battery can't hold a charge

[SDPreRunner]

The symptoms my truck has is the battery can't hold a charge and the battery slowly drains while i'm driving, this has been happening the last week and a half or so, i've replaced the battery and alternator and it's still dying... i noticed today that i have a blown 10 amp fuse in the fuse box inside my cab, so i'm about to replace that and see what happens, but i doubt that will fix the problem. Also i took a voltmeter and measured the amount of current going into the alternator, which is about 12 volts... but there aren't that many volts coming out of the alternator, if i'm not mistaken that would mean that it's the alternator, but this is my second replacement alternator cause the first replacement i got was defective and i don't my luck is bad enough to get another defective alternator, i doubt that's the case but is there anything in my car that could be killing my alternator? any other suggestions to what i could go to fix this or what i could do to figure out the problem.

[Geoffreyster]

This might be a bit obvious, but- are your battery cables & terminals free of corrossion? Nice and tight? How bout the alternator belt? I haven't had any problems w/ my toyota's electrics, but I used to own a fiat (still do, but in storage)... I had lots of trouble w/ the alternator. Eventually I replaced it only to find the problem slowely returning. In this case it was water spray getting into the engine compartment because the previous owner had for some reason decided to remove a splash shield.

[fourwd1]

SDPreRunner said:

Also i took a voltmeter and measured the amount of current going into the alternator, which is about 12 volts... but there aren't that many volts coming out of the alternator

OK, first of all current is in amperes, voltage is in volts, and the voltage is coming out of the alternater into the battery, thus charging it.

A GOOD, charged battery will measure around 12.6V with the engine off, and around 13.8V with the engine on and a GOOD alternator charging it. Try this with your voltmeter and see what you get, it may help narrow things down.

Places like Pep Boys will test your battery and alternator for free, you just have to carry them in.

If the battery drains while you are driving, this usually indicates a charging (alternator) problem. The engine is running off of the battery instead of the alternator.

Depending on year, you may have an external alternator, which could be bad.

[dman33733]

I've had that problem with my 1980 P/U recently and had the alternator replaced, which didn't solve the problem. The load on the negative side of my battery was immense, so I had the negative line rebuilt which, knock on wood, has solved the problem for now. I think my problem was tied into my foglights however. Hope this helps ya at least a little!

[bikeguy18974]

I had a short in some wiring to a previously installed power seat that would drain the battery. Removed it and everything was fine. If you have a short somewhere that could be the problem.