Car battery ground fault detector.

Guys,
Need your help in designing a car alarm that would detect if car battery terminals have accidentally touched the car chassis. This would be added on the circuit that i am building... a car battery voltage stabilizers... mainly a capacitor circuit. This is for a dissertation project that i am currently engaged with.
Thanks,
Dirk Bowitski

kasmot69 said:

Guys,
Need your help in designing a car alarm that would detect if car battery terminals have accidentally touched the car chassis. This would be added on the circuit that i am building... a car battery voltage stabilizers... mainly a capacitor circuit. This is for a dissertation project that i am currently engaged with.
Thanks,
Dirk Bowitski

Click to expand...

Hi Dirk,
If you ever short the car battery to the chassis you will not require an alarm to tell you so!.

As you may know one side of the car battery is conected to the chassis of the car.
The current drawn from the battery will only be limited by the resistance of the wiring/contact which is shorting,
so the wiring will catch fire.

One thing that can happen is that heat at the point of the short can 'weld' the shorting object to the chassis, making it difficult
to disconnect also you can't pull the wires off as they get red hot very quickly.

If I have mis-undertsood your post let me know.

Regards

It's called a fuse.

Normally the car chassis is used as a return current path for everything in the car so an RCD type earth leakage detector is useless.

However on military vehicles, which often carry explosives, a two wire system is more commonly used. The battery is bonded to the vehicle chassis at one point but it is not used as a current return path. All electrical appliances are connected to 0V via a separate wire. The vehicle chassis is only used for EMC screening purposes and nothing else. This improves reliability, as conductivity between the chassis and other appliances isn't an issue and EMC as gound loops are less of a problem. A similar system is used in domestic and industrial mains supplies, the neutral is bonded to earth and the separate protective earth isn't used as a current return except for filtering and earth leakage faults.

Earth leakage detectors are sometimes fitted to military vehicles but only those that have a single point ground and as all cars I've seen don't they're useless.

You could place a current measurment shunt immediatly after your voltage stabilizing circuit, or at any other point you need to detect shorts, and just use analog (or digital) circuitry to monitor the rise and fall times of the voltage to detect shorts. PC switch mode supplys do this to prevent the supply from going up in smoke if the current draw increases too fast.

The idea is to have a detection circuit that would indicate a battery terminal has made an accidental contact with the chassis, once this is happened the car owner would react immediately by stopping the car safely and correct the situation. Funny it happened to me before (a 1995 mitsubishi lancer) i was going for about 60kph and i hit a pothole, next thing i knew my engine light went on then off... next thing that happened the engine went out and smoke came billowing out of the hood, the battery came loose and hit the chassis causing the negative wire insulator to burn off. fortunately i was still able to drive it to a nearest car repair shop after removing the shorted cable. Thats the long story he he he. Anyway thanks for the ideas and keep them coming.

That would happen immediately anyway, as soon as the positive touches the chassis the battery a huge current will flow causing the battery to boil and the engine to stop. Modernautomovive systems are fused to prevent this from happening but I don't know if all the connections to the battery are fused; I don't think either the alternator or starter motor are fused.

Hero999,thanks for the reply but i am going to use this circuit on an old car, here in our country 10year old cars are still a common sight (Phils.) so the circuit might still be variable. Could a voltage level detection circuit might work? a short ciruit would in theory yield low voltage and a high current.

Even if you could build this circuit, how are you going to react fast enough? What do you even plan to do? Pull one of the cables off the battery while its arcing and hot from the current? By the time you come to a stop from 60kph and get pulled over and out of the drivers seat, the damage will be done.

You need a fuse to react for you. Make sure the battery is held in with the appropriate hold down. A loose battery is asking for trouble. Make sure all leads are insulated and away from sharp edges that might rub though the insulation and you shouldn't have any trouble.

I hav never owned a car that is newer then 10 years old, and I have logged thousands of miles in trucks 20 years old or older and I have never had a battery short out.

If you're really concerned about this happening with the car all you have to do is an extensive inspection of the cars wireing system from the fuse box to distribution panel and the alternator/starter. That's the only place a failure that would cause the battery to explode could happen, anything after the fuse box should be fused quite obviously. That's not a lot of wireing to inspect, and just outright replaceing it with new wireing would be more practical and less expensive than creating some elaborate warning system that wouldn't actually save anything anyways.

kasmot69 said:

The idea is to have a detection circuit that would indicate a battery terminal has made an accidental contact with the chassis, once this is happened the car owner would react immediately by stopping the car safely and correct the situation. Funny it happened to me before (a 1995 mitsubishi lancer) i was going for about 60kph and i hit a pothole, next thing i knew my engine light went on then off...

Click to expand...

Kasmot, you should fix the problem instead of finding a kludge. Batteries should not move when the vehicle drives over a pot hole. Fix the broken or missing battery hold-down clamp on your older vehicle.

1st: Fasten your battery properly with a clamp so it can't move.!!

2nd: You can add an ammeter to monitor your charging / discharging current.
This one measures the total electrical loading off the system exept for the starter motor.

3rd: Fit a voltmeter, a healthy state of a charged accu while engine is running is between 13.8 and 14.4 volts.