Car won't start, it's like the battery is dying, but it tests 100%

[1vicissitude]

So I have a strange issue occurring on my 72 240z.

 

I have been driving the car without issue for the past  2 months since I bought the car. The car has triple Weber 40s, l24 block, n42 head, mechanical fuel pump still, etc. I didn't drive the car for about 9 days, and I am used to needing to pump the gas a bit and modulate the throttle to get it running once it has sat a bit. 

 

I went to go start the car and the car is completely dead. The car is acting like the battery is 100% dead. Well I have an Optima redtop in the back of my miata (that's in pieces) that fits just fine so I dropped it in. I got a couple 15 second cranks out of it, but then the battery acted like it was dying too. I first attributed this to the battery having sat a bit so i threw it on the trickle charger for 5-6 days. I took it back out and the car could only crank for maybe 20 seconds total before acting like the battery was dead again. I repeated this one more time, same results.

 

When I first put the Optima the battery into the car fresh of the trickle charger it cranked really fast, but slows down to the point it can't even crank the motor with in 30 seconds or less. The lights dim, the dash dims, the head lights barely function. 100% symptoms of a dead battery.

 

This is where it gets odd. So I take the Redtop down to my local Autozone. The guys there have this fancy load tester and it has a specific setting for Redtop Optimas. The two guys at this location really seem to know their stuff BTW, I know some locations have some rather shady unknowledgeable guys behind the counter. Well the battery tests better than new. Read 12.87 volts, passed the load test with flying colors.

 

The guy there suggested that it could be either the starter solenoid or the starter, but he was kind of guessing at the problem because that is the only thing the battery is directly connected too. Now I am inclined to try swapping it out, but I wanted to check if this is a common issue or if anyone has had such a weird situation happen before.

 

 

 

TLDR: Car acts like battery is dying very fast, cranks full speed down to a near stop in 20-30 seconds. The battery is testing out perfect. What could be another cause?

Edited August 30, 2013 by 1vicissitude

[Domzs]

_{20 sec crank time is too long . You can damage the starter by over cranking . It needs cool down time between cranks . Make sure the battery is fully charged and in good condition , check your cables and terminals ( cleaned and tight ) . If everything checked out , the problem could be the starter .}

[1vicissitude]

Sorry, I should have clarified. 20-30 seconds of crank time total. As in I will crank it for 5 seconds, stop, and repeat. Each 5 second set progressively slows down and the lights dim darker and darker until it just can't turn the motor over anymore and the lights are nearly dead. So it takes about 4-6 5/second cranks before the car is dead. With the battery testing out perfect it should be able to crank the car for a loooong time. I mean it's rated for 800CCA, 1000CA, 50aH, and has 100 minute reserve time. Something is definitely wrong. It should be spinning like a top.

 

Is there anyway the test performed can come back with a false reading? Or the battery can read proper voltage with no amps? 

[johnc]

Check your battery cables, the chassis, and engine grounds. Run the biggest cables you can.

[beermanpete]

I suspect 2 bad batteries. The load testers most part stores have don't load the battery as much as the starter does and can give misleading results. Have you tried a know good battery from a car in daily use? You could also try the suspect batteries in another car.

[1vicissitude]

Well the Optima would read steady voltage on the miata's VDO volt gauge, would start it very strongly, and on the AEM EMS everything was also steady. I usually had it on a trickle that had an auto cut-off. Unfortunatly I can't test on this car anymore because the motor is dismantled.

 

I have not tried another battery though. I was planning on buying one when it tested dead, but instead the positive test just left me puzzled. I pulled it straight out the car when it wouldn't crank anymore and brought it in for testing. I was 100% sure it would fail because it seemed like it was completely dead at the time of the test, but it came back ~12.87v. 

 

The battery ground and positive cable are 2 awg, look pretty new too. A lot of stuff in the bay has been replaced recently, I can tell just by the condition of the parts. The car has never been a garage queen so the newer appearance of a lot of misc parts leads me to believe a lot has been changed out. Albeit, the history of the car is largely unknown to me. Other than the carfax there was a language barrier between the previous owner and myself so I couldn't harass him over too many details.

 

It seems to be firmly grounded. (this was my first guess after the battery didn't solve anything)

 

I'm thinking of replacing the starter and cables anyways, it's cheap enough to be nearly inconsequential. More of an annoyance than anything. This way i can reaffirm all the connections as well and clean everything.

 

I just wanted to double check there isn't something else that could be the cause of this. I suppose I can yank the battery out of the Accord and swap it in, but in all honesty I'm worried that there is some sort of short or something. Last thing I need is having all my cars stuck haha. (plus that darn battery was just replaced under warranty, it's a 12' on lease) 

 

I appreciate the replies so far fellas. Thanks.

Edited August 30, 2013 by 1vicissitude

[1vicissitude]

Well, it was the battery after all.

 

Lesson learned, don't trust the autozone battery test.

 

Thanks again for any input.