How many volts/amps needed to start 7.3? - Ford Powerstroke Diesel Forum
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Old 06-09-2011, 06:10 PM
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How many volts/amps needed to start 7.3?

Chasing a hard start/long cranking issue. Just checked batteries but can't find volt/amp minimums required. The batteries are almost 3 years old and have a 850 cca rating. Both are showing roughly 12.3 volts (57% charge) and 750 cca when tested separately. (Tester shows 1120 cca when both are hooked up to the cables in parallel). Starter draws less than 160 amps. Alternator output is 13.29 volts - 60 amps. Am I in the ballpark or do these numbers sound weak?
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Old 06-09-2011, 06:44 PM
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You're in the ballpark, but there are a lot of if's that come with that.

If both batteries are at 750cca, then they should be 1500cca together.
850cca is a good size for these trucks, but it doesn't hurt to go higher.
12.3v is the surface charge, you need to load test them to see the actual voltage.
Is the 57% SOC after charging the batteries? If so then your batteries are only holding half of their original capacity which would explain the 1120cca and hard starts.
60a for the alternator and 13.29v is a bit week, but could be because the batteries are drawing it down or could just be the RPM it was tested at. Realistically, I would prefer to see 13.6-14.2v from the alternator and at least 90a from a stocker.
The current the starter draws depends on its condition and the load. The starter on my 2000 is marked 3600w which would be 300a loaded. If the truck is cold, the fluids will be harder to move and require more from the starter.
You also have glow plugs that come on every time you turn the key to run for up to 120s depending on temperature. So that's another ~100a draw.

My guess is your alternator is getting weak and not fully charging your batteries.
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Old 06-09-2011, 07:48 PM
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Thanks for the quick reply. I'm trying to do some layman's diagnostics, before taking it to a shop and getting ripped off again.

The 57% charge I mentioned is actually the average of what I found when I ran the test on a borrowed AutoMeter Battery Analyzer - one battery was 53% and the other was 61%. I dont know if that tester puts any "load" on the battery. The batteries had not been charged, prior to this test and the truck has been driven very little in recent months.

Ref the starter "draw", are those numbers OK or too low? The analyzer spit out those results before the starter really even got rolling.

If I have to replace a 10 year old alternator and 2 batteries to get this thing to start right, I'll be a happy camper.
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