12V circuit breaker - polarity sensitive?

A diode will prevent reverse connection. 30 amp ones are not that expensive. But will introduce about 0.6v voltage drop.

Dave Plowman (News) expressed precisely :

Thanks but I don't need a diode. The socket is keyed so that a reverse connection can never happen. I need to know if the breaker will work as an over-current device when the direction of current flow is from battery to socket, but also allow current to flow in the opposite direction if I need to use the jump start battery pack.

It's the fact that one leg of the breaker is 'Line' and the other leg is 'Load' that's throwing me - is there something in the breaker that needs it to be wired that way? A fuse would blow if there was over-current in either direction but will a breaker allow current flow both ways and still work as a breaker?

Unlikely its not designed to operate in that manner

The line in is the power supply the load is the source of the power requirement , its the load that's protected foam over voltage or over current (depending on the type of breaker you are using) not the supply . These can be protected electronically or electro magnetically or a combination of the two , reversing the polarity of the breaker may mean it will not work , or just trips out

Some hybrids have 12V jump start terminals as part of the inverter pack under the bonnet. I think most Priuses have this.

D A Stocks submitted this idea :

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.

What kind is the circuit breaker? I assume it's suitable for DC, but is it thermal, magnetic, etc?

For example these thermal ones:

I would assume are bidirectional (it doesn't matter which direction the current is flowing in, it'll still get hot) but a magnetic one: is assymetric according to the diagram, although exactly what that means is unclear.

In those designs, it looks like current will flow both ways unless tripped.

You could test it by applying a >30A load to find out.

Theo (pondering whether the lead acid battery could be replaced entirely by one of those jumpstart packs)

Thanks Theo. It's one of these but I don't know if it's thermal or not:

It rather depends on how the breaker works. If it is electronic, it may only function correctly if inserted in circuit as labelled,

I'm a bit surprised there isn't a 'jump start' terminal already provide. My Hybrid Outlander also has its 12V aux battery in the boot and, like yours, isn't easy to access. (There is a removable cover to allow you to disconnect the -ve/earth but the +ve side is tucked away.) However, under the bonnet, there is a special terminal provided to allow you to connect a 12V external battery to jump start the vehicle. Like yours, it only really jump starts the electronics, not the engine, the starter motor is powered from the main drive battery under the floor on the Outlander. I've read of people using small (in terms of capacity) batteries to jump start Outlanders as only a relatively small current is required (I'm not sure what but I suspect far less that 20A.)

As for a fuse vs breaker, given it will only be used 'once in a blue moon' (hopefully) I'd opt for a fuse and carry a spare, just in case.

Ah, that makes more sense now- I would have been surprised if a jump start point wasn't provided. I wonder, does your hybrid not charge the aux battery when the vehicle is parked for an extended period from the main (drive) batteries? The Outlander does this (I think at 2pm, why

2pm I don't know) if the vehicle hasn't been driven for (I think 24 hrs). I've left our Outlander, main (drive)battery charged but not quite fully for 5 / 6 weeks and it started first time, with no noticeable drop in drive battery charge (ie range). I don't leave in hooked up when we are on holiday etc. Whether it will do the same when the batteries are older time will tell- it is only just over a year old.

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.

15.32A for my Lexus RX450h to reach the 'READY' state.

Cheers, think I will.

I'm surprised it is as high as that but I assume you've checked / measured it so accept what you say.

I'm curious what it is all powering- 15A at 12V is 180W- a couple of laptops worth.

Yep, here's a photo:

I wasn't doubting you but thanks for the photo.

I'm still curious what is is being used for. A decent laptop (in terms of performance) would have perhaps a 60 or at most 90W PSU so we are looking at between 2 and 3 times that. But a laptop has all the graphics, a car only has one set, and graphics can be needy in terms of power. (I'm still not disputing your 15A, just trying to play with the numbers- I'm a cross between an Engineer and a Mathematician, 25 years as one and 11 year teaching Maths, degrees in both, makes you rather a tinker with such things ;-)

I like you idea of a jump start point accessible 'in the warm'. I'm planning to add some wiring to the 12V battery in my Outlander to power some amateur radio kit and, while I have the boot floor up etc, I may run an extra cable, just in case.

We used to have a CRV and, when it was left parked during our long (motorhome) jaunts, the battery often ran flat. In the end, I ran a cable to the rear so I could link it to a float charger- all protected with diodes and fuses in case we forgot and tried to drive it away or someone tinkered with it one the drive (at the time, we had another car in the garage).

No worries my friend, I know you weren't doubting me but yes I agree,

15A does seem a lot just to power up a couple of ECUs.

Good idea on running a spare cable just in case - from John G7IFM :')

In this case:

it's powering the electric brake pressure pump which takes 15A for a few seconds.

There will also be inrush currents for loads like capacitors and contactors.

Theo

Thanks again Theo, very interesting.

Us radio types seem to get everywhere, there are a few in this group.

73 G8OSN