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Old 03-07-2019, 10:32 PM
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Default Measuring my starter cranking current?

While I'm tied up with some mechanical/fabrication stuff in the garage, I've been kicking around some future projects. I want to pull out some more battery weight this year (I'm currently running a 16# 360 CA battery), but I'd like to do it a smarter way rather than just putting in something smaller and seeing if it works. (I'm looking at either a lithium battery or a supercapacitor bank, both which can get me down to the four pound range). I'd like to measure the starter current draw and see where I'm at. (I'm running a C4 PMGR starter in the '69.) If someone has some good ammeter ideas that would save me some time from looking all over the web, I'd appreciate the help/input.

Thanks!
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Old 03-08-2019, 06:32 PM
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I did that search already!

The sports racer was giving me some starter problems. Would spin, but not engage. I was thinking "ahh" maybe not enough grunt to fully push the drive into the flywheel. So, I had this link:
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But, I bought a new starter and installed it --and still had the problem. So, I found another flywheel and shaved 1 pound off it. I will install in a week or so. Then I'll really know if it was a mechanical issue. I pretty much eliminated it as an electrical problem - from the diagnosis. I reckon it should be less than 200-250 Amp draw.

Good luck! The ASE Guide is pretty good explanation.

Cheers - Jim
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Old 03-09-2019, 11:59 AM
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I am by no means knowledgeable about this but I have thought about this. I just bought a new battery for my truck and it is rated at 600 "cold cranking amps" which I assume is 600 amps while your cranking the starter. What I don't get is this, if the current draw on the battery is 600 amps and the voltage is 13 that is 7800 watts. That is 10HP! There is no way the starter is capable of 10HP or the demand is 10HP.

However, starting a DC motor under load does have a "starting current" that is usally many times it's running current. I worked on a medical device years ago that used a small DC motor that supplied an angular displacement based on a variable pulse (poor mans stepper motor). What I found is that the power supply had to supply much more power than the steady state power requirements based on torque@speed.

Doesn't answer your question but maybe indicates that this starting current transient might be very difficult to measure.
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Old 03-09-2019, 04:54 PM
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Quote:
Originally Posted by BBShark View Post
I am by no means knowledgeable about this but I have thought about this. I just bought a new battery for my truck and it is rated at 600 "cold cranking amps" which I assume is 600 amps while your cranking the starter. What I don't get is this, if the current draw on the battery is 600 amps and the voltage is 13 that is 7800 watts. That is 10HP! There is no way the starter is capable of 10HP or the demand is 10HP.

However, starting a DC motor under load does have a "starting current" that is usally many times it's running current. I worked on a medical device years ago that used a small DC motor that supplied an angular displacement based on a variable pulse (poor mans stepper motor). What I found is that the power supply had to supply much more power than the steady state power requirements based on torque@speed.

Doesn't answer your question but maybe indicates that this starting current transient might be very difficult to measure.
I agree with your comments. The CCA and CA ratings are the capability of the battery, not what the battery's going to be required to actually put out with every type starter and engine combination.

I expect a larger initial current draw when the starter motor just starts to turn, but I'm also trying to see what the "steady state" current is after the engine has started to rotate. I "half guess" that my clamp on ammeter might be able to tell me the initial current flow (as an increasing DC current looks a lot like part of an AC waveform), but once the starter motor is turning, I suspect that my meter will only display the ripple current, not the total DC current out of the battery. (I've looked into getting a high-current DC current meter, but I can't justify the costs they're asking for those things.)

So far my 300 CCA battery seems like overkill in the summer, but I'd sure like some actual data to give me an idea of what lighter/smaller size battery (or capacitor bank) I could put in the car.
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Old 03-09-2019, 10:04 PM
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Well, I was wandering the electrical aisles of a large hardware store this afternoon looking for boring house repair stuff, and happened to notice their VOMs. In the middle of a bunch of clamp-on meters that only measure AC current, I found one model that will also measure DC current (up to 400A). It was $80, so I took a chance on it. I'll keep my fingers crossed that it will actually be halfway usable for this particular project.
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