12v dc power supply for gps

Discussion in 'The Projects Forum' started by maura, Dec 26, 2008.

  1. maura

    Thread Starter New Member

    Dec 26, 2008
    3
    0
    Hi folks!

    I have a garmin 2730 gps, which does not have an internal battery. I have it wired direct to my motorcycle with a 1 amp fuse. I want to be able to use it in a restaurant or rest stop off the bike, using a small portable battery.

    The gps is designed to run on 12v vehicle power, which is actually usually 13.8v.
    From the gps I have garnered the following information:
    usage: 12W max @ 13.8 v DC
    12v DC 1.25A

    With my primitive knowledge of these matters, I have imagined 2 scenerios:

    1. 8- 1.5v AA or AAA batteries or 10- 1.2 v rechargable AA or AAA batteries
    in series to a connector (sae type that I have on the gps) to the gps
    -easy, 12 volts, but would need to locate and buy some sort of battery holder and a bit bulky

    2. 2- 9v batteries in series to a plug to gps
    -more compact, easier, already have the 9 volt battery connectors from old discarded radios-my problem is 2 - 9 v batteries are more than 12 v and I do not fully understand the implications of that. It is supposed to be 12v, but we already know they are expecting 13.8 v. What about 14.4v for 2 NiCd 9v batteries?

    My questions:
    -Do I need to step down the voltage on two 9 v batteries?
    -How would I do that?
    -Do I need the 1 amp fuse?
    -Is there a better way of doing this?
    -Am I going to ruin my $1000 gps?
    -(less important) How long will these battery set ups last?

    Thanks for your attention. :)
     
  2. Napoleon

    Active Member

    Dec 21, 2008
    45
    0
    Carrying the GPS + battery pack would be a hassle for me.
    More crap to haul on the bike and keep track of.

    I suggest getting a cheap TomTom.
    That is what I use on my motorcycle, wired to the bike.
    It has a built in rechargeable battery that lasts a few hours.
    So when I get somewhere, I just slide it off the mount and take it with me.

    They can be had for less than $150.
    And, if you break it or loose it, you wont be out much.
    Plus, they are very capable.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'd go with Napoleon's advice.
    AA batteries would be barely sufficient to power your GPS for 45 minutes, if that. An exception would be Energizer Lithium Metal AA batteries; you might even get a couple hours run time from them - but then you'd be buying all brand new batteries.

    Forget trying to use 9v batteries - they wouldn't have enough current output capacity to even turn your GPS on.
     
  4. maura

    Thread Starter New Member

    Dec 26, 2008
    3
    0
    Thanks for your comments, great to have such a rapid response! I already own the garmin 2730, and it suits my specific needs, the tom tom does not. I have two other gps units, with almost identical power consumption. My garmin streetpilot III runs on 6 AA batteries for approx 6 hours, so this is my basis for believing that 8 or 10 AAs might work. The other gps, a garmin 376c, has an internal rechargable battery that I cannot get at, it runs for approximately two hours. I am not looking for more than two hours. There are devices for this purpose on the market, but I think it is a simple enough problem that I can do it myself. I can power the unit off the bike with an AC adapter when in a hotel or at home. I am trying to make a power source that will allow me to get off the bike at a highway rest stop, take the gps off, and rearrange my route, without being dependant on an AC source, or forced to fiddle with the gps on the bike in torrential rain. I would like to use rechargable batteries because I already am carrying a battery charger for AA and 9v. If these types of batteries are inadequate, what would work? I carry a laptop, with a 15v battery. Maybe I could rig that battery up to work?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    According to the product specs, the Streetpilot III will run on 6 AA batteries from two to 20 hours, depending upon the backlight setting.
    https://buy.garmin.com/shop/shop.do?cID=164&pID=158#specsTab
    Six alkaline AA batteries in series will output a total of 9v when new. Not sure what the mAh rating is on them at the moment, since the website I usually refer to seems to be down.

    Energizer makes AA rechargeable batteries that have a 2500mAh capacity. You might get as much as 1 1/2 hours use per charge out of them. Use their charger for maximum battery life.

    Better to pitch the charger you have and use the Energizer charger.
    Better to not mess with that. If you should goof up the connections, you'll be buying a new laptop batteries, and they are expensive.
     
  6. HwT

    New Member

    Mar 21, 2009
    2
    0
    I'm looking for a similar solution. The maximum draw from my Garmin 2720 is less than one amp and I can reduce that very substantially when I'm not in bright sunlight by reducing the screen brightness. I'm thinking of a sealed lead acid 12 volt battery of the type used in UPS systems. The 1.5 / 2.5 ah sizes are small enough to put in the tank bag and they can be had very reasonably. I believe they can be charged right from the bike's 12 volt system and have spade terminals so they can easily be connected to the bike and the GPS.
    I agree with Maura, I've had a number of GPS units and the 27xx series are just about perfect for bike use so I'd like to find a way to power it off the bike for a short period. Also like Maura, my experience with battery life is similar as one of my older Garmin models with a larger screen size used 6 AA alkaline batteries and gave about 10 hours of use with a maximum draw of about 1.5 amps.
     
  7. maura

    Thread Starter New Member

    Dec 26, 2008
    3
    0
    I put together two battery holders (2 AA and 6AA) for a total of 8AA and thus 12v. I decided to forgo rechargeability because I really am not going to use this too much, it is just great to have when you need it. I put them in a little project box that I got at a local electronics supply, put an SAE connector on so it mates with the connect I use to power the GPS on the motorcycle, I used the garmin dash mount that I was not using otherwise to mount the GPS on the project box, and Bob's your uncle, I've got a compact little box that does the trick for less than $10. I am using lithium batteries, more expensive, but the amount of actual time I will use the battery power this will last me the season.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    While your post is similar to the original poster's topic, your choice of battery is different. You really should have started a new topic of your own, and referenced this thread by a link, since it is somewhat similar.

    I strongly advise against attempting to charge your small-capacity SLA battery from your motorcycle's electrical system directly. Attempting to do so will at best permanently reduce the capacity of the battery due to pockets being formed in the gel electrolyte, and at worst, the SLA battery rupturing/exploding from overcharge.

    You really need to limit the charging current to about 1/8 to 1/10 the AH capacity of the gel cell battery, and ensure that it is not overcharged. Doing so will be somewhat complicated; it will require a voltage boost circuit plus a charging circuit. I don't have a quick solution for you offhand.
     
  9. HwT

    New Member

    Mar 21, 2009
    2
    0
    Thanks to you both. Of course, SgtWookie is correct, the rule of thumb for charging gell cell motorcycle batteries is 1/10 the ampere rating, just wasn't thinking clearly.
    The AA battery solution seems more practical but I was still concerned about the run time allowed. I found the answer (~ 5 hours) on this site http://www.cycoactive.com/gps/powint.htm#065-0700. Elsewhere on their site, Cycoactive also has some excellent information on preventing power supply problems.
     
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