Convert 21s BMS into 20s BMS

[420b]

I have a BMS configured for 21s, but I only have a 20s pack. Can I make a connection to trick the BMS into thinking there is a 21st cell with the same voltage as the 20th cell? I was trying to come up with a way to do this, but the 21st cell is based on the 20th cell so I cant just wire the 20th cell twice. Can anyone think of a clever way to do this?

 

[Sunder]

Most BMSes are smart enough that if they detect zero volts on all cells closest to the positive terminal, that it means that the battery is smaller than the BMS design, and it lowers the total pack LVC and HVC.

For example (See below), on my 24S BMS, if you wanted to install only 21 cells, you would simply leave off cells 22, 23 and 24, and connect cell 21 to the final . Some don't even need that final step. It will take the last non-zero voltage as the final cell. You should try this first to avoid any potential shorts. (Unlikely)

You will want to test this on a lab in case your specific one isn't this smart, but my understanding is that most of them are. At least, over the last 10 years, every single BMS I've used has had this capability.

 

[420b]

Most BMSes are smart enough that if they detect zero volts on all cells closest to the positive terminal, that it means that the battery is smaller than the BMS design, and it lowers the total pack LVC and HVC.

For example (See below), on my 24S BMS, if you wanted to install only 21 cells, you would simply leave off cells 22, 23 and 24, and connect cell 21 to the final . Some don't even need that final step. It will take the last non-zero voltage as the final cell. You should try this first to avoid any potential shorts. (Unlikely)

You will want to test this on a lab in case your specific one isn't this smart, but my understanding is that most of them are. At least, over the last 10 years, every single BMS I've used has had this capability.

Short BMS.jpg

Thanks, I'll give it a shot.

 

[420b]

Most BMSes are smart enough that if they detect zero volts on all cells closest to the positive terminal, that it means that the battery is smaller than the BMS design, and it lowers the total pack LVC and HVC.

For example (See below), on my 24S BMS, if you wanted to install only 21 cells, you would simply leave off cells 22, 23 and 24, and connect cell 21 to the final . Some don't even need that final step. It will take the last non-zero voltage as the final cell. You should try this first to avoid any potential shorts. (Unlikely)

You will want to test this on a lab in case your specific one isn't this smart, but my understanding is that most of them are. At least, over the last 10 years, every single BMS I've used has had this capability.

Short BMS.jpg

Unfortunately mine does not. I built a 20s pack and it behaves like there is a low voltage cutoff.

Edit: I just reread and realized I frocked it up. I removed the 21st BMS wire, ending on the 20th as the most positive terminal. I'll have to move the 20th to the 21st and we will be good

Edit 2: unfortunately it does not work. I put wire 21 on 20th positive and skipped wire 20. I put wire 19 on 19th positive and so on.

 

[Sunder]

Sorry mate. It was worth a shot.

Not sure there's much else you can do. Simulating a fake voltage isn't a good idea. It'd need to be fully isolated.

Unfortunately, BMSes of that size aren't all that cheap either. (At least in my experience, once you're over 12S/48V, things start getting pricey).

 

[420b]

Unfortunately I blew it up while messing around removing resistors that I figured may be sense resistors on the board. I will buy a new BMS. There are some cheap 20$ 20s ones from Macau that I purchased.