540i starter current draw

[84318i]

Hi,

Does anyone know how much current stock 540i draws from the battery when cranking ?

I am asking because I'd like to add a circuit breaker in the trunk right near the battery but have no idea how much current it draws and don't have an a claw style amp meter to measure .

Thanks

[zubbie]

Never measured it but I'm going to guess in the 300 amp range for the starter, 15 -20 amps for the solenoid. Why do you want to put a breaker in the starter circuit? There's a reason why your starter is directly connected to the battery with a thick copper wire

[84318i]

I really doubt it's that high though , 300 amps by 12 volts would be over 3000 watts ...

I want to add a circuit breaker for simple safety reasons, I don't like having an unfused high current directly connnecfed thick gauge wire going from the trunk of my car to the engine bay. Circuit breaker won't affect anything in a negative way if it's spec'd correctly .

[moroza]

I'd guess it's in the hundreds of amps. FWIW, the starter on my 2.2L Toyota with ~8:1 CR is rated for 2kW IIRC. 3kW for a 4.0 V8 with 10:1 seems reasonable. Also, isn't there a reason our stock batteries are rated for ~800-950 cranking amps?

RealOEM says starter 12411736921 is rated at 1.7kW, which to me seems an underestimate.

A quick search found this.

"A starter motor from a normal 2 Litre petrol car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin... Under winter conditions, this current can easily double when the oil is thick.

Diesel engines have a very high compression ratio e.g. 22:1 and require more powerfull starter motors. On average they draw between 300 and 500 Amps for average sized car engines 2 to 3 litre while on truck engines the current could easily reach 1000 Amps at initial turn over."

It's a random guy on the internet but sounds knowledgeable, and their guesses are in line with ours.

The problem you'll run into is that just about any short circuit in the starter circuit will be well under its normal draw. A short of, say, 50A could well blow something up but the breaker won't trip. The saving grace is that there are few opportunities for a short on that wire anyway. (Unlike the TxD data link... ask me how I know... )

[84318i]

That's helpful , I was thinking it would be under 2kw . I guess I'll just have to measure it or try a couple different high amp fuses in series and see which one is enough ...