12VDC Backup supply

[partyanimallighting]

Hi wizards, I am installing a GPS tracker in my vehicle and I will be connecting this tracker to the 12V ACCESSORY, probably from the ignition feed. However, I would like to include a rechargeable battery (I would think a small 9 VDC would be fine) to provide power to the tracker when the ignition is keyed off. My reasoning for this is that, if the vehicle is stolen and the thieves disassemble it partly or in it's entirety, power would still be sent to the tracker even when the main battery is disconnected and the vehicle or what's left of it, can still be located, once they do not discover the tracker. Would the 12VDC vehicle battery feed to the tracker damage or overload the smaller backup battery? How do I prevent the backup battery from discharging back into the vehicle's electrical system? Would a diode off the backup battery + prevent this discharge? (I learnt here that diodes only allow current flow in one direction!! Yay!!!)

 

[(*steve*)]

You can use diodes on both the auxillary battery and the main 12V supply to prevent either from feeding into the other. This is fine and will do what you want.

However, the main problem is the auxillary battery. Wouldn't it be best to use a rechargeable battery? A small 9V battery may not be sufficient to power the tracker (how much power does it require?)

 

[partyanimallighting]

Hi Steve, I'm a real novice at electronics but I really enjoy this forum, especially all the advice I get here and more importantly, everything I've learned from each and every one of you who have assisted me in the past. I did state "rechargeable" battery in my post as that was the initial plan. As far as the tracker itself, it has a wide voltage range of 9 ~ 48VDC and the standby current is 0.2mA, which I think is really low. As far as the circuitry goes, I would assume that a diode will be on the vehicle's battery 12VDC feed, allowing current to flow to the tracker and the rechargeable battery, powering the tracker and charging the backup battery at the same time. When the ignition is switched off, this diode will prevent the backup battery from discharging throughout the vehicle's electrical system. Does this sound correct? If I'm on the right track, what value diode should I use? Any specific type of diode? Can I simply solder in into the 12VDC vehicle battery feed and seal it in some heat shrink?

 

[Audioguru]

The diode can be any little diode like a 1N4148.
The diode is not a battery charger. Get a battery charger IC that is made to charge the type of battery that you will use.

 

[Kiwi]

Do you have a link to the specifications for the GPS?

Is the GPS the type that waits on standby and only wakes up when you send it a text? It replies with a message containing GPS location coordinates, and then goes back to sleep. This type can safely be connected directly to the battery.

 

[partyanimallighting]

Found this image online. It's similar to this and all I want to do really is to be able to track the vehicle's whereabouts via an app on my phone which shows it's location on a GPS map.


You DO NOT want to try to decipher the English in this user manual.

 

[Kiwi]

Should be able to just connect it to a battery supply.

 

[partyanimallighting]

Here's the thing. Will it run down the battery if I do not use the vehicle over a one or two week period? And, if I install a backup battery, what type, size and voltage should i use?

 

[WHONOES]

A small 9V rechargeable battery such as a PP3 in the UK or 6LR61 in the US would have a capacity of about 100mA hr at a discharge rate of 0.2mA you would get about 500 hours run time. But that assumes that your device will still run at 7V which is the level it will be at after 500 hours. You are more likely to get a 100hrs or so before you device became unreliable. If needs must, you can use a larger 9V battery (within reason) for which the current limiting device described later on would still be suitable.
Further, you cannot just connect a 9V rechargeable battery to a cars 12V system even with a diode in series as a large current will try to flow form the 12V batt' to the 9V batt' probably causing a fire if the diode survives the long enough to do so. You will need a current limiting circuit in series with the 9V battery to limit the current passing into it to 10% or less of its capacity. The circuit you require is actually very simple and could be built on either a bit of strip board or just soldered together.
See circuit diagram attached.
The car battery is represented by the 12V supply and, the 9V battery goes where the IPROBE is on the diagram. The circuit delvers about 9mA of current.
A blocking diode should not be necessary as the 9V battery would need to be of a higher voltage for current to flow from it to the car battery. Not only that but the current limiting circuit would also prevent it from doing so.

 

[partyanimallighting]

Hello Whonoes, based on what you're saying, a 9V rechargeable should give me about 20 days backup at 0.2mA current draw, so I think that would be fine. If the vehicle is stolen and I can't locate it within 20 days, I'll probably never locate it!! As for the large current flow from the 12V to the 9V, this is the issue I was wondering about initially, hence the inquiry post to get some findings on this. My assumption was that the heavy load would overcharge the 9V battery, causing overheating or basically a meltdown. Now here's the most important thing......as I state in all my posts here all the time, I am not a tech, I am a tinkerer, and I consider myself a very good one too. I post my problems and with your assistance (explained by you guys in the most layman terms possible), I try to and most often resolve any issues I have. Hence, because of my lack of experience, I am not very versed at reading circuit diagrams etc. so it'll take me a while to diagnose the diagram to figure it out, as simple as it may seem (to you, not to me). The other problem I'll definitely encounter here is the unavailability of basic parts so I'll do some searching and see where that goes. I'll post an update as soon as I know something.

 

[(*steve*)]

Yes, and as per my first reply, a diode connected to both the 12V and the 9V battery should be used to isolate them from each other.

 

[partyanimallighting]

Hello Steve, should diodes be used or should I use the circuit above? What about what Whonoes stated about the high 12V battery output burning out the diode and probably causing a fire? NOTE: The vehicle is NOT insured for fire damage. Also, if I use 2 9V batteries instead of 1, in parallel, won't I get twice the storage capacity? ie: 200 hours definite runtime instead of 100 hours based on Whonoes calculations at 0.2mA for 1 9V battery?

 

[(*steve*)]

Something as simple as this should work:



D1 and D2 can be 1N4001 or similar.

For simplicity, a small 12V lead acid battery would be best for the auxiliary battery. R1 allows a slow trickle charge of the aux battery from the main battery to keep it topped up. A 100 ohm 2W resistor should be sufficient to keep the battery topped up and be safe even if the auxiliary battery is shorted.

A 1.3Ah 12V SLA battery is pretty small, and will power the device for about 9 months. The greatest advantage is that it has the same chemistry as your main battery so no special magic is required to charge it.

 

[partyanimallighting]

Hi Steve, I googled the 12V SLA and, although it's pretty small, I still prefer the 9V option as it's a lot smaller and will be a lot easier to conceal. It's a panel van and these are notoriously tight on space so there is absolutely no way I can conceal the 12V SLA easily (from thieves) whereas the 9V can be hidden behind inside a panel a lot easier so I would like to focus on utilizing the 9V rechargeable instead if that's OK with you. Plus I'll have to order the 12V SLA (i doubt it will be readily available locally) whereas I should be able to pick up a 9V and future replacements easily if and when the original battery dies. What do you think?

 

[(*steve*)]

In that case, why not use a normal 9V battery and the same circuit (minus the resistor). Use a good battery and replace it once a year.

The problem with the circuit is that unless you disconnect the main battery you won't know if the other battery is dead.

 

[partyanimallighting]

Hi Steve, I suppose once I occasionally log into the GPS with the ignition off I'll know if the backup battery is fine and functional. Just to confirm, I've attached my "layman's" drawing of the circuit to be made (don't laugh too much). Let me know if it looks right to you.

 

[(*steve*)]

There is no positive connection to the GPS in your diagram. It should connect between the diodes.

 

[Audioguru]

A little 9V rechargeable battery today is 7 cells of Ni-MH and its fully charged voltage is about 9.8V and its average voltage during a discharge is 8.4V. Its load should be disconnected when its voltage has dropped to 6.3V so that its weakest cell does not become reversed and destroyed. Energizer battery company says that a Ni-MH battery should have a very low current as a trickle charge that is 1/40th or less of the capacity which would take 2.5 days or more to fully charge.

 

[WHONOES]

No. I have done a lot of stuff with rechargeable batteries from the MOD down. NI MH and NiCad batteries can be charged indefinitely at the 14 hour rate which is essentially charging at 10% of capacity. The 14Hr rate comes from the charge process only being 60% efficient.

 

[(*steve*)]

There manufacturer may say that indefinite charging at the 14 hour rate is ok, but then they're in the business of selling more cells.

If you want to go down that route, the circuit I have you could also be used, just change the resistor.

The resister value should be

3000h / m

Where h is the number of hours a full charge would take, and m is the capacity in mAh. This assumes the battery under charge is nominally 9V and the main battery voltage is nominally 13.8V.

For this use, I'd recommend using the 200 hour rate since you're doing little more than replacing self discharge. And remember that self discharge is much lower these days.

For a 100mAh battery, that resistor would be 5k6, producing a charge current of just under 1mA.

Not that in this application you are not considering the actual charge time, since the battery is not expected to discharge in an environment where it will expect to be recharged. If the vehicle was "lost" with the main battery disconnected, and then recovered, you would probably have to charge the battery externally (or wait a couple of weeks).

However, all discussions so far have centered on the 0.2mA drain when the device is in standby. What is the current when it is active? Using a little battery like this might only give you a couple of tries, if that, depending on the transmit power.