What is the short circuit current of an VTC5A 18650 cell? - Page 1
Forum section: electronics / projects

author: Losky
author_rank: Newbie
timestamp: December 28, 2017, 04:20:12 am
content: I am trying to spec out a fuse for my lithium-ion battery pack and I want to check that the interrupt current rating of the fuse was sufficient. I am using a VTC5A cell (72 Series, 3 Parallel). What would be the short circuit current of the battery pack? Attached is the data sheet for the cell. I can't find an internal resistance for the cell.Thanks!Erik

author: Damianos
author_rank: Frequent Contributor
timestamp: December 28, 2017, 06:49:48 am
content: Your question indicates that you are unable to use these batteries in a safe manner!What you describe is not a toy! Even a single cell can become dangerous...There is a lot of information in the data-sheet, but it seems that you can not understand it!

author: Losky
author_rank: Newbie
timestamp: December 28, 2017, 03:22:22 pm
content: I understand that these cells can be dangerous and I was hoping to clarify something I don't understand well here on the forum. If someone has a helpful answer that would be very appreciated.

author: amyk
author_rank: Super Contributor
timestamp: December 28, 2017, 06:28:21 pm
content: Given that it's rated for 35A continuous discharge per cell, the short circuit current will be well over 100A, maybe even 200-300, but your fuse should open at only a little higher than the expected current draw in your application.

author: Gyro
author_rank: Super Contributor
timestamp: December 28, 2017, 10:36:14 pm
content: Quote from: Losky on December 28, 2017, 04:20:12 amCan I just clarify? Are you talking about setting up a string of 72 series connected sets of 3 parallel cells, ie. 216 cells total, a 266 Volt stack???If so, you are talking about a massive amount of stored energy which would take very specific design measures to be safe, including cell balancing, high voltage DC break fusing etc. A very serious undertaking.Please provide some more details.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 28, 2017, 11:18:28 pm
content: You should probably be speccing the fuse based on your expected load ( and using a battery rated for that load) , not the capability of the battery.

author: uer166
author_rank: Super Contributor
timestamp: December 29, 2017, 05:10:45 am
content: He mentioned interrupt current specs. E.g. a 250A automotive fuse can support ~2kA for hundreds of milliseconds. But it also has an interrupt current (say, 3kA), and if the battery pack is able to generate that, the fuse might be unable to quench the plasma arc. If the battery ESR is guaranteed to limit the current to below the fuse's interrupt current, there is no problem.The issue is those cells have no ESR specs, and really, if they had it it might not be trustworthy. One thing that you might rely upon is your wire's resistance to limit current, as that's more reliable and changes less with temp. Or you can do what Tesla did and use some sort of pyrotechnic fuse with active circuitry with super high interrupt current ratings.

author: uer166
author_rank: Super Contributor
timestamp: December 29, 2017, 05:19:43 am
content: Also, AC impedance is quoted in datasheet as: 7mOhm~15mOhm after a standard charge within 3 days. So 4.2/7mOhm is 600A per cell, or 1800A for 3 parallel. That might not mean much since that's at room temp.Maybe you can add interconnect resistance values to get an upper bound?

author: Damianos
author_rank: Frequent Contributor
timestamp: December 29, 2017, 09:43:24 pm
content: Quote from: Losky on December 28, 2017, 03:22:22 pmWhat is the question that ...waits for an answer?You stated that you need to calculate a fuse, but missed the internal resistance of the battery cell!