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bronsonj · 03-25-2020, 10:17 AM

Most people run a BMS. The issue is "Catastrophic failure doesn't happen slowly". (I think that was a quote from Sergio)

So today you charge the cells and when the charger cuts off all cells are 4.12vpc (57.68v for the pack). If you have on cell go bad you can still charge the pack to the same V but you may end up with 13 cells at 4.05v and one cell at 5v.

Similarly on discharge, if you have your controller set to shut off at 47.6V and today all cells are in balance at 3.4vpc. In the future you may have 13 cells just fine with one cell at 2V.

And you won't know this... So the weak cell is now being worked far beyond the V it is designed for so it gets weaker and weaker. Then very soon you get 13 cells charged to 3.95vpc and one cell at 6.33v and on fire.

Also, even if all your cells are healthy, the further you discharge the cells the more variation you will see in individual voltages because of the lithium discharge "knee". You may be within 0.01vpc when charged to 4.1v but could well have 0.5v difference when discharged.

Also, fuses are good, but they will not help you protect against overcharge. So do use fuses, but that's not the only thing you need. That's my nickel's worth.

03-25-2020, 10:17 AM	#6
bronsonj Gone Wild Join Date: Jan 2013 Location: Saint Petersburg, FL Posts: 2,089	Re: Couple lithium questions. Most people run a BMS. The issue is "Catastrophic failure doesn't happen slowly". (I think that was a quote from Sergio) So today you charge the cells and when the charger cuts off all cells are 4.12vpc (57.68v for the pack). If you have on cell go bad you can still charge the pack to the same V but you may end up with 13 cells at 4.05v and one cell at 5v. Similarly on discharge, if you have your controller set to shut off at 47.6V and today all cells are in balance at 3.4vpc. In the future you may have 13 cells just fine with one cell at 2V. And you won't know this... So the weak cell is now being worked far beyond the V it is designed for so it gets weaker and weaker. Then very soon you get 13 cells charged to 3.95vpc and one cell at 6.33v and on fire. Also, even if all your cells are healthy, the further you discharge the cells the more variation you will see in individual voltages because of the lithium discharge "knee". You may be within 0.01vpc when charged to 4.1v but could well have 0.5v difference when discharged. Also, fuses are good, but they will not help you protect against overcharge. So do use fuses, but that's not the only thing you need. That's my nickel's worth.