sixtysix

I have a stock alternator(rebuilt from autozone) and I dont think it is keeping up. My car battery dies every 3 weeks or so- anyway I took it out and it was checked 4 times and it passed.
I think it is not strong enough to recharge my battery-I have a amp in the trunk,fog lights and a power top....I have checked everything-re wired,checked grounds, ect... and I really believe that the alternator is the problem even though it keeps passing


here are my questions:I want to upgrade to a stronger amp alternator

what was the stock alternators rated at? 42 amps 45 amps?
I want to get a stronger alternator.....how strong...60 amps 100 amps
what is the highest I can go that my wires will handle?

Thanks Scott

MustangBradley

You can put a 500 amp alternator in if you want. It won't ever put out that amount of amps because you won't ever need that much. The voltage regulator keeps the alternator's output at around 14.5v, and as you draw more power (say when you are raising the top) the voltage regulator "tells" the alternator to put out more amps. You'll notice your rpms drop slightly when you operate the top, and that is because your alternator is working harder to supply the increased "need" for amps which makes it harder to turn, so your engine has to work harder, thus the rpm drop.

You need to determine if your alternator is putting out over 14v at idle. Use a multi-meter for this. If it is not putting out enough volts at idle, there are several routes you can take. If you rev the motor slightly, and the output jumps over 14v, you could just put a smaller pulley on the alternator. This will increase the speed that it spins at idle, thus giving you more volts.

If it never gets over 14v, you might have a bad voltage regulator, or a lame alternator. If the alternator is rated at 60 amps or more, you should be fine. On a modern car, with its higher electrical needs, they need much more reserve amp capacity, so they often have 100 amp or more in stock form.

One more thing to check: With the car off, and the doors/trunk closed, if you pull off a battery cable, then touch the cable to the post, do you get an arc? If so, you are leaking power whenever it is parked. If it is a tiny arc, you might just be supplying the clock in the radio with juice, but if you get a large arc, you have a short to ground somewhere that is draining your battery. There are threads on this site dealing with the procedure for tracing those down.

Good luck!

sixtysix

Thanks so much for your help-scott

jlg2002

Your stock alt is rated anywhere between 38 -55 amps-(woefully undersized for today's electrical loads) and the typical charge voltage at full field ( alter is full on and the Vreg is fully limiting is around 14.5 Vdc as measured across the battery terminals at roughly 3000 RPM. However your wiring is also rated to those same specs. You can in fact run a 100 amp 1 wire alternator but you need to resize the feed wire from the Alt's Bat + terminal to the starter relay(black-yellow stripe) and if you are using the stock wire harness to power your accessories (the red or yellow wire from the starter relay) it wil need to be re sized also.
I also agree with the previous poster that you should check for drains. but before you do so you should also do a battery load test and a hydrometer reading on the individual cells to make sure your battery is actually able to accept and retain a charge.
Regarding a drain test, another way to accomplish this is the use a test light between the neg terminal and the BATT cable connection (test probe to the battery and the clip on the cable). If it has a light you have a pretty good drain going. BTW-the best way to check for drains is with a DC ammeter.