Thread: 3G Connected OBD2/GPS Device

Start first with the feasibility. When you say, "always on with low power draw" what, exactly, does that mean? 1 watt? .5 watts?

It's probably easier to translate that from a requirement like: Must function continuously for 2 weeks on standard 12 volts, X amps battery without consuming more than X% of the battery's capacity.

Then you can do a quick back of the envelope just to see if it's feasible.

Updates:

So it seems that people have already done this with arduino, so I think that is the platform I am going to use for now. Link

As for the power requirements, let me scroll through and let me see what the power consumption is...:

1. ELM 327 - 5V @ 12mA = 60mw
2. Arduino ATMega328 - 5V @ 12mA = 60mw
3. GPS GPRS/GSM board - Unknown, but 3G modules use about 2-4W during data transfer

So the biggest draw here is the 3G connectivity. However, that is only during data transfer. However, I will limit the device to only use the 3G once every x amount of time. X will be user-settable, so it could be anything. The ELM327 could also be switched off as well and the ATMega also has a super-low sleep mode that only uses ~1mw.

Lets assume that I set X = 15 minutes. It will take approx 1 minute for the GPS to get a fix, gather all the OBD2 values, and send/recieve all data to OSDash. So thats 4 minutes an hour or 96 minutes a day that we are using the full 2-4W.

A normal car battery is approx 45 Amp/Hours in capacity. However, we can only go down to approx 70% capacity before we will not be able to start the car. That leaves us approx 13.5 amp/hours to work with. Using approx 3W a day (96 min at 2W) and the car battery having 162 watts to use (13.5A * 12V) is close to around 54 days of use time at this rate.

However, this might be calculated wrong, can someone check my units and math?

Originally Posted by ioi8

A normal car battery is approx 45 Amp/Hours in capacity. However, we can only go down to approx 70% capacity before we will not be able to start the car. That leaves us approx 13.5 amp/hours to work with. Using approx 3W a day (96 min at 2W) and the car battery having 162 watts to use (13.5A * 12V) is close to around 54 days of use time at this rate.

However, this might be calculated wrong, can someone check my units and math?

3watts for 96 minutes of the day is around 0.4amp hours at 12v.

Edit : math fail

Originally Posted by malcom2073

3watts for 96 minutes of the day is around 0.4amp hours at 12v.

Edit : math fail

So..... .4 amp/hours per day will still leave us with around 33 days of active usage at this rate if we only use 13.5 amp/hours of the car's main battery. Still pretty good.

Check the details on the GPRS/3G data link.

If you can drop it to only using a 2G radio link (AKA, dropping back to GPRS), then the power draw drops significantly.

Remember, you aren't trying to download a torrent on this link, just a couple of k in status updates / commands. So if you can use a 2G link, you will find that the power draw drops.

Bearing in mind, this is just what I have found with using mobile phones for data transfer, if you kill the 3G radio and just use 2G, the battery life jumps by 20-30%

I have two free usb ports on my router that I can connect a usb obd2 interface to...

http://www.mp3car.com/vbulletin/gps/...ml#post1347351

it's not running gpsd anymore since it's crashes all the time, had to write my own multiplexing tcp server that shares the gps com port over tcp.
up to 100 clients can connect and use the same gps data over wifi or 3g. the same server could share the obd2 com port in the same way.

the router needs 5v 1.5A to run and power the usb devices.

more info:
http://www.koppel.cz/conferences/viewtopic.php?p=10552

n2k, 7.5W is waaaayyyy too much to draw for this device. It needs to be <.5W while in operationg mode (allowed to go up to 2-3W while in 2G/3G mode). Anything more than an embedded device will draw too much power.