Stored codes, fuses and possible alternator sensor issue

So I had a bit of a dumb moment last night/today which could have ended up being costly, but managed to figure it out in time and learnt something new :icon_bonk: :D

So here goes:
I recently blocked my EGR and as a result have had the engine light come up a few times which I've managed to clear using the Torque android app. However recently it kept coming up almost on a daily basis and kept showing as a stored code. I couldn't clear the stored code but could clear the engine light.. anyway this got annoying, so I went over to threads searching for some answers.

I found a post that talked about erasing stored codes by shorting pins 1 and 8 on the OBD without the need of a OBD II device. So I went ahead and did this using a paperclip. All was good until one end of the paperclip slipped out and touched the chassis.. I saw a tiny spark and thought that's no good! :( but I left it as it was late and went to bed thinking "she'll be right". Mind you, this did clear the stored codes.

I usually leave my car unlocked in the garage and didn't notice anything till the next day when I got to work and tried to lock the doors with the remote. I noticed there was no response from the remote key or the drivers side master lock/unlock button. Then I noticed the headlight warning buzzer also didn't go off when my door was open and key out. WTF type of thoughts start swirling in my head.. hmm shorted a fuse I started thinking.
I thought I'll quickly check the fuses and see anything obvious. Checked the internal fuses and didn't notice any blown ones, then checked the engine bay fuses and also didn't see any blown ones.. ended up leaving it and going in to work.

At the days end, I jump back in the car and start her up, suddenly I hear this frighteningly loud warning buzzer. Scared the crap out of me as I've not heard it before.. Next thing I see is the Projecta dual battery monitor flashing 15.1v and 15.2v on both batteries and realised this is where the loud buzzer was coming from.. WTF now!! I know an overcharged battery is no good, but don't know enough. So I stopped a few times on my short trip home and notice when the car is off, both batteries are reading 12v. At this stage I knew that the batteries are fine, so started suspecting the alternator. I managed to get home keeping the charge at exactly 15v.

I decide to look over the fuses once more and this time I notice a very faint dark mark on the "Bat Elec" (7.5A) fuse inside the car. You beauty, I found the problem replaced the fuse and fixed the remote door lock/unlock as well as the headlamp warning buzzer. Started her up again and the battery monitor goes bezerk again jumping to 15v... damn. back onto the forums.

After reading up about multiple blown batteries, loose earth cables, bad alternators, fuses and sensor wires (crapping myself reading this stuff), something finally clicks... When I checked the engine bay fuses before work, I may have moved something or not plugged the fuses in properly. So off I go to check those fuses and discover that I fked up. I've plugged the 7.5A fuse in a spare slot nearby rather than the original plug marked "ALT S". I now realise that "ALT S" must stand for "Alternator Sensor". Start her up again voltages are sitting nicely at 14.2v :cool:

This got me thinking a bit.. what if the ALT S fuse was blown and I didn't have a battery monitor... the battery would have definitely fried over time. Is it me or is this a design flaw???
Also, how long would it take for a battery to overcharge on average, before it blows?

Anyway after all that, I learnt 2 things.
1. Never rush a job and always take care (I should already know this, but..)
2. You never know when something comes in handy. Up until today I thought I wasted $80 on the battery monitor. Now I realise if I didn't have it, I would have been totally oblivious to the problem and I would have blown one or both of my batteries as well as cause other damage.