I bought a 1995 project car, rebuilt engine install that was 1/2 done and done incorrectly. Total mess...
I can’t seem to figure out why the battery appears dead after driving 7 to 10 miles.

- battery on charge over night
- next day start up drive 10 or so miles. Park and it won’t restart, acts like battery is low power. Spins over one time. Voltage drops to 9v or less while cranking.

I have to jump it or put it on a charger for 20 minutes and the engine will turn over and start.

- with engine running voltage at alternator post is 14.5
- with engine running voltage at battery positive post is 14.3
- with engine running voltage on gauge cluster is 14.3, 14.4 ish.

After driving the battery (engine off) is around 11 to 12 volts.

I can drive further than 10 miles. Head lights are bright, radio works. Windows go up and down. Doesn't misfire or display anything negative.

but if I stop and turn off the engine. It won’t restart without a charge or jump

Could I be missing a grounding strap?

Could the alternator produce the correct charge volts but not able to output enough amps? A missing engine ground would limit the flow of electricity. Amps, right? As a test, i was going to use 4 gauge copper wire from the alternator bracket to the frame. See if an additional ground helps. Otherwise could it be the alternator itself? I'm just confused as to why I would see Volts go up when it runs but not enough to keep the battery charged.

I installed two interstate batteries. Same issue. I just put in a new battery today. Same exact issue. I can go about 10 miles or so.

 

Starter heat soak? Checking all the connections and grounds can only help.

 

I thought that too but when it won't start the voltage drops to as low as 6volts. ?? could the starter cause such an excessive load to draw down voltage like that?
The other thing is if i jump it within seconds or minutes it will start. A new starter wouldn't hurt.

My plan for today was clean grounds I can find but I have a feeling one or more grounds are not connected correctly. I was going to add a ground strap, and pull the alternator and have it tested at Autozone or wherever I can find that can test them these days.

 

Have the battery tested. I have had that happen a couple times over the years. The old/weak battery gets heat soaked. Once I drove 1 mile, wouldn't start afterward. Took it in to test and it had a bad cell, must have been on its last leg.

 

Could it be the "rebuilt" engine itself? After being driven, things heat up and begin to seize? Like too-tight piston to wall clearance? What you describe is excessive load on the starter, not a low state of charge for the battery.

Drive it for your 10 miles, but end in your garage and shut if off. If it won't turn over fast enough to start, put a bar on the crank pulley bolt and see how freely the engine turns with a bar. Compare to when the engine is cold.

As previously mentioned it could also be a heat soaked starter.

 

Sounds like a bad battery.
I agree with 95wht6spd. Have it tested.