Bump start a car with no battery?

[braddo]

If the battery has been removed from a car is it correct that it can’t be bump started?

I am guessing the loose battery leads mean a break in the electrical circuit if nothing else?

Maybe an old car with points ignition could still be started but for a car with an immobiliser and electronic fuel/ignition, no battery means no go?

Basically, the reason for asking is whether a car is any more or less secure with its battery removed (needs a recharge...) compared to a charged battery and immobiliser active.

[liner33]

Yes you cannot bump start a modern car without a battery fitted .

Are you aiming to immobilise the car ? There are much easier ways

[braddo]

The main reason for asking is that I had to take a battery out of a car to recharge it, and does doing so make the car any less resistant to theft?

Removing the battery means the engine immobiliser is no longer active. But then if it's impossible to get the car going without fitting a battery, then the theft risk hasn't risen.

The odds of theft are low (touch wood), but I don't want to inadvertently make the car easier to steal and risking insurance cover.

[liner33]

braddo said:

The main reason for asking is that I had to take a battery out of a car to recharge it, and does doing so make the car any less resistant to theft?

Removing the battery means the engine immobiliser is no longer active. But then if it's impossible to get the car going without fitting a battery, then the theft risk hasn't risen.

The odds of theft are low (touch wood), but I don't want to inadvertently make the car easier to steal and risking insurance cover.

Whilst the immobiliser is as you say no longer active neither is anything else.

[anonymous-user]

For those who are interested,
Old cars had a "dynamo" to charge the battery. A dynamo has physical magnets in it and can only generate a small amount of DC electricity. Any time the shaft is turned the rotating coil produces electricity as it moves through the magnetic field. Hence a car which is pushed along the road in gear will have electricity to fire the coil and plugs as soon as the clutch is released.
Modern cars have an "alternator" to charge the battery. An alternator has no magnets, just two different sets of coils which interact with each other and generate a huge amount of AC electricity. (Hence the need for diodes/rectifier to convert it to DC for your car.) One set of coils must be "energised" with electricity from the battery to produce a magnetic field and the other set of coils then rotate through that field to generate electricity. Hence if there's no battery installed or if it's completely flat your alternator won't work at all and your car can't be started by being pushed.

[300bhp/ton]

rockin said:

For those who are interested,
Old cars had a "dynamo" to charge the battery. A dynamo has physical magnets in it and can only generate a small amount of DC electricity. Any time the shaft is turned the rotating coil produces electricity as it moves through the magnetic field. Hence a car which is pushed along the road in gear will have electricity to fire the coil and plugs as soon as the clutch is released.
Modern cars have an "alternator" to charge the battery. An alternator has no magnets, just two different sets of coils which interact with each other and generate a huge amount of AC electricity. (Hence the need for diodes/rectifier to convert it to DC for your car.) One set of coils must be "energised" with electricity from the battery to produce a magnetic field and the other set of coils then rotate through that field to generate electricity. Hence if there's no battery installed or if it's completely flat your alternator won't work at all and your car can't be started by being pushed.

As this is 2021 and alternators have been mainstay for many many decades. You may need to clarify what you mean by 'old' and 'modern'

[anonymous-user]

And then, of course, there's the Magneto. Essentially an alternator with permanent magnets that produces a small amount of high voltage electricity. Usually found these days in your lawn mower or chain saw which can be started without any battery.

Although Magnetos are ancient technology (e.g. early aircraft) they are still very much in use today....

[braddo]

rockin said:

For those who are interested,
Old cars had a "dynamo" to charge the battery. A dynamo has physical magnets in it and can only generate a small amount of DC electricity. Any time the shaft is turned the rotating coil produces electricity as it moves through the magnetic field. Hence a car which is pushed along the road in gear will have electricity to fire the coil and plugs as soon as the clutch is released.
Modern cars have an "alternator" to charge the battery. An alternator has no magnets, just two different sets of coils which interact with each other and generate a huge amount of AC electricity. (Hence the need for diodes/rectifier to convert it to DC for your car.) One set of coils must be "energised" with electricity from the battery to produce a magnetic field and the other set of coils then rotate through that field to generate electricity. Hence if there's no battery installed or if it's completely flat your alternator won't work at all and your car can't be started by being pushed.

[Huff]

300bhp/ton said:

rockin said:

...
Modern cars have an "alternator" to charge the battery. An alternator has no magnets, just two different sets of coils which interact with each other and generate a huge amount of AC electricity. (Hence the need for diodes/rectifier to convert it to DC for your car.) One set of coils must be "energised" with electricity from the battery to produce a magnetic field and the other set of coils then rotate through that field to generate electricity. Hence if there's no battery installed or if it's completely flat your alternator won't work at all and your car can't be started by being pushed.

As this is 2021 and alternators have been mainstay for many many decades. You may need to clarify what you mean by 'old' and 'modern'

Other issue is - an alternator's output regulation can be wild without a battery to stabilise it/ deal with inductive load dumps, and that's likely a very good way to fry a car's electronics.
Don't start and then try to run a modern car without a battery in place.

[anonymous-user]

Old cars had a mech fuel lift pump + carbs and points + coil, one of which needs no power at all to work, the other which needs very little. A modern car with ECUs, electric fuel pumps, DI injectors, electric throttle bodies and the like is not going to start without power....

[Clifford Chambers]

Max_Torque said:

Old cars had a mech fuel lift pump + carbs and points + coil, one of which needs no power at all to work, the other which needs very little. A modern car with ECUs, electric fuel pumps, DI injectors, electric throttle bodies and the like is not going to start without power....

I doubt dynamos will produce enough power at bumping speeds, maybe a tow start.

[stevieturbo]

braddo said:

Basically, the reason for asking is whether a car is any more or less secure with its battery removed (needs a recharge...) compared to a charged battery and immobiliser active.

You can tow it away regardless, fire it onto a trailer etc etc.

Is it at any particular risk of theft ?

[braddo]

stevieturbo said:

braddo said:

Basically, the reason for asking is whether a car is any more or less secure with its battery removed (needs a recharge...) compared to a charged battery and immobiliser active.

You can tow it away regardless, fire it onto a trailer etc etc.

Is it at any particular risk of theft ?

Indeed, I would agree the bigger risk is that the car is lifted away. I don't think there's any particular risk of theft (touch wood); it was more a question of is the car any less secure by having no battery.

One could imagine an insurance company taking the angle that because the anti-theft protection (immobiliser) was not 'active', it was therefore easier to steal.

Not sure there any many car thieves out there who can be arsed trying to drive off in a Caterham with no steering wheel, no battery and a recalcitrant immobiliser. It would be enough of a hassle just getting into the driver's seat with the hood on.

[anonymous-user]

Max_Torque said:

Old cars had a mech fuel lift pump + carbs and points + coil, one of which needs no power at all to work, the other which needs very little. A modern car with ECUs, electric fuel pumps, DI injectors, electric throttle bodies and the like is not going to start without power....

Your point is well made in that "voltage drop" can be fatal to a modern car.

In other words the battery may still deliver enough current to turn the starter but during the cranking cycle the voltage drops too far because the battery is weak, i.e. below about 9 volts. At this point the various computers don't have enough voltage to run properly and either the car will fail to start at all or it will start with a dashboard full of error messages. In the latter case, if you run the engine for a few minutes then switch it off and re-start it the error messages may well clear. But you still need a new battery a.s.a.p.

IMO if you have battery problems spend your money on a new battery, not on a charger.

[Huff]

Max_Torque said:

Old cars had a mech fuel lift pump + carbs and points + coil, one of which needs no power at all to work, the other which needs very little. A modern car with ECUs, electric fuel pumps, DI injectors, electric throttle bodies and the like is not going to start without power....

Exactly - all of which prob needs sustained 30A+ off the 'key' to do their thing.

and then, the car likely will not attempt a start unless a minimum sustained rpm of what, 150 or so, is registered by -already alive body electronics. So as not to chuck injected fuel at cold cats, and all sorts of other things you know far, far, far better than I.



tl;dr: start a duff car off a spare fully-charged battery. 'bump start' = no longer meaningful.

[Mave]

braddo said:

Removing the battery means the engine immobiliser is no longer active.

Is that true? I always thought that an immobiliser is actually a mobiliser. Ie you need it to be active to be able to start the car rather than needing to be active to disable the car.

[braddo]

Good point. I don’t know. By inactive, well, I mean the red light doesn’t flash any more! But it makes sense that if I immobilised it before the battery went dead, it won’t just mobilise itself by reconnecting the battery.

[GreenV8S]

braddo said:

Basically, the reason for asking is whether a car is any more or less secure with its battery removed

Most cars will have an alarm as well as an immobiliser. I'm not convinced a car alarm going off actually makes any difference at all but your insurer might feel differently.

[anonymous-user]

When I was in school, early 60s, I discussed this with my Dad, because my uncle had done old bike without a battery, in thought it should work for a car. We took the battery of his Westminster and tried to bump start it down the hill, and in Wales we have proper hills, didn't work, put battery back on at bottom of the hill and drove home.
May be theoretically possible but it doesn't work in practice.

[Chris32345]

braddo said:

Indeed, I would agree the bigger risk is that the car is lifted away. I don't think there's any particular risk of theft (touch wood); it was more a question of is the car any less secure by having no battery.

One could imagine an insurance company taking the angle that because the anti-theft protection (immobiliser) was not 'active', it was therefore easier to steal.

Not sure there any many car thieves out there who can be arsed trying to drive off in a Caterham with no steering wheel, no battery and a recalcitrant immobiliser. It would be enough of a hassle just getting into the driver's seat with the hood on.

At imboliser is always active
It needs to be energised to turn it off



Or else they would be too say to bypass