Circuit To Accomodate Dual Power Supplies

[cs2000]

Hi guys,

My first post here after watching Dave's exploits for about 2 years, I finally needed some EEV assistance and there was clearly no better place to turn! Apologies if this isn't how this forum works, or is in the wrong place, but let me know if so

Basically, I have purchased a car GPS tracker, specifically I was looking for one without an internal battery as they all seem to be something silly like 1000mah which only gives a few days of runtime. My car can be parked up for a week at a time or longer so these were useless.

The tracker itself takes a standard ~12v from a car battery, but in the box it also includes a little magic black box which contains a boost converter and it lets you power the thing from a 5v USB power bank as the boost converter steps this up to 12v.

What I'm looking to do, it provide a system whereby when the car if off, the included boost converter powers the tracker from a USB battery bank, however, when the car is on, it needs to instead draw 12v power directly from the car (which I will provide), and start charging the USB battery bank itself. Once the bank starts charging, output is terminated anyway. I was going to accomplish this step by hacking up a standard car cigarette lighter USB charger thing as it already does the 12v to 5v step down which I would need to charge the USB bank, but its the switchover which has got me confused. . .

My initial thought was to try and do it through relays, but I soon got lost down a rabbit warren from which io couldn't recover. I'm a optimistic amateur when it comes to this stuff, I can do basic breadboard work, possibly down to soldering IC's as long as they come with sockets if this helps.

I'm sure in my head its probably a simple circuit, but not being my field of experience, I cant wrap my head around it!

Any replies at all would be amazing, thanks

[Avacee]

Can't you wire it into the car's 12V circuit so it powers from the car battery even when the ignition is off?
The car battery will easily run the GPS tracker for considerably longer than the 1 week you mention.

Can't see why the USB battery bank is needed when you've a relatively massive car battery sat there already.

[Metatronic_Mods]

What Avacee said. Or if messing with your car's internal wiring is too complicated, surely there are USB power banks which can be charged from a cigarette lighter, while still being plugged into the device you want to power. In other words just let the internal switching and regulating circuitry of the power bank do the work for you.

[skarecrow]

GPS trackers are meant to be powered 24/7 from the vehicles electrical system. The reason they have the battery backup is in case a thief disconnects your battery so it can still track and report for up to several days. Many can even alert you if the power is disconnected so you can check on it before the thief gets very far.  We have them in all of our tow trucks.  Even if you yank out all the wires the internal battery will let us find the truck (or the tracker if somebody took apart the whole dashboard to get to it and removed it) up to a week later. Adding additional batteries seems kinda pointless. Is there a reason you want to set it up that way instead of how it was intended to be set up?

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[cs2000]

Fair point. my concern was the standby power draw, I guess it would be smart to measure it, last thing I want it to have a dead battery, id rather have a flat USB bank

@Metatronic_Mods - I have no problems at all meddling with the cars wiring, that's what I'm good at! All of the battery banks I have found so far seem to disable the output when they're being charged.

@scarecrow - The reasons above really, concerns about flattening the cars battery is my main one to be honest. Of course I plan to stealth install the tracker anyway so its doubtful anyone would find it, but also using the USB bank would allow me top do what you mentioned and still provide power to the device if the cars battery gets pulled.

Full disclosure, this has come around after having a GPS tracker with an internal battery for a few years, I noticed recently the battery had puffed up (as the unit stopped functioning) hence my search for an alternative

EDIT: Current draw during start-up phase was pulsing, but somewhere around 0.075a to 0.100a. Once booted up, it seemed stable around 0.050a I gather this isn't a lot of current...
Further dismantling also revealed an undocumented 3.7v 180mah battery (assuming Lipo) which you can make the thing run on by flipping a internal switch. I assume therefore I can buy a larger battery and solder this in on the same points?

[skarecrow]

I wouldn't worry about a GPS tracker killing your car battery unless you plan on letting it sit for more than 6 months at a time. If you still insist on using a power bank, I have one by Omaker. It's 15600mAh, has 2 2.1A outputs, and works while charging. Not sure what they cost or where to buy one, but you should be able to find that online with ease. I found mine in a parking lot a couple years ago.

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