I have found numerous times with so called "smart" battery chargers as needed for Li-Ion but also NiMh or even lead acid leisure batteries that they refuse to charge a really discharged battery. Their internal logic does not cope for a very flat battery. Happened only last night with my BL-5CB in my Nokia105 which was only reading 3.05vdc. First time I just bought a new battery online and away it went. This time I used the trick mentioned by an earlier poster. Just get a few 1.5v alkaline batteries (enough to exceed the 3.6v of the BL5CB) and connect them together (I use a battery holder from some item I scrapped a few years ago) and then just connect the BL-5CB to it +ve to +ve and -ve to -ve and leave it for just a minute or two. This will give enough charge to bring the BL5CB voltage out of "deep discharge" and then I put it back in the Nokia 105, DON'T turn the phone on as not enough charge to power it, but straight away plug in and switch on the charger. Up popped the little charge icon on the screen and a few hours later the phone was back to full functionality showing 100% charge.
Similar approaches worked with my 12v leisure battery that my smart trickle charger/maintainer refuses to recognise unless the battery voltage is over 8v. Takae care not to use any power supply which can provide too much current as it will probably blow the protection fuse inside the BL-5CB. I measured my charge rate last night with an inline multimeter and it was only 30mA. but that was enough as I explained above.