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Car lock project - Batteries?
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noctorum View Drop Down
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Posts: 14 		Post Options Post Options   Quote noctorum Quote  Post ReplyReply Direct Link To This Post Topic: Car lock project - Batteries?
    Posted: 04 June 2009 at 1:46pm
I was reading over the info for the KL042, and a quick google search
found me this;

http://www.adafruit.com/index.php?
main_page=product_info&products_id=80

I know the KL042 is rating at 200mA, I was thinking of hooking up a few
of those 9v batteries in series in order to avoid tapping the wires in my
ignition harness and having a cleaner look without power cords running
across my dash.

My concern is that I'm not sure what battery life would be like though. I
was considering 3 or so standard 9v batteries being used.
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amal View Drop Down
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Posts: 1530 		Post Options Post Options   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 05 June 2009 at 9:52pm
A 9v battery has virtually no amp hour capacity compared to a car battery. You won't get much more than a couple hours out of a 9v, or three 9v batteries in series (series = same amps/more volts, parallel = same volts/more amps).

I think toward the back of chapter 10 is a paragraph about the battery life of a 9v when used with a 250mA RFID reader. Check it out when you get a chance.
Amal ;)
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Whiterabbit View Drop Down
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Posts: 32 		Post Options Post Options   Quote Whiterabbit Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2009 at 10:42pm

If you are consideing a car lock project, the mains 12V battery is the ONLY way to go. Not only is it a huge current source, but it automatically charges every time you get into the car.

The load in my car with the key out is around 500 ma, I can last over a week with no dead starts.

I'd be worried at a load over an amp, but under? never been a problem.

and FYI I keep a passive rfid scanner on 24-7 above my C-pillar in case I dont have my keys on me, lock my keys in the car, etc etc. As long as I have my wallet, I can key card into my car.

great emergency entrance method. And the only way I can dead-start my car is by installing a 4 year old battery and running the windows up and down repeatedly with the car off.
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Posts: 52 		Post Options Post Options   Quote ecnal Quote  Post ReplyReply Direct Link To This Post Posted: 13 January 2010 at 5:20pm
An old thread, but in my mind I've always wondered about a solar charger. Such as running the RFID off a 9v, or a 12v separate battery that's on a constant solar trickle charge. A decent enough panel should be sufficient to keep enough power in a battery or capacitor to run a reader/basic stamp.
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Whiterabbit View Drop Down
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Posts: 32 		Post Options Post Options   Quote Whiterabbit Quote  Post ReplyReply Direct Link To This Post Posted: 13 January 2010 at 7:24pm
totally unnecessary. Modern cars have lots of parasitic drains on their batteries. these circuits aren't drawing all that much at any given time at idle. Given a daily driver, the mains battery easily last so long that the starter motor which IS a massive raw on the battery, can still start with ease. The batteries recommended for cars can still start the car even after several days of no drive time, such as a weekend trip or even a week vacation.
 
If your goal is trying to run an RFID and a micro, have at it. Your car battery can handle it even without a solar panel. Just make sure you drive at least once a week.
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Posts: 1530 		Post Options Post Options   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 14 January 2010 at 11:24am

I don't know... I had a solar panel on my car and it worked out nicely. An RFID reader can have quite an effect on a lead acid car battery. Most parasitic loads pull microamps or at most a few milliamps (3-10). An RFID reader can (and typically does) pull upwards of 100ma to 250ma, which is a big difference. A quarter amp draw on a car battery designed for short burst high cranking amp output can wreck a battery's ability to start a car, even though there would be plenty of energy left to slowly drain off power to lesser loads.

 

My 2004 VW with a year old battery in it is no exception. If I leave my car sitting for a week as I drive my other car to work, with my RFID reader powered up and no alternative supplemental power source, I have to cross my fingers when I get in to start it. If, however, you drive your car regularly, then you don't really have to worry about it. Unless you live in a place where there is 24 hour sun, if you get a solar panel you'll only be prolonging the inevitable, but you might be able to let your car sit a couple weeks and still be able to start it. Or you could get a bigger panel that could power loads and charge during the day, but putting a giant panel on your roof just to balance out your RFID load probably isn't something you'd want to do :) Just drive your car once in a while and you’ll be fine.



Edited by amal - 14 January 2010 at 11:26am
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Posts: 52 		Post Options Post Options   Quote ecnal Quote  Post ReplyReply Direct Link To This Post Posted: 15 January 2010 at 4:45pm
An idea I had last night: what if there were a magnetic switch on the car, hidden, perhaps in the upper-left hand side of the windshield, on the inside. Cars are metal, so it would be easy to keep a small neodymium magnet stuck to it somewhere easy to reach. I actually have a magnet on my roof, for no reason though.

You take the magnet, and stick it to the windshield. Two things happen. The switch is activated and thus the RFID reader is powered on, and the magnet stays in place because it's attracted to the metal in the switch. You scan your implant (or keyfob, or whatever), say, at the bottom of the windshield. Doors unlock. Grab the magnet and toss it onto the roof, where it patiently will wait for next time.

Hell, you could have two hidden magnetic switches. Or even better, now that I'm thinking about it more:

The magnet switch powers on two separate RFID readers. One unlocks the doors when you scan your hand, one locks them. Once the magnet is removed, both readers power down. Magnet goes on roof. Or hood. Or door, quarterpanel, etc. It's genius!

BTW, would a neodymium magnet's field affect an implant? Like, if you passes a neodymium (or any magnet) over a tag, briefly or prolonged, would it erase or distort the tag?
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Posts: 52 		Post Options Post Options   Quote ecnal Quote  Post ReplyReply Direct Link To This Post Posted: 15 January 2010 at 6:07pm
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Never mind! Tag works fine after being exposed directly to a neodymium magnet for twenty minutes.
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Posts: 1530 		Post Options Post Options   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 15 January 2010 at 10:02pm
Originally posted by ecnal

Never mind! Tag works fine after being exposed directly to a neodymium magnet for twenty minutes.

Yeah, the tag would not be affected by a static field, only a modulating one... just like putting a magnet up to a wire coil does nothing. Only by rapidly moving the magnet across the coil can you induce a voltage. The same goes for a tag... however most all tags have DC blocking and over-voltage protection built in.
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Whiterabbit View Drop Down
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Posts: 32 		Post Options Post Options   Quote Whiterabbit Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2010 at 10:48am
I'd suggest a switch that does not require a prop. pushbuttons are good, many cars have doorhandles you can purchase that contain switches.
 
There are also capacitive touch sensors, vibration sensors, proximity sensors, any number of sensors.
 
That as you say, will only enable the RFID, not actually perform a function. Without having to drag around a magnet or additional device.
 
Afterall, isnt the point to carry LESS? may as well strap a key to the car, or just carry one around.
 
----------------
 
But really how many secondary switches do you need? The RFID can be powered all the time, that's no problem. So why add a switch to power it on or off? Just ads overhead to the process. And at that point, again, you may as well carry a key.
 
If it is not CONVENIENT, then it does not improve anything.


Edited by Whiterabbit - 16 January 2010 at 10:51am
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