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really daft battery wiring question
jabbahutt - 2/4/07 at 12:50 PM

Afternoon

This has to be one of the esiest wiring questions but being completely inept at wiring I thought I'd check.

I believe the +'ve goes from the battery to the starter motor. does the -'ve just go to a decent earth on the chassis? It just seems a bit simple to be true.

Any help greatfully received as ever.

Thanks

BenB - 2/4/07 at 12:52 PM

On my BEC it goes straight to the battery negative terminal. It could go via the chassis but you'ld need to be really really careful to ensure a good contact at all points- starters pull serious current.... I'd go straight to the battery unless it was totally impossible...

theconrodkid - 2/4/07 at 12:57 PM

go straight to the engine for earth and a thinner one from engine to chassis

Macbeast - 2/4/07 at 01:03 PM

As ^^^^ but use flexible braid cos of engine vibrating. I think all cars I've dealt with in 40 years have had battery neg straight to chassis but it would be lower resistance to connect to engine first.

02GF74 - 2/4/07 at 01:04 PM

see my pos about wiring battery in something like "shold my leads smoke"

it is cpommon sense if you take a moment to think about it.

electrochemical battery is capable of supplying huge currents; the only part of a car that needs those currents is the starter motor.

therefore you want to wire he start motor directly to the battery with the shortest leads possible (those leads need top be capable of the carrying the current with ease).

Now it gets more complicated.

the starter motro doesn't have an earth terminal (I think) and that is made vi the meat of the gearbox/engine block. So you would wire -ve of battery to one of those points. The chassis is also connected to -ve as other compontnets use it as an earth.

Then the start motor is switched by a solenoid - the +ve battery goes to that switch.

jabbahutt - 2/4/07 at 01:20 PM

so let me get this straight.

+'ve from battery to starter
-'ve from battery to engine (say bell housing bolt)

then another earth from engine to chassis

have I got that right?

smart51 - 2/4/07 at 01:35 PM

The -ve from my battery goes to the bolt that bolts the starter motor to the engine. The engine is bolted solidly to the chassis and makes an excelent earth for the rear lights

Mr Whippy - 2/4/07 at 02:20 PM

quote:
Originally posted by jabbahutt
so let me get this straight.

+'ve from battery to starter
-'ve from battery to engine (say bell housing bolt)

then another earth from engine to chassis

have I got that right?



you got it.

02GF74 - 2/4/07 at 03:54 PM

quote:
Originally posted by jabbahutt
so let me get this straight.

+'ve from battery to starter
-'ve from battery to engine (say bell housing bolt)

then another earth from engine to chassis

have I got that right?


almost. the earth (-ve) you're fine with; the chasis to engine/gearbox strap shouldn't need to be a big cable as the majority of current, for starting should be going in the lead from battery to engine/gearbox.

The +ve side to starter - the is a switch in this circuit otherwise the starter will be runing all the time. Due to the big currents involved, there is a sloenoid that is in effect the switch.

So the fat cable from battery +ve needs to go to one end of the switch; the solenoid can eitehr be a separate device or part of the starter motor (I don;t know where yours is.) if separate unit, then continue the fat cable to the start motors terminal.

hope that helps.

caber - 2/4/07 at 07:55 PM

WRONG! Current remains the same the voltage drops across the starter motor or anything else! The return path needs to be able to sustain the full current otherwise you get that annoying smoke thing!

I plan to run the engine earth strap and the battery negative to the same point on the chassis and that will be at least an M8 bolt onto clean metal,not paint.It will get a good coat of vaseline to stop it rusting up. I will also be linking all the metal bits that bolt together with some wire earths particularly the dash, petrol tank, radiator etc. it is surprising how many car wiring problems are caused by bad earth paths!

Caber