using a bms with less cells than it it's made for

[marcello]

One of the cells of my 16 cell lifepo4 pack has died. It's an old pack and rather than buying a new cell I'd like to use it as a 15 cell pack. I managed to adjust the charger and I'm now only wondering what to do with the two 8 cell bms boards that i use with this pack. Letting the cable for the missing cell hang loose does not work because then the bms does not switch on. Could I just connect the loose cable to any of the other cells? While riding it would sense a high cell voltage and not switch off the bms. While charging it would just double the balancing capacity for the cell it's connected to. Am i right? Any thoughts before I try it?
This is the bms I use:

 

[James340]

I think you will have to install a 15s bms.The bms leads go in series so voltage goes up from 1 to 16 .1 being 4.2 for example so 16 is 60v.I have seen bms that cover a range of series instead of a fixed number

 

[amberwolf]

Some BMS can be used this way, some can't. You'd need to check the manufacturer's wiring diagram to find the specific way to wire yours for this configuration, if it is possible.


Here's a link to the page for a BMS I have here that does have this ability, and they do have the wiring diagram. Don't use this info to apply to your BMS, it's just an example; you'd need to search for the markings on your BMS to see if you can find a page like this for it. (I found mine after only a few searches and alternate terms)

::::: 春豆科技股份有限公司 SOONTOP TECHNOLOGY CO., LTD.:::::

春豆科技股份有限公司 SOONTOP TECHNOLOGY CO., LTD

www.soontop.com.tw

Experimenting has a chance of damaging the BMS in a way that prevents it from monitoring the cells for HVC and/or LVC, which would mean it can't detect when the cells are full or empty, and so it can't do the job it is there to do, which is to protect the cells from damage that can lead to a fire.

 

[marcello]

@James340 Darn! Of course! You are right! The cell voltage is measured from one balance lead to the previous lead and not to ground. Which made me think: Since I have two 8 cell BMS, what if I connect both BMS to the battery cell in the middle of the pack (See the images below to better understand what I mean). Would that work?

 

[James340]

If the voltage is the same perhaps .A bms is not that expensive but if you want to try to fool the 16s bms check to make sure voltage is correct

 

[docw009]

Like Amberwolf said, some BMS can be used for different S-strings. I have a JBD 10S-14S unit. I can use it on 36V, 48V, or 52V packs. The restriction is that all the lower series groups must be used in sequence. I believe the unused balance wires are tied to the last one used. Haven't tried that though.

That means that if I lost a cell in the middle of a 14S and wanted to make it 13S, I would jumper or remove the bad cell and have to rewire all the balance wires above that cell, so they stay in sequence, allowing the last wire to be free.

 

[fechter]

If two BMSs "share" a cell, something bad might happen if one of them trips.

I've used several BMSs with fewer cells by having the unused tap wire at the positive end of the string and tie the unused tap wire to the positive. This might not work with the fancier Bluetooth units.

 

[marcello]

@docw009 Thanks for the tip. I think I'll end up buying one of those JBD's. They cost a fraction of what I payed for mine.

@fechter Thanks for your response. Do you mean tying the unused tap like below?

 

[fechter]

Correct. I learned the hard way you can't just leave the wires floating. By tying it to the cell above, that channel is effectively turned off and won't trip the BMS. But there are many different BMS designs, so some may not work like that.