I have a 2004 XC90 AWD 2.5. It died back in October and I replaced the alternator and the battery. It was turning over just fine, but I had not been driving it because it also needed new struts. About a week ago I tried to start it and it was completely dead so my boyfriend gave it a jump. Started just fine after that, then a couple of days after it wouldn't start but was making the flutter sound of a dead alternator. It was a 50 dollar alternator and I was told there was no guarantee it was any good so I bought a remanufactured one from Advanced auto. I bought the 140 not the 160. Today my boyfriend's mechanic friend fixed the struts and put in the new alternator and it was running fine, I drove it around the driveway and then shut it off. I went back out a little while later to drive it around the block and it was completely dead. So, new battery, new alternator. I have no idea what could be wrong. I just watched a video that said an alternator could cause parasitic drain. Also the battery that Advanced matched to my vehicle is actually smaller than the one that was replaced. I was wondering if alternator voltage could be the reason or if I was sold the wrong battery. Or could his friend have installed the alternator wrong? I don't want to waste anymore money.

 

The alternator amperage is its ability to sustain voltage during peak load. The battery "size" is first about what fits in your car physically ie terminals in correct location, correct width/length/height to fit in the mounts etc. Battery models have varied capacities measured in cranking amps (max power output at a given temperature) so if you live in a cold climate you want a bigger number since the power output goes down with temp. The battery should be able to hold a charge for a week or more unless there's something draining it. Say your glovebox light is staying on... all looks normal except it could drain the battery in 2-3 days time. Another possibility is that your altnerator is not fully charging the battery - but you don't have a warning light on...

So with that said, there's some simple tests to determine what's going on.

1) buy or "rent" (say from Autozone or similar big box store) a Volt-ohm multimeter. With the engine off, measure volts across the two battery terminals (or the jumper terminals for cars the with battery in the rear). You should see 12.6V. Now start the car (even if you need a jump). with the engine running measure across the battery terminals again. You should see 14+V. Now turn on every electrical device on in the car - high beams, set the AC blower fan on high (in manual mode), turn on the rear defroster etc. Measure across the battery again - you should see 13.5V or more. If you are seeing these numbers, your alternator is working fine. If you are seeing less than 12.6V with the engine off, you have a battery issue. Now the last test is the most complex. Your multimeter needs to be able to measure current. To do that, it needs to be in the circuit path or needs to be a more sophisticated "inductance" meter which clamps around the outside of the wire and measure current externally to the wire. To determine the parasitic drain, first you'd need to measure the total current draw with the car off/key out etc. If it exceeds normal, then you'd need to go circuit by ciruit pulling fuses and relays until you find a circuit that when cut off, returns the current draw to normal. Once that's known, then its off to see what devices are hanging on that circuit.

True story: Once I helped a friend figure out what was killing his car's battery. Isolated the path and it turned out to be an orange peel was stuck in the 12V lighter socket - and it had enough moisture and acid to make a conductive path and drain the battery over a couple of days. removed the peel and car was back to normal. So, it doesn't take much :-)

 

Something is clearly causing a drain to Earth - faulty starter motors can do this, and the drain with those can be significant. Either way, any half way competent auto electrician will figure it out quickly.