I left my 99 LS400 sitting for three months while traveling. When I came back the battery was dead as I didn't think to disconnect it before leaving. I charged it up all night using a trickle charger. It started up great the next morning and I drove it to the store. When I went to leave the battery was too low to crank the engine so I got a jump and drove it back home and charged it some more. Today I went to start it and it cranked slowly but still started. So I figure my battery might be bad. I went to a couple auto parts stores and had it tested and they both think it's the alternator, which I now suspect as well. When I got home I measured the voltage and it's a little over 10V with the engine off, and just a little over 11V with the engine running. This tells me it's the alternator. But what is throwing me off is that during all of this time, there have been no warning lights of any kind on the dash. Is this normal?

 

Depending on how old the battery is, running it down like that may have just finished it off. Do you have a battery charger/ alternator tester? The battery should be at 12.7 volts 20 minutes after you turn the car off (with headlights off). I cannot remember what the voltage should be at idle. I took the alternator out on my RAV4 by running it with a bad battery. I knew better, but I was trying to make it home so I could work on it in my garage. Did the trickle charger bring the battery all the way up to full charge?

 

Charging a dead to very low battery from a trickle charger it is suggested to disconnect the battery from the car. Driving it for 20-30+ minutes should bring the battery back. If not could be two things; battery is gone and or the alternator is also a suspect.

Three tests:
Car not running 12V. A few ticks above is fine, battery ok. 11V or lower isn't looking good.
Running 13.1-13.6V+ range. Alternator ok.
AC/accessories like lights on 13V-13.4V+ range. Alternator ok.
There will be some variables higher or lower, getting 13V+ is what you are looking at for a functioning alternator .

 

I'm starting to suspect it's both as the battery is now not holding a charge at all, and with the vehicle running it's still only showing about 11V. But it seems weird that both would die from just sitting for a few months, and also strange that there is still no indicator on the dash of any charging problems. I'm going to start by replacing the battery first since it's under warranty. And then hope the voltages are magically better with the new battery in.

 

That is odd about the charge light not coming on. I've read several posts on this forum and other forums I use for our Toyota vehicles about the 'Trac' light and 'ABS' light coming on when the battery is going south. I always think to myself, "Why isn't the charge light coming on instead of the TRAC and ABS lights"? Hopefully someone here has the answer.

 

When I've experienced alternator failures the lights come on just as the car begins to die. That leaves about 5-15 minutes of runtime on the battery. Plug it in and see if there is an error code.

 

I got it all fixed. Replaced the battery and everything is back to normal. Strange that it was reading 11V while running before the battery swap. Now it's back to 14.4V.

 

It seems to me that one of the battery cells out of 6 was internally short-circuited. That's the reason why the voltage meter showed 10V+ to max 11V.