I bought my Tello few months back ... it was initially fun, but then we got a Mavic Air and the Tello did not get much usage. A couple of weeks back I pulled the Tello and the battery now won't charge. I did buy 2 extras from Amazon, but would very much 'revive' the original/old battery. Any safe way to do that?

The voltage of the battery probably went too low and for safety reason the charging circuit will not allow charging.

Does this mean the battery is toast ?

JB63 Posted at 9-24 10:00
Does this mean the battery is toast ?

Yep - leave it on the charger for about 30 minutes as it be in hibernation mode but if charging doesn't start that it's dead. Aim to charge, discharge and recharge batteries at least every month.

Good day, sorry for this unpleasant experience this issue has caused you. Per it, may I know if it can be charged via the aircraft? If yes, kindly check whether the issue is fixed after the charger’s charger/charging cable is replaced. Check whether the issue is fixed after the battery is removed and inserted without connecting to a charger. Hope the info above would assist and look forward to your reply.

I think I have the same problem... Did not use it for several months, now charging and all I get is slow blue light (that is good..) for 35 seconds. Then solid blue for 17 seconds. Then repeat. For at least 2 hours it is doing this.
Of course it does not respond to start button, all the tricks with placing, taking that I read around here in the forum and YouTube.
Battery dead? I just have one...

RYZE Posted at 2019-9-29 17:52
Good day, sorry for this unpleasant experience this issue has caused you. Per it, may I know if it can be charged via the aircraft? If yes, kindly check whether the issue is fixed after the charger’s charger/charging cable is replaced. Check whether the issue is fixed after the battery is removed and inserted without connecting to a charger. Hope the info above would assist and look forward to your reply.

I tried several times charging on the drone itself, and I also tried with an external charger. In both cases, no success. Any chance to 'disable' the circuit protection and charge the battery ?

djiuser_3UcgFQLn0gaj Posted at 3-5 15:02
I think I have the same problem... Did not use it for several months, now charging and all I get is slow blue light (that is good..) for 35 seconds. Then solid blue for 17 seconds. Then repeat. For at least 2 hours it is doing this.
Of course it does not respond to start button, all the tricks with placing, taking that I read around here in the forum and YouTube.
Battery dead? I just have one...

I'm not sure what technology DJI is using, but this is slowly becoming irritating. I've also seen my MavicAir batteries drain after a while. I always charge them fully before putting them away, that way (in my thinking) they're ready the next time I need them. To my 'unpleasant' surprise, they always have less charge, typically 40-60%. I have invested in multiple DJI products (7+), and I don't use them year round. Does this mean I have to have them all out and recharge the batteries repeatedly/periodically? ... That sounds like utter non-sense to me. I've had multiple DSLRs and other cameras (primarily Nikon) ... and the batteries do not drain as fast as those from DJI.

djiuser_3UcgFQLn0gaj Posted at 3-5 15:02
I think I have the same problem... Did not use it for several months, now charging and all I get is slow blue light (that is good..) for 35 seconds. Then solid blue for 17 seconds. Then repeat. For at least 2 hours it is doing this.
Of course it does not respond to start button, all the tricks with placing, taking that I read around here in the forum and YouTube.
Battery dead? I just have one...

Dear customer,
The blue indicator blinks irregularyly indicates battery charging problem, it is suggested to have a test with another adaptor or Micro USB. If it still not work, please contact us via support@ryzerobotics.com.
Please pay attention to the storage and maintainance of battery to prolong its lifetime:
1. Don't put battery inside of aircraft when not used.
2. Keep battery of 45%-60% charged for a storage over 10 days, that is you can fully charge the battery, power on aircraft and hover it for about 3 minutes.
3. It is suggested to charge and discharge battery per 2 months.
For more details, please click the following link and look through the manuals.
https://www.ryzerobotics.com/tello/downloads
Hope my anwser could help you.

Leave on a charger long enough it will eventually start charging.  Make sure the surroundings does not easily catches on fire just in case it overheats.

just wanted to share my experience; got my Tello unused for a year or maybe even two (because in between I got Spark and Mavic Mini), but recently I took it and wanted to charge - no luck. Blue blinking slowly indicating that it's charging, but never got to the point that it could be powered on. I basically gave up and left it on charger for 2 days ! Now second morning from starting of charging I see blue light steady ! Which means that it has charged. So yeah... the longer you leave the battery unused the longer it will take to revive. I am not sure about its performance, but I flew with it and it works. So yeah... just be patient and if blue light is blinking leave it for some extended period of time. It will eventually charge. Even maybe after 1-2 days.

Thank you, I'll try as you recommended. I've only left the battery plugged in for a couple of hours .. I'll try longer and see what happens.