Hello all! My car had to sit for about a month recently and the battery died completely, I discovered this after it stalled on me (due to a bad fuel pump relay) and wouldn't start back up. So I replaced it with a yellow top optimum battery, which had very low charge when I got it. I drove the car for about half an hour and the charge needle went to its normal position but for the last 10 minutes hovered between A and L. When I got home and shut the car off it started again on a dime, but when I went to move the car later that night it didn't have enough juice left to start again. The battery had been disconnected for the few hours the car had been off. I cleaned up the battery ground and tested the alternator which is doing its job, my theory is that perhaps the car has a bad ground somewhere either in the engine ground strap or one of the alt connections. I was hoping one of you might have had a similar experience and have advice for me? The car is a 1983 V12 coupe with a 115 amp alternator retrofit I did about 2 years ago.

 

Any alternator will NOT fully charge any battery after 1/2 hour.

Maybe after 4 hours of non stop driving, but I doubt it.

I suggest a proper out of car overnight soaking charge.

The alternators are designed to maintain a charge, and top up a tad, after many hours of running, BUT, every time you restart the engine, any alternator takes about 30 minutes to replace what you just used to start it, and then, providing you keep running, will very slowly add more charge.

 

You might have a poor ground...or several poor grounds. Most old Jags do. They're usually the root of the "Lucas, Prince of Darkness" stories. Whenever you find a ground point take a few minutes to clean and tighten just on general principles.

But....

On your battery/starting issue, I agree with Grant. Before doing anything else fully charge the battery and see what happens.

Cheers
DD

 

" I replaced it with a yellow top optimum battery"

As in a NEW battery or a battery just laying around? An old battery may be bad. Either way, charge the battery first and then put it in, don't ask your alternator to fully charge a battery.

Verify voltage when charging/running and maybe take the battery to the store to get tested if you are really in doubt.

 

Brand new. I'll take it to get fully charged. Thanks guys.

 

Don't I know it. I already do this religiously. Anyone know where the engine block ground strap is?

 

Battery ground to the RHS of the battery
Engine grounds via the front subframe, bottom LHS of the engine (a sandwich plate bolt) to the subframe to the LHS bottom chassis rail. This will inevitably be pretty poor by now if never cleaned up, and the subframe join in the groins strap is impossible to get at engine in car. The best and easiest thing is to run a new ground from the alternator fixing bracket to the chassis rail on the bottom RHS near the alternator somewhere.
Greg