Starter motor current draw?

[CLB]

Starter motor is 1.5hp
Engine is 4 cylinder Petrol 2.3 litres.
Battery is 12v

Anyone want to hazard a guess at the sort of current flow when the turning engine over?

Best I can come up with is around 90a based on 1.5hp being equiv 1100w at 12v, but I know that initially, at least, it will draw more. The question is, how much?

In case you are wondering why I need to know, I am thinking of fusing the circuit and trying to work our fuse size. Would 150a be too small?

 

[Daverw]

Not sure why you would want to, for a few seconds of running I would like my engine to start and not a fuse blow. Direct short unlikely if cable and connections well protected

 

[jim.howes]

I cannot answer your question but I am sure your local auto electrics engineer might be able to help after all its really only a floating car. You have the right idea to put a fuse in the system as you cannot guarantee the wirering/connections at all times. Better a fuse than a fire. Jim

 

[Mistroma]

Just a thought, shouldn't you also consider the cable. e.g. 16mm2 is probably around 110A and 25mm2 about 170A.

No point fitting 200A fuse if cable has a lower rating.

 

[halcyon]

Starter motor is 1.5hp
Engine is 4 cylinder Petrol 2.3 litres.
Battery is 12v

Anyone want to hazard a guess at the sort of current flow when the turning engine over?

Best I can come up with is around 90a based on 1.5hp being equiv 1100w at 12v, but I know that initially, at least, it will draw more. The question is, how much?

In case you are wondering why I need to know, I am thinking of fusing the circuit and trying to work our fuse size. Would 150a be too small?

All depends on starter motor design, but stall current ( when it starts to turn engine ) could be 500 amp for a small engine, as revs go up amps come down 250 amp possibly, but again it can vary due to many factors. The 1.5 hp is output of starter and not related to input current, not as in your calculation.

Brian

 

[pvb]

Starter motor is 1.5hp
Engine is 4 cylinder Petrol 2.3 litres.
Battery is 12v

Anyone want to hazard a guess at the sort of current flow when the turning engine over?

Best I can come up with is around 90a based on 1.5hp being equiv 1100w at 12v, but I know that initially, at least, it will draw more. The question is, how much?

In case you are wondering why I need to know, I am thinking of fusing the circuit and trying to work our fuse size. Would 150a be too small?

You're fusing the cable, so choose a size appropriate for the cable.

 

[CLB]

Well, that didn’t go as planned ?

I am thinking of using 35mm2 cable but was kinda hoping that the answer to my initial question would confirm, or otherwise, that decision. Surely I need to know the current draw before I can spec the cable, and therefore the fuse?

 

[VicS]

Starter motor is 1.5hp
Engine is 4 cylinder Petrol 2.3 litres.
Battery is 12v

Anyone want to hazard a guess at the sort of current flow when the turning engine over?

Best I can come up with is around 90a based on 1.5hp being equiv 1100w at 12v, but I know that initially, at least, it will draw more. The question is, how much?

In case you are wondering why I need to know, I am thinking of fusing the circuit and trying to work our fuse size. Would 150a be too small?

I have some figures which might be of interest

A 1.1 HP ( 810W) Bosch starter motor fitted to a 2.1 litre, 4 cylinder, petrol engine

Unloaded 30 -50 amps at 11.5 volts
loaded 185 - 220 amps at 9 volts
locked 400 - 490 amps at 7 volts

 

[bedouin]

Well, that didn’t go as planned ?

I am thinking of using 35mm2 cable but was kinda hoping that the answer to my initial question would confirm, or otherwise, that decision. Surely I need to know the current draw before I can spec the cable, and therefore the fuse?

I think you will usually spec the cable based on voltage drop not maximum current.

I am not even sure that the current rating for the cable is particularly appropriate when talking about starting current as ratings are likely to be a continuous rating whereas starting is much more about peak current

 

[pvb]

Well, that didn’t go as planned ?

I am thinking of using 35mm2 cable but was kinda hoping that the answer to my initial question would confirm, or otherwise, that decision. Surely I need to know the current draw before I can spec the cable, and therefore the fuse?

35mm2 cable should be fine, in which case use a 250A fuse. MegaFuses are good.

 

[PaulRainbow]

35mm2 cable should be fine, in which case use a 250A fuse. MegaFuses are good.

Spot on, 35mm cable and 250a fuse

 

[bedouin]

Spot on, 35mm cable and 250a fuse

How can you say that without knowing how long the cable run is?

 

[PaulRainbow]

How can you say that without knowing how long the cable run is?

Because i'm clever.

 

[cpedw]

Can we calculate the maximum possible current from the resistance of the motor measured with a multimeter? As I understand it, the motor's impedance (resistance when it's running) can only increase from there. I could be wrong though.

 

[CLB]

I have some figures which might be of interest

A 1.1 HP ( 810W) Bosch starter motor fitted to a 2.1 litre, 4 cylinder, petrol engine

Unloaded 30 -50 amps at 11.5 volts
loaded 185 - 220 amps at 9 volts
locked 400 - 490 amps at 7 volts

That’s interesting.

 

[CLB]

35mm2 cable should be fine, in which case use a 250A fuse. MegaFuses are good.

Thanks. I was just about coming to a similar conclusion. In fact already got the mega fuses in my eBay basket. ?

 

[CLB]

How can you say that without knowing how long the cable run is?

It’s a short run, maybe a metre from battery to switch to starter.

 

[VicS]

Can we calculate the maximum possible current from the resistance of the motor measured with a multimeter? As I understand it, the motor's impedance (resistance when it's running) can only increase from there. I could be wrong though.

You need specialist equipment to measure such low resistances. .

 

[JumbleDuck]

Spot on, 35mm cable and 250a fuse

That's what I fitted (well, the fuse is a circuit breaker, but still 250A) for my 1GM10 and kept for my Nanni N2.14. Nothing has gone bang, fizz or pop yet.

 

[Rappey]

Not quite a starter motor but similar, i had a 12v winch, 13,000 lbs.
Was 5.3kw and drew about 400A. No load just to spin it was 80 A.
My 2.1 car diesel draws about 250 amps, measured with a clamp meter. the car diesel starter motor is tiny compared to my old bmc starter motor.. the solenoid is nearly as big as the car starter motor ! guess thats improvements in technology ?