voltage drop problem
 

I have converted my sbc to a 3 wire alt and a hei from points. My voltage is dropping from 12.35 to about 9.40 when cranking. Then back to 12+ in the run pos. Any thoughts? Thanks

Re: voltage drop problem
 

It does take some voltage to crank an engine over...then once its going the alternator maintains the battery from there. A voltage drop while cranking is normal...long as it doesn't stay that way when the engine is running. :chevy:

Re: voltage drop problem
 

That number seems low, is that alt putting out enough? Don't you have to wire an HEI directly to a 12v source and get rid of the resistor and fusible link?

Re: voltage drop problem
 

I agree. Plus the available voltage without the engine running and when cranking doesn't have to do with the alternator but the battery. Unless of course the alternator is not maintaining the battery.

Re: voltage drop problem
 

Where are you checking the voltages at ? what size are your battery cables? and did you run new wire (what size) to the HEI?

Re: voltage drop problem
 

how is your ground? i like a good ground to the frame and to the body.

Re: voltage drop problem
 

12.35V is between a 50% and 75% charge. Should be 12.65V.

http://theamcforum.com/forum/battery...opic52935.html

With the vehicle running I look for between 14V and 14.5V across the battery posts.

Re: voltage drop problem
 

12.3 to 12.6 is a standard resting voltage...14.1 to 14.3 is good voltage with the vehicle running. Newer batteries should rest about around 12.5 or higher.

Although a slight drop in voltage is normal during cranking, getting down below 10 volts is definately a problem and needs to be checked out.

Put a good trickle charge on that battery overnight to see if the battery can be resurrected. If not, go and get a new battery. I just got an Optima red top for $153 from Summit including shipping...Autozone, etc has good batteries for less than $100 bucks. Heck, I think Wal Mart does too!

Also, make sure you get something with 800 Cold Cranking amps...or more. Anything much higher than that is overkill...
Tom

Re: voltage drop problem
 

I've got to disagree with you Tom.

Although a hydrometer is more accurate, 12.3 volts is a discharged state. Close to 50%.

Starting at 12.3 volts is going to result in a lower cranking voltage. 9.6 volts is considered acceptable but higher is better. Even the best, freshest, batteries usually cannot support 11 volts or more while cranking.

Re: voltage drop problem
 

Thank you Dave...I should have been a little more specific...a good battery that is reasonably fresh should be 12.5 to 12.7. Below 12.5 is showing a battery that is perhaps getting tired. 12.3 to 12.5 is probably a bit tired... But should still be usable. Below 12.3 and there could be challenges under high draw conditions.

Of course there are many other variables... Strength of connections, conditions of cables, impedance, gauge of cables, starter amperage draw, how long you are cranking it etc... I was trying to give some help on resting battery conditions.

Best,
Tom