[Verizon Samsung Galaxy S III] Blue6IX:
Discharging is what the battery always does when not plugged into A/C power and turned on, and even sometimes when it is plugged into A/C power (when you get in the top 10% of full it starts going through discharge/charge cycles to keep it within the top 10% of power but never totally full. This prevents overcharging and explosions)

That said:

A sudden reduction in the runtime for the battery under the same average load is almost always the battery going bad.

If you mean the battery doesn't last as long because you now use OTG - those things you plug into the phone need to get their power somewhere. The phone battery is it.

So now you are powering the phone + whatever other things you attach to it. Some of them can drain a lot more power then you might think. You just never noticed much (if at all before) because your total reserves weren't such a tiny amount.

If you stop using OTG devices and your battery capacity bounces right back up to where it was before, then you can reasonably infer the extra gadgets are the culprit.

(if you stop OTG stuff for a few days and the battery still sucks when before it didn't, probably need to buy a new battery because yours is most likely shot.)

Assuming it seems to work fine...

So two choices, right? No accessories and lots of battery life, or lots of accessories and no power to run them with.

well...not really:

It's probably not always possible - but how often do you use OTG near A/C power?

Whenever you are near somewhere you can plug in, the first thing you should plug into your OTG cable (besides the phone) is a USB hub with a power cord.

...and really, if you want to use those power hungry USB hard drives and things like that, then you really can't without taking the stress off of the phone of supplying that power - that hard drive just half spins up like a car that cranks but won't start.

Every crank in that car wears the battery down and beats up the starter when it doesn't fire up the engine. Just like every spin up but not quite read of the hard drive puts incredible load on the S3 to try to throw enough current at it fast enough to keep it happy. Just beating it up for nothing.

Which brought me to what I wanted to bring up for some consideration:

Think about trying to use a powered hub whenever you can as the first thing off the OTG cable, should take an immense amount of stress off of the S3 in making it all work.

Bet at the least you reduce the risk of popping since the power spikes will be much less - that powered hub is a pretty solid buffer for that kind of stuff when you are constantly popping flash drives in and out.

I remember way back when for the Nook Color when some really sharp people made OTG happen out of the thin air - no way Barnes & Noble had that on even a remote list of possibilities when they were ready to throw the device to the masses and see what happened.

If you wanted to use more then the smallest of thumb drives you needed a powered hub. The device wasn't designed for it, and simply wasn't able to squeeze more then a paltry amount of power out in reverse. (but it was just enough to talk to a powered hub...)

Anywhere that you can plug in your powered buffer should contribute to avoiding the problem the OP posted about. The thought experiment (hopefully) growing from this post? How much truth does this theory exhibit?

Edit: Totally missed this post:

Here you say you made an OTG cable, the first post I quoted you in you said you bought one. If you made it, i'd say that is your problem.

The quality of the cable you use absolutely matters. Higher quality parts = higher quality results.

I have a bunch of micro USB cables (as i'm sure many of you do also - ) ... and from some of them I get an absolute night/day result in charging.

Some cables (mostly the ones from the dollar store or older cables) just plain suck. They can't bring enough power from the same A/C adapter as my OEM HTC/Samsung cables. (I have some motorolla OEM cables from around the time of the droid X launch - they are closer to the dollar store cables in quality - haven't tried recent motorolla OEm cables, hopefully they got better or I got a bad batch over a six month period)

Using the low quality cables I can absolutely lose power plugged in just being a wifi hotspot with the screen on, until it eventually goes critical and dies. (this I tend to notice when my pc banished to the corner of nowhere loses all network connectivity) - whereas with the HTC/Samsung cables from within the last two years keep me near full with that being the only difference.

I'd rather have a few good cables then a bag full of POS dollar store ones. That little bit better material, little bit tighter QC adds up to a big difference in the final outcome.