12V circuit breaker - polarity sensitive?

[Paddy Dzell]

I have a hybrid car and the 12V battery is under the boot floor, which is awkward to get to if I ever need to jump start the car so I've installed a socket in a more convenient and accessible place and I've taken two wires directly from the battery to that socket.

The socket is keyed so there's no chance of any reverse polarity accident occuring if I ever need to use my jump start battery pack but I need to protect that cable run from any sort of accident resulting in a short-circuit, so I bought a 12V circuit breaker rated at 30A (the computers draw less than 20A to get the hybrid system to the 'READY' state).

My question is - the circuit breaker has one connection labelled as 'Line' and the other one as 'Load', so is there anything in there that means it will only operate with current flowing in that direction?

The wires from the battery to the socket will have a permanent 12V supply on them. If I were to plug anything into that socket then current would flow from battery to socket. However, it's purpose is to do the reverse and provide 12V from an external source to the flat battery so current flow would be from socket to battery, so will the breaker work 'going one way' as an overcurrent device in case of a fault but also allow current flow in the opposite direction when/if needed?

Or should I just put a 30A in-line fuse in instead of the breaker?

[Michael Chare]

Google for 'starter motor current'. If a car wont start, I either put it on charge or just go an buy a new battery using a different vehicle.

[harry]

Are you brain dead? There IS no starter motor.

[harry]

[Paddy Dzell]

A normal starter motor draws upwards of 300A when cranking the engine but there's no starter motor in a hybrid and all the 12V battery does is power the entry/exit system and boots the computers to get the hybrid system to the READY state, a draw of less than 20A.

[Dave Plowman (News)]

How does the IC engine start without a starter motor? it may well be a combined generator and starter, though.

[Michael Chare]

I suspect to much depends on the way a particular car works. Some of the battery packs are 100 to 200 volts which would allow the motors to draw less current than a normal starter motor.

[Bob Minchin]

So how does the petrol engine in a hybrid vehicle start?

Does it initially drive on battery and engage the drive train somehow to turn the engine and bump start it?

[The Natural Philosopher]

The drive motor is the starter motor. Ona Pious at least IIRC. Its a weird power train designed to share torque betwen electric conventional and output

[Bob Minchin]

Thanks I shall be 86 when the 2040(?) petrol/diesel ban comes in and AIUI, this will also ban hybrids. If I am still driving by then, my plan will be to buy one of the last "proper fuelled" cars to see me out

[Paddy Dzell]

Dave Plowman (News) expressed precisely :

The traction battery is located under the back seat and is around 288V. There's an invertor/convertor that changes that to 650V to feed the two motors (well, motor/generators) underneath and it's MG1 that acts as a starter motor for the internal combustion engine.

There is also no alternator and both the power steering and the aircon compressor are electric, driven from the traction battery. Because the internal combustion engine (ICE) doesn't run all the time, you can't have anything that in a 'normal' car would be belt-driven from the engine. There's no reverse gear either - reverse is achieved by turning the electric motor in the opposite direction to normal.

Because the 12V battery doesn't have to provide a big cranking current they tend to be smaller and less capacity than usual and if the car is stood, say, in an airport car park while you're away on holiday for a couple of weeks, it's not common but not exactly unusual either, for the battery to be flat and the car won't start, hence why I fitted this convenient jumping point.

[The Natural Philosopher]

I will be dead.

95% probability :-)

[charles]

Like bump starting?

[Paddy Dzell]

Dave Plowman (News) formulated the question :

The traction battery is located under the back seat and is around 288V. There's an invertor/convertor that changes that to 650V to feed the two motors (well, motor/generators) underneath and it's MG1 that acts as a starter motor for the internal combustion engine.

There is also no alternator and both the power steering and the aircon compressor are electric, driven from the traction battery. Because the internal combustion engine (ICE) doesn't run all the time, you can't have anything that in a 'normal' car would be belt-driven from the engine. There's no reverse gear either - reverse is achieved by turning the electric motor in the opposite direction to normal.

Because the 12V battery doesn't have to provide a big cranking current they tend to be smaller and less capacity than usual and if the car is stood, say, in an airport car park while you're away on holiday for a couple of weeks, it's not common but not exactly unusual either, for the battery to be flat and the car won't start, hence why I fitted this convenient jumping point.

If there was ever an accident or something caused a short-circuit on these two new wires that I've run to the new socket, the potential short-circuit current could run into the hundreds of Amps, and those two wires won't handle that current for long before they melt and perhaps start a fire, which is why I need an overcurrent device in there. But I also need said device to allow current to pass the other way too, when or if I ever need to jump start the car, which is why I'm asking if a circuit breaker will be alright or should I just use a fuse?

[Theo]

Essentially there's an ICE, two motors and a planetary gear system. Depending on the field applied to the motors, they can either apply or receive (regenerate) power. One of the modes is that a motor can turn over the ICE and so start it. The design is actually simpler than a regular car (ICE+gearbox+starter+alternator)

The control system is such that it's smooth through the power curve - it can move power between the two motors, starting and stopping the ICE as needed, depending on how much power is required and the state of charge of the battery.

The Weber Auto YouTube channel above is great for explanation of how transaxles and hybrids/EVs work.

Theo

[Michael Chare]

Thank you for the explanaion. I suppose an overcurrent device might not trip if it was damaged in an accident. So maybe a fuse and spares.

[Andy Burns]

I have a normal* car, with the 12V battery under the boot floor, but it has + and - terminals in the engine compartment in case I ever need to jump start the car (or someone else's).

• actually it describes itself as a "light hybrid".

[Andy Burns]

My mistake, it's a "mild hybrid", it's still just a bollocks marketing term though.

[Bob Eager]

It was always a real pain jump starting a Mini...

[Paddy Dzell]

Yes, my car has one too. However, Sod's Law states that it'll be a cold, wet, windy night when I need to get the engine bay covers off and risk losing the little plastic clips while getting cold and wet. My new socket is in the centre console cubby hole under the arm rest so that I can do it all from the dryness of the drivers seat.