Greetings - I have been asked how I changed the TSL60-10 charger to handle charging 21 LFP cells on my XM-3500LI from the original 20 cells.
First, let me mention you undertake this at your own risk, you will void your warranty, and this info is free so I do not assume any responsibility for how it is used or any damages that occur from it! ;)
Now, below is what is needed for changing this charger to scale its output by 5% high to cover 21 cells in series from the original 20. I hope it makes sense (note - if you want to go more than 21, I would hesitate as the electronic components and design might run out of margin and you will get a failure - I feel comfortable with just a 5% increase).