Troubles charging BP-511's, what's wrong?

So I have these three batteries for my 50D:

- the BP-511A that came with it
- another original Canon BP-511 from my Powershot G3 (several years old offcourse, but still OK)
- a knock-off BP-511A from an imo trusted local store

The battery that came with the camera will charge just perfectly fine in the 50D's charger. The knock-off would also charge perfectly fine before, I have probably charged it for about five times now.
The BP-511 from the Powershot camera however, will not charge in the 50D's charger. When I put it in, the charger will start charging normally and the light will flash. However, within 10 minutes the battery is "full". Offcourse, the battery not full and is only charged for about 10 to 20 percent.
Charging that same battery inside my Powershot (the G3 has a built-in charger with a power adaptor connected to the camera) will allow me to charge it properly and it will still last for a couple 100's of shots with the 50D.

However, since yesterday, the knock-off battery appears to give the same result. I did use it all the way until the battery symbol on the camera started flashing, something I haven't done with that battery before.
After putting it on the charger, it says "Full" within a few minutes and the battery will only be charged a little bit. When getting the battery off and back on the charger, it indicates full within one minute.
I haven't tried charging the knock-off in my Powershot camera yet, will do that tonight to see what happens.

To me, it appears as if the 50D's charger has a different (more picky) measuring method compared to the G3's charger.
Almost as if the two failing batteries have a higher internal resistance compared to the 50D's BP511A which fools the charger into thinking the battery is already full since the charging voltage will be higher to supply the same current. This results in an improperly charged battery. Strange thing is that the G3 is able to charge at least one of the two failing batteries properly, as I said I haven't tested the other one yet.

Any ideas of what's going on here?
Anyone experiencing the same issues and perhaps anyone has a solution to this problem?

I have had a similar charging problem with my old BP-511s and what it boils down to is the battery cells are dying. I have two batteries that will charge in about 10 minutes from dead to full and I can go and take a couple hundred pics with them. However, the same batteries will drop from a full charge to dead in about 3 days when they are just sitting there. I use 50D, 30D and 10D chargers and have noticed no difference in their charging.
Replace the dead cells with new ones and they function as normal.

It might also be that the batteries or the charger have bent contacts so aren't connecting well. But old age is the likelier.

Hmm... I've bought this knock-off battery about six months ago (just found the receipt), I just haven't used it that much. So it really isn't that old, or worn because of frequent charge/discharge.
It probably also isn't old stock since its label says 2000mAh and afaik all earlier ones were 1800mAh at most.

I've checked the contacts, they don't look bent to me, but I can't really move them a bit to be sure. The battery charged OK before and I was able to take lots of pictures with it since the previous charge (about a month ago).

At this moment it is charging inside my G3, which immediately indicates that it's charged to 90% (steady orange LED). However, it keeps trickle charging (already for 15 minutes or so), so I will see where this ends up. The battery definitely is bad, but I might find out whether it's a bad cell or bad protection circuitry inside the battery preventing a proper quick charge.
Offcourse I'm leaving for a holiday right this weekend, so I'm kind of hoping to still be able to have this one available as a spare battery, since I can't get a new one in time.

I've sent the store an e-mail, will just have to wait for them to see if they will replace it.

edit: After two hours the charger indicates "full", but the battery's voltage is still only 7.60V, way below the voltage of a full battery (8.3 - 8.4V).
So charging it in the G3 doesn't solve anything.

I would not risk a bad generic battery catching fire in my camera.

2000 mAH BP-511 clones have actually been around for a while. IIRC I got some from Sterlingtek around 2006.

So it might be possible they gave me a "new old stock"-ish battery.
I'd still expect it to work badly from the beginning, but I don't have enough experience with li-ion cells to be sure.

Got a reply from the store; they will replace the defective battery.
They told me to always store these batteries half-full when storing them for a longer period, while I stored them empty for just a few days.
I've found some more info on this here.

Now, I can understand that storing them empty for a month or so could cause trouble. But just a few days would still be really quick I'd say.
Like when going on a holiday, forgetting to bring your charger and ending up with dead batteries. I've done this with the original G3 battery before and that one is still functioning perfectly fine.

Now that I think of it, I did have a somewhat similar problem with a knock-off battery a few years ago. It was inside my G3, which was in my travelcase between some clothes. Somewhere during the trip its power button got activated, the lens came out halfway and got stuck between the clothes. A few hours later I've found the camera switched off, with its lens out halfway and a dead battery.
That battery was completely dead and it wouldn't charge anymore.

@ Camel Toe Coalition: I didn't think charging the battery inside my G3 (which I hardly ever use anymore, but still) would be harmful, since the battery doesn't draw much current while charging. The charger doesn't get hot, the battery doesn't get hot. And a high current would be needed to heat up the battery and have it catch fire. Yes it was a guess, but I kind of knew what to expect and look out for.

So in general I suppose it would be safe to never discharge the batteries until the battery symbol starts flashing? And recharge it ASAP?

Worth checking the pins on the charger, one of mine got stuck depressed (due to being dusty) and made little contact with the battery. It would indicate fully charged when the battery was almost dead.
I just played about with the pin until it poped back up and all works OK now.

I've tried two chargers (both the 50D charger and the G3) and none of them seemed to work. But perhaps the battery's contacts are somewhat dirty, so I'll clean those first and try again.
Thanks for your hint, you got me thinking!

Edit: Cleaned the contacts and I was able to charge the battery a little better.
But I still doubt its capacity after a full charge. It's not what it's been before.
The store has sent me a new battery to replace the defective one, so the defective one can be disposed anyway.