I had one of these fail . The charger just stayed as it was and didn't recognise the battery and would not charge it . A multimeter across the contacts showed no voltage . So I opened up the case and was pleasantly surprised.
It had a bank of 5 Lithium Ion 18650 cells in series so I was made . These are very easy to replace . The foil packs are a PITA.
So using a multimeter I measured each cell and marked the bad cells . Half a volt on the bad ones and 3.7 volts on the good. I had to replace two so I used a watchmakers screwdriver pushed between the battery case and the welded terminal and prised it off. Once both ends of the cell were disconnected the cell just pushed out of the holder easily. I used a fairly good 60 watt solder iron to pre tin the terminal ends while off.
I got two replacement 18650 cells and pre tinned both ends. Watch that they are fairly close to the original capacity . my originals were 1300 mAh so I used 1500mAh as replacements . If you used 3000mAh you would need to discharge them to match the existing cells first . That way when the battery runs out of charge they cells reach roughly the same low voltage together.
Slid them in and then held the terminal to battery end and touched it with the solder iron holding it with a screwdriver while it set. Make sure the polarity is right (don't be spooked by others they won't explode of catch fire ..thats crap, Samsung solid phone batteries got greedy)
Job done ,checked the overall voltage and screwed it back together.
18650 batteries are notorious for frauds where they don't fill up the cell so the weight is down . The fraud cells don't last 5 minutes because they are not packed tight enough.
Dont buy anything that claims to be more than 4000 milli amps in capacity and what you buy should weigh about 44 grams.
Get them from computer packs and other sources as they are more likely to be genuine.For this job you want Lithium Ion batteries (charge to 4.2V) not Lithium Iron Phosphate.(LIFePo4 charge to 3.5 volts)
The BMS (Battery management System ) details are readable from the pics but I don't know anything about it. Most of them just work like a switch. If the pack is within set limits the MOSFETS will be turned ON , if not they will be OFF. Thats why the charger does not see a voltage or circuit at all.
If you want to know more about Lithium cells go here and join up this group
The roses are for you Kristian.