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CAn you run a BMS with missing cells correctly


NoordSolar
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Need to build up a 16- 18 cell Lithium battery pack for my Victron MP1 - however was considering adding an extra cell or two in case one or two go rouge after a few years .  The problem is will a BMS be able to handle a missing cell ? or will you have to use say an 18 cell BMS initially and then replace with a 16 cell one if in the case when 2 cells go off. How else do you manage this possible scenario?

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Some BMS can work with a variable set of cells. You will see them advertised as a 16s-20s BMS or something similar. With a BMS like that you can just remove the defect cell and adjust your cell connections accordingly. 

If you keep the defective cell in your battery however it will pull limit the battery. The BMS will stop discharging as soon as one cell falls below the min. voltage and stop charging as soon as one cell goes above the max voltage. So the range of the worse\smallest cell will limit your battery.

In the end it most likely will not be worth it to go that route.
The scenario that you are thinking off is more what will happen to lead acid type batteries, as these do not normally have a BMS to look after each cell.
Lithium batteries with a BMS will mostly be managed by the BMS that their life duration will be very similar, so if one fails then the other cells will be close to failing (end of life) also. So it would be better to replace all the cells in a Lithium battery.

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For lead acid a BMS is not required.

For anything lithium you have to have a BMS.

 

Lithium cells that are overcharged is a fire hazard. The Victron charger charge a set of batteries and do not look at the cells. Even if you think you initial balance the cells it does not always work 100%. The risk is too high.

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On 2020/07/29 at 3:12 PM, NoordSolar said:

Can you get away without a BMS if you do the inital balance well and then manage max /min voiltage with the victron charger and perhaps as a victron disconnect.  

No.

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