starter amp ???

[John Dinkel]

About 200-250

[Tom keedle]

Originally Posted by John Dinkel

About 200-250

really?
that seems awful high for a gear reduction starter..
guess i'll have to check mine if and when it ever gets a battery hooked up to it.

[Bill Harris]

Those starter are usually 1.4KW (around 1.9HP). At 746 Watt/HP, Mr Ohm sez that with a 12V battery it takes 116.67 Amps to dissipate 1400 Watts. That, of course is at 100% efficiency. If you assume a 70% efficiency the starter will draw about 152 Amps at rated output. The mini-starters use permanent magnets for the field so they draw a lot less current than the old style wound field (electromagnetic) starters which draw a lot of current just to create the static field.

Note the importance of having really good cabling and connections between the battery and the starter. If you drop voltage across the cabling, the amperage will go up quickly. If you only supply 10V to the starter due to drops or insufficient battery capacity, the amperage will go up to 182 Amps to get rated output from the starter. This is a vicious circle since as more current is drawn, more voltage is dropped across the resistive cables/connections so the voltage at the starter goes down, the current tries to go up.... and so on.... leading to the dreaded "click-----nothing".

Sorry for the lecture.. I'm bored....

[FED 387]

BILL---OK so based on your info what would be the smallest battery Amp hr or CCA or whatever would be needed to use this starter with a #6 cable with a maximum total length of about 48 inches probably closer to 36----all new stuff connectors ,cable , eye rings etc.---just trying to determine what is the smallest that will work satisfactorily-----can always get a larger battery if needed just trying to see what is the smallest for now-----any other suggestions you can make??--- is the battery/starter cable tthe correct size to big too small just right?? your thoughts please---yes,weight is a consideration here too---thanx FED387

[Run to Rund]

I suggest you check on yellowbullet.com and look at the wire sizing chart:
http://www.yellowbullet.com/forum/sh...ad.php?t=47266
it looks llike 4 gauge would be better. I use 1 gauge for the 3' to my RobbMc mini starter from the junction block because too big won't hurt, and the junction is 24' from the battery (2/0 to the junction from the battery). Also, the initial surge needed may be a lot more than the running amps.

[FED 387]

Run 2 Rund ---excellent site for the cable info --thanx FED387

[Bill Harris]

From a battery perspective, a lot depends on whether you have a functional alternator and if you always put it on a charger in the pits. A little battery like a group 35 Optima red-top at 31 lbs would certainly start the engine with good cabling, but how many times? I have always had two batteries in my racecars, and a functioning alternator, but I still worry about having to start the damn thing 10 times while moving up in the staging lanes. If you don't have a working alternator, and have drained the battery before staging the car, it might not have the capacity to properly run the ignition system during the run, much less start the engine after the scales or fuel check.

As for the cables and connections, just keep one thing in mind. At high amperages like those associated with a starter, very small resistances can result in large voltage drops. Automotive electrical systems are plagued by having to deal with a relatively low voltage and therefore you have to deal with a lot of current if you have to supply a lot of power (hence the 16V batteries that are out there). The ear-bleeding car stereo whackos are very familiar with this situation and have developed some pretty good electrical stuff to deal with multiple 1000W power amps and such which have to work off 12V.

So lets say you cheap out an use that old battery shutoff switch you have laying around. You wire your car in a way that all current from the battery goes through the cutoff switch which is the usual (and legal) way of doing things. Say the switch has 0.01 ohms of resistance. In general, 0.01 ohms is a very small amount of resistance. You probably couldn't read that low value on the usual cheapo volt-ohmmeter, it would just say zero. Assume all the other connections are perfect. That stinking little 10 milli-ohms of resistance is going to drop 150*0.01 = 1.5V all by itself, reducing your 12V to 10.5V at the starter. But now if you need the full 1.9HP from the starter to crank the engine it will take 173 Amps at 10.5V with the additional current dropping the voltage even more... and so on. You can tell that the switch is a POS since it will get hot. If it drops 1.5V at 150A it will be dissipating 225 watts, more than two 100 watt light bulbs, and you know how hot a light bulb can get. If your cut-off switch is even warm, it should be in the garbage.

The point is that it is absolutely imperative to make the best electrical connections you can possibly make when you have to carry a lot of current. Tiny amounts of resistance can cause huge differences in the ability to deliver power to the intended load (the starter motor).

For your 4 foot cable length, a 6 gauge cable will present around 410 micro-ohms, which has a voltage drop of 0.0615 volts at 150 amps, well within the "don't care" region. A 4 gauge is about half as much. That is all hunky-dorey, as long as the connections between that cable and the battery and starter are very, very small resistances too, and that is where the problems can arise. If you crank your engine over for a few seconds and then touch the cables or cable connections and they are hot, or even warm, you have a problem.

One area that people forget is the "ground" path. If you are supplying 150A from the battery to the starter motor, then 150A has to go back from the starter motor to the battery. There are two current paths to deal with, not one. Remember that strap from the cylinder head to the firewall that you didn't bother to put back on because it was a pain? Put it on. Even with solid motor mounts and a torque strap, use a good large gauge strap between the engine and the chassis. Solid motor mounts are mechanically solid, not electrically solid. The starter is probably the highest current draw you will encounter in a race car. Run the same sized cable from the starter to the chassis as you do from the solenoid to the battery, then make sure that the negative side connection from the battery to the chassis is excellent, nothing less.

Can you tell that I'm still bored?