Disconnecting battery when car is running ...

[Dom]

There was a conversation going on about this in another thread last week that it was :

a) safe to disconnect the battery when the car was running and ....
b) the car would not run when the battery was disconnected.

Point b) was false - the car would still run while the battery was disconnected however .......

Point a) could cause a phenomenon known in the automotive electronics trade as a "load dump" where because the battery isnt there to buffer the spikes from the vehicle electronics large spikes on the car can occur. These spikes can vary from 50 volts to over 200 volts and last for almost half a second in some cases.

If any of your vehicle electronics are not protected against this spike then their life will be greatly reduced if not ended instantly.

A similar thing can happen if you've got a faulty car battery so if your battery is on the way out then change it before the rest of your car electronics start doing weird things !

[azza69]

also not all cars will run with the battery disconected,ive seen many that dont when doind a live battery swap

[Dom]

Some cars have ecu's that measure the battery voltage and will turn off the ECU if an undervoltage or overvoltage condition occurs.

I have been playing with some linear regulators that basically cut off the circuit in the event of a load dump so if I use these in my circuits, they will work fine unless there is a severe over voltage or undervoltage in which case they will be protected (and cut off) from the car electrics.

[azza69]

and a linear regulator is???where does this live?

[Dom]

lmao lmao - its a small chip that you put 13-20 volts in and it gives you out 12 volts (or 5 Volts) to within a couple of %

[Rob_S]

didnt care whether it was safe, was only a corsa

[Andy K]

don't you just fit a series resistor to your equipment, the then fit a diode across the supply on the equipment side.

[Makis22]

The battery acts like a capacitor and helps the alternator to generate a clean 14.4V regulator voltage. Without the battery the voltage will still be 14.4V max conrolled by the regulator, but the voltage itself will be flactuating a lot from 0-14.4V. Most electronic components in the car have their own regulation and as such they should cope with crap DC voltage. The components that will suffer will be the ignition and the supply to the injectors. When this happens it is possible the car to stall. If you are lucky and you have sufficient residual capacitance and or inductance in the system the car will actually carry on even if the battery is disconnected. I crap battery will always have sufficient residual capacitance to allow the car to run OK.

[Mr. Efficiency]

If you disconnect a battery and the car dies dosent that mean your alternator is toast? Maybee cars are diffrent in the UK (I'm in Canada) but from were I am every car can run fine with no battery it just uses the batery to start and thats it, you can disconect it and nothing will happpen ever, I've never disconected a batery from a car and had it die, except for a few with screwed alternators but once you fix the alternator it will run fine without a battery.
But maybe things are just built diffrent over here for diffrent road specifications.(like the test a car has to pass to get on the road over here its called DOT)

[Dom]

If you disconnect your battery with the car running you can create a phenomenon known as "Load Dump" - have a search on Google for this.

It can cause the voltage across the alternator to soar to over 200 volts for over half a second that can cause damage to other equipment in the car.

[Dom]

don't you just fit a series resistor to your equipment, the then fit a diode across the supply on the equipment side.

You can do but with a lot of stuff I'm making they require more than an amp of current so you need a beefy resistor which in turn dissipates a fair bit of heat so it doesnt become feasible.

Best way I've found is using an LC input stage, fuse, MOV and a Zener to keep anything nasty at bay then using an automotive grade 5 volt regulator after this stage to make doubly sure.

The more you read up on automotive electonics (not electrics) the more complicated it gets lmao

[BigbaRd]

I was well confused when I disconnected the battery with the engine running and it kept going (cav engine). I needed to turn the engine off quickly for something (forget what) and the battery was the quickest option.

[Dom]

Pulling the king lead is the quickest option normally