Can bms work with 8s 48v lifepo4 battery?

[yves31]

I am thinking to wire bms in a weird configuration for my battery, and would like to know if anyone had similar setup.
I have two 24v DIY batteries and they have been working well. Each is wired as 8s 24v with Daly smart bms, and all 16 cells are identical in capacity and size.
Now I'd like to make 16s 48v battery. A 16s48v bms should be used for this setup normally. However, I like to use the existing 8s24v bms for new 48v battery. To do this, I will wire bms to every OTHER cell, instead of every cell. In such configuration, bms should show 8 x 6.4v (48v battery) instead of 8 x 3.2v (24v battery). I will need to double all voltages in bms settings. For instance, charging limit to 7.3v instead of 3.65v.
Does anyone think this configuration would work?

 

[RCinFLA]

You need to be sure the BMS's are built with 75-100v breakdown MOSFET's.

Many 24v BMS's are built with 40v breakdown MOSFET's.

 

[yves31]

You need to be sure the BMS's are built with 75-100v breakdown MOSFET's.

Many 24v BMS's are built with 40v breakdown MOSFET's.

Thank you for the reply. I have daly 8s24v 100A smart bms and will dig out the information.

 

[Sharky722]

You need to be sure the BMS's are built with 75-100v breakdown MOSFET's.

Many 24v BMS's are built with 40v breakdown MOSFET's.

Hey RC this is a subject I'm interested in atm and trying to work out why some can or can't be series connected.

I did work out the higher voltage fets are a must, but does it also include any MCU and ICs on the board also?? I think it does, the whole board has to be the higher voltage? Would that be a fair assumption?

The cheaper 4s ones I've noticed the chips and fets are around 30v so not suited for 48v but maybe only a 24v setup, would this be a fair assumption also?
Or are the ICs clamped down to a lower voltage somehow?.

 

[RCinFLA]

Chinese manuf. usually squeeze every bit of cost out. Power MOSFET's cost goes up with lower Rds_On and higher breakdown voltage. Sometimes they build with higher voltage MOSFET's for reasons like they sell into 12v self-contained batteries that may be series stacked or it simplifies their inventory carrying cost not to have too many different variety of parts to stock.

On 12v self-contained LFP battery you have to be careful. Lowest cost 12v LFP may not be series stackable for 48v systems. It should say in their specs if they are.

Most other IC's are on regulators.

Only other potential issue may be input muxes that sample the cell voltages and transfer sample voltage down to base level for microcontroller ADC.

There is a big step in mux cost going from 8s to 16s voltage but most of the low cost BMS's are using discrete small SOT packaged MOSFET's to do the mux'g and sample and hold function which normally take higher voltage.

 

[acdoctor]

If the breakdown voltage checks out. Just series the packs as is. I don’t think it will work as described. BMS is really pointless only seeing half the cells.

 

[RCinFLA]

If the breakdown voltage checks out. Just series the packs as is. I don’t think it will work as described. BMS is really pointless only seeing half the cells.

Same situation as putting multiple 12v LFP self-contained batteries in series for 24v or 48v systems. Any of the BMS's can shut down the series stack.

If they are smart BMS's you have to deal with looking at different application screens showing eight cells at a time.

Balancing is done on individual cells so no difference having multiple series BMS's. Only slight difference is an active balancer BMS only transfers from its highest voltage cell to its lowest voltage cell so it cannot take from highest cell in other BMS group of cells or push to lowest cell on other BMS.

 

[BradCagle]

However, I like to use the existing 8s24v bms for new 48v battery. To do this, I will wire bms to every OTHER cell, instead of every cell. In such configuration, bms should show 8 x 6.4v (48v battery) instead of 8 x 3.2v (24v battery). I will need to double all voltages in bms settings. For instance, charging limit to 7.3v instead of 3.65v.
Does anyone think this configuration would work?

No, just don't even do this.
First I don't even think the BMS will work with cell voltages this high.
But even if it did you have no way of knowing that one cell is 3.1v, and the other is cooking at 4.2v

Yes the cells can, and will get this unbalanced. This is why a BMS is required to monitor EACH cell.

 

[Goboatingnow]

It’s not a workable idea nor is it safe.