jfb

87 vette, charged battery, 50F outside temp. Peak current recorded at 313 amps. Fluke Y8100 clamp on dc/ac ammeter, Fluke 177 peak holding digital voltmeter.
I am suspicious though of the calibration of the clamp on ammeter and am in the process of checking it. I may revise the peak current.
I am also going to do this measurement again but with a storage oscilloscope or a Fluke recording voltmeter.
The clamp on ammeter is in need of calibration. After checking it, I have to revise the peak cranking current to be 240 amps.

Sandpiper59

That sounds very feasible.. Of course a lot of variables come into play on top of that measurement. Aside from temp (which you have and will assume the block was at ambient) is battery voltage at the starter during cranking, cranking RPM (a stiff tight motor will slow the starter, which in turn will draw more amps) etc

You can back peddle all your numbers using basic ohms law. Measure the DC resistance of the starter, measure the resistance of the soleniod and add them together (it's in parallel so 1/starter + 1/soleniod = 1/total), devide that number into your cranking voltage at the starter and you now have the amps you're drawing.. You need a good meter though as the ohms will be 0.something ohms.

ie, .05 on starter, .8 on the soleniod = (1/.05) + (1/.8) or (20)+(1.25)=1/21.25= .047ohms. If there is 12 volts at the starter during cranking, 255 amps are being used. If only 9 volts during cranking, only 191 amps are being used. Next, see how much voltage is at the battery during cranking, divide the difference by your ohms and you now know how many amps is being lost in the cables and connections. (for those interested, multiply your volts times your amps to get watts, ie the starter above would use over 3000W at 12V and only 1700 at 9V, that's 1/2 the cranking power with a 3V drop)

dizwiz24

This is great information. I recently relocated my battery (93 supercharged vert). Since I used heavy 1/0 gage wiring I wanted fuse protection. I had some issues deciding what 'size' fuse to get. I settled on a 300 amp ANL fuse back there as a balance between protection (dont want the battery to blow up in the rear compt behind the passenger) vs. being able to start the car under all conditions.

Ive started the car up quite a bit in 30-40F tempts and it hasnt blown or anything so it must be good. I frequently take my vette on 200 mile work trips and would hate to be stranded somewhere bc the fuse blew.

But yeah, the better way is to measure it, in a variety of temperatures, cranking conditions (ie. cold oil vs. hot oil) as jfb has done.

jfb

Take a spare fuse and tools to change it with you!

dizwiz24

Q: Ive got a question...

What if you are running (at which point most vehicles put out about 14.2 volts), and you accidently turn the start key (grind, grind, grind! ).

Wuuld that draw significantly more amps than a starting-up car cranking at 12 volts?

Sandpiper59

No, amp draw drops as motor load drops (note, the above calculations would be a stalled starter, or max draw, should have mentioned that). In a 'grind' situation, the starter really is free spooling and will draw much less current (and spin much faster than if cranking)

Sandpiper59

Probably a safe size fuse for 1/0 wire, but possibly too small in the long run. A battery can easily offer up 600 amps (check your cold cranking amp rating). Depending on the ohms across your starter you can have problems. Lets assume stalled, the starter can draw 450 amps, then if your engine decides to jam briefly (like they can do sometimes with a weird ignition fire) then you'll pop your fuse.

Flip side is the American Wire Gauge table says 1/0 is safe up to 245 amps. Since your starting is not continuous then 300 amps should be safe but is over the recommended limit...


I used to deal with Telephone power supplies (main central office supplies).. We used 500MCM and 750MCM cable (750 is about 1 3/4" for just the copper) and even the 750 would run warm to the touch with 500 amps continuous going through it (48vdc)

dizwiz24

It handled my compression test, in 40F weather, the other night without blowing. 3 readings per each cylinder, about 3 seconds each time.

THough you do bring up a good point...

What happens if I go to start it in cold weather, radio on, headlights on, parking lights on, etc.

I should prob test it out under those circumstances.

I have 100/130 (hibeam) watt headlight bulbs those could draw some amps. Not a lot, in comparison w/ starter, but still an additive effect.

jfb

Almost all electrical power goes through the ignition switch and everything the ign sw controls is off during cranking.