I've been building custom batteries out of 18 650 s and different modules for a while now. I'm currently building a 3kwh 7s 24 volt battery with 18650s for backup power and I just came across some 3.7V 120ah 0.45kwh lithium ion cells...

I'm having a brain fart at the moment lol. Can I take one of those cells and add it to my existing pack? No, correct? I would have to take 7 of them put them in series then I can add that in parallel.... Rite?

 

Yes, you can connect a series string of 7 cells and add them in parallel to your 24 volt pack. Before you do, make sure they are charged to the same voltage as the pack or there will be a whole lot of current dumped from one to the other in a very short time, possibly damaging the cells or causing an explosion.
The problem then will be how to equalize the cells when you are charging them. How do you do that with the existing pack?
Regards,
Keith

 

Yes, you can connect a series string of 7 cells and add them in parallel to your 24 volt pack.
Keith

The reason this is not the way to do things is because the cells must be balanced and kept within voltage limits ... and if you had two 24V batteries you would need 2 BMS circuits .....

This is the way ....
You have a 7s battery ..... Seven separate 3.7V packs each containing many 18650's .... for optimum performance your goal is to have the capacity (AHrs) of each pack ideally the same , so just add these big 120Ahr cells to the existing 3.7V packs , if you don't have 7 of them (or 14 or 21 etc) no problem , just shuffle the small 18650;s around so as to make each 3.7 pack close to the same capacity

 

The reason this is not the way to do things is because the cells must be balanced and kept within voltage limits ... and if you had two 24V batteries you would need 2 BMS circuits .....

This is the way ....
You have a 7s battery ..... Seven separate 3.7V packs each containing many 18650's .... for optimum performance your goal is to have the capacity (AHrs) of each pack ideally the same , so just add these big 120Ahr cells to the existing 3.7V packs , if you don't have 7 of them (or 14 or 21 etc) no problem , just shuffle the small 18650;s around so as to make each 3.7 pack close to the same capacity

Thank you. I Knew the answer but was having a lapse in memory... Lol. But yes you are correct.