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Please give your advice, it's much appreciated: I have read the blog: https://www.apogeeweb.net/electron/bad-alternator-vs-bad-battery-vs-bad-starter.html I have a 2002 GMC Envoy SLT. I have it currently parked and unable to start, due to an unknown reason. Recently it's been difficult to start, (as in, takes a little longer for the ignition). Today it won't start at all (stranded in a parking lot). My lights will come on, power locks, dashboard, everything- except it's all dim and slow.. fairly under-powered and no ignition at all. Nothing turns. So I figured, hey, I'll jumper cable it. (Which I do properly) I've had to do it before, but only once when I accidentally let it drain. This time, it would not jump- it would not charge- and will not start. I heard a clicking noise the first time I tried, however... and since then nothing. I'm going to try and replace the battery myself, but assuming that won't work (since it wouldn't even jump start) do you think it could be the alternator? If so, and I buy a new 150A or whatever alternator, can I replace it myself?

I'm not much of a mechanic anymore, but if your battery has gone flat, I would recommend replacing it no matter the case.

Lead acid batteries are a fickle sort, especially automotive batteries. Batteries store energy chemically in 3 parts. A lead-antimony alloy anode grid, lead-oxide cathode, and these are both immersed in an acid electrolyte.

During the discharge process, the antimony will break away from the anode and apply to the cathode. This process is reversed during charging. This chemical back-and-forth maintains your battery.

When a car battery is fully discharged, the antimony becomes bonded to the cathode. This antimony on the cathode will impair the battery's ability to recharge. While it may recharge back to a usable point, but it's capacity will be severely diminished.

Reference: https://www.doitpoms.ac.uk/tlplib/batteries/batteries_lead_acid.php