #4
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USB uses 5 volts at a max of something like 1 amp. Considering that watts = volts x amps, this is a fairly low draw - 5 watts max.

So you have a couple of options.

1. Get a 5 volt lithium ion battery - for any device. Then simply make a harness that connects it to your GPS USB input (assuming your GPS uses a USB charger of course).

2. Take four D cell batteries and find a battery holder at Radio Shack that holds the four D cells positive to negative (they all do this really). Install the batteries, then verify that you have 6 volts. Connect the USB charger and you are probably OK. Wait till you are low on power then connect your external power at a rest stop.

3. If you can, get a second battery made for the GPS. Not all GPS units have a user serviceable battery however. Most car GPS units do not. I HATE THIS!! Not making something so I can get to the battery is LAZY PRODUCT ENGINEERING designed to SELL MORE CRAP, because when the battery goes you just throw out the unit. And please - no excuses about "water proof". The GPS I use on my WAVERUNNER (ever see a dry Waverunner? Me either) has a battery I can get to. It has survived skimming over the waves at 50 MPH. So please Garmin - tell me why the only Nuvi with a user replaceable battery is the 500/550 - which also happens to be the only waterproof unit you make? Why not make all the other car GPS units with a battery that can be replaced (or swapped out when it gets low)? Whats that? All I can hear are crickets chirping!

You could also MAYBE use a flexible 12v solar panel (or two) and then just connect a cigarette lighter socket (available at Radio Shack) and plug in your car charger. But this is a lot of stuff and not always easy to pull off on a bike. Probably not a good option for a road bike on a long ride.

And lastly, you could just buy an older Garmin E-Trex (can get em cheap now). They use AA batteries - one set will probably be enough but you can also carry another set. You can create your own map by entering the waypoints for the turns and rest stops in advance (assuming the ride organizers provide this info in advance - which is usually the case).

For the longer rides here, I think they all drive the route in advance and mark the turn locations with some bright paint on the street anyway.

Good luck! Enjoy that ride!
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Old 04-23-10, 08:22 AM   #5
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Thanks for all replies. I'm just wondering if anyone had used the 2/4 AA type battery boosters that I've seen that connect with a usb type cable? also I didn't think that paint is used on the pavement for brevets. I would need I think to keep the unit on as my biggest concern is not missing a turn. I would hate to turn it on at an intersection and find out I'm way off course. As one poster commented on another thread, I have that too many birthday eye disease and it is hard to read a computer while riding hence the need for voice prompts. :-(
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Old 04-27-10, 08:38 AM   #6
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I use a 2 AA battery charger that I bought for my USB cell phone on my TomTom GPS... Kind of a pain (because the cord is short), but it works... Sorry I can't tell you how long a pair of AA batteries last... never used it long enough at one time to run down a set in one go.
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Old 04-27-10, 09:02 AM   #7
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I've been using a cheap 2 AA battery booster with USB-out with my iPhone when I go riding. The phone's internal battery will suffice for almost all the riding I do, so this is a belts-and-suspenders approach.

You can get a idea of how many batteries you'll need by looking at the voltage and amp-hours of the GPS' internal battery, and its run time. If (for example) your GPS has a 3.7 V battery with 1500 mAh, that means it's producing 5.55 watt-hours of power. And let's suppose it gives you 6 hours of run time on that. So you know you need about 1.1 WH per hour of run time. You'll need to recalculate these figures with your actual device, of course.

NiMH batteries are typically about 2200 mAh at 1.2 V, or 2.64 WH, so each one in theory will add about 2.5 hours of run time. Obviously you'll need to use them in pairs or fours, depending on the charger you use, and there will probably be some loss of efficiency using an external power source.
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