Disconnecting battery while engine is running.

Only if you want to buy a new alternator (possible damage) , and possibly fuel computer etc. That battery is like a big capacitor to make your electricity S m o o t h. direct from alternator is very dirty and has loads of noise and ripples.

Any electronics present- DO NOT.

Tim..

It's said the output voltage can rise *way* above the nominal 12 volts, so could fry things - including possibly the alternator itself.

Don't see the difference given that modern diesels have engine electronics too.

No my car is one of the most simple cars you can find in the market. there aren't any alarm systems, central locking, etc.etc. the engine is carburettor so there's no computers present. In fact, when the lights and air cond is off there's no other accessory consuming electric power except for maybe fuel pumps and radiator fans. that's all. It's a 660cc 3 cylinder engine.

Is it still not OK to disconnect the battery while engine is running?

Why would the alternator's output rise above normal? Wouldn't the regulator on the alternator itself regulate the voltage for me so it stays within range?

Thanks.

It probably would be, insofar as there is nothing much which is voltage- spike sensitive. If you have a radio or similar, that might not like it.

It depends on the regulator design, but some, if they don't have a load, can produce _much_ higher voltages than spec., which can damage the regulator. Whilst the engine is running, the ignition coil will draw an average of a few amps, which may be enough to load the regulator, but it is an inductive load, which is not ideal. I would suggest that if you are going to do this you put some sort of resistive load on. I would suggest headlights and/or heated rear screen.

HTH

Andy

Done this before, you can and will get away with it until something goes awol. Battery does act as a capacitor, disconnect the battery and start turning on electrical services and you'll give the alternator a hard time.

I've got a battery indoors for this purpose (wont hold 12 volts for more than a couple of days) use it as a hot swap battery if I need it. Good battery in, start car get the jump leads out and hot swap with my iffy battery whilst running.

No Dont do it. You risk causing lots of problems. Potentially very expensive ones.

Ed

If you got a large 1farad audio cap you could do the same :)