BMS voltage lower than pack voltage?

[electrominer]

Hi, this is my first post as well as building my first battery pack and I have an issue with the BMS voltage.

I've built a 48V 16S2P battery pack using used Headway LiFePO4 batteries from Battery Hookup.

I checked all the cells when I received them and they were all around 3.2V - 3.3V each. I assembled the cells into a 16S2P pack and I measured 52.2V across the pack. I've connected a BMS and I measure 3.2V - 3.3V between each BMS wire. However, the problem occurs when i measure the voltage through the BMS where I only record 46.6V. Do you know why this would occur? And how can I get the voltage to 52V?
Thanks

 

[john61ct]

Maybe the BMS is calculating from summing the series? Do you still get the same pack-level voltage measuring with your DMM?

Cells can be completely shot and still show a normal voltage.

Each should be load tested and residual capacity measured before assembly, both for go/no-go and warranty returns, and for grouping into balanced parallel sets.

 

[amberwolf]

Put a load (like your controller) on the BMS output, and it will probably drop to nothing, because that symptom usually means the BMS has shutdown to protect the pack due to low voltage cells or some other problem.

If the voltage does drop to nothing under load, then you will want to recheck your connections to the BMS, and verify that you do have cell voltages at the actual BMS board.

If you do, then you should check to make sure it's the right kind of BMS (meant for LiFePO4, rather than other chemistries).

If it is, then there may simply be something wrong with the BMS.


If the load doesn't cause the BMS output to drop any further in voltage, then there's something else going on inside the BMS; either something might be hooked up wrong or it might be defective.

 

[john61ct]

how can I get the voltage to 52V?

Charging of course, once properly tested and assembled.

55V would be a conservative initial target for 16S.

How low can you adjust the BMS "start balancing" voltage?

But really, precise top balancing should be part of the pre-assembly commissioning process.

Have you got PSUs, DCDC converters or chargers capable of precise voltage / current adjustments? If so, in what ranges?

 

[electrominer]

Maybe the BMS is calculating from summing the series? Do you still get the same pack-level voltage measuring with your DMM?

Cells can be completely shot and still show a normal voltage.

Each should be load tested and residual capacity measured before assembly, both for go/no-go and warranty returns, and for grouping into balanced parallel sets.

Thank you for the reply.
I haven't checked each individual cells capacity and I will take your advice and do that. I don't have any equipment so I'm going to go and buy an IMAX B6 V2 balance charger/discharger today. Seems to have decent reviews and appears to be able to do what I need.
Thanks.

 

[electrominer]

how can I get the voltage to 52V?

Charging of course, once properly tested and assembled.

55V would be a conservative initial target for 16S.

How low can you adjust the BMS "start balancing" voltage?

But really, precise top balancing should be part of the pre-assembly commissioning process.

Have you got PSUs, DCDC converters or chargers capable of precise voltage / current adjustments? If so, in what ranges?

Thanks for the reply.
I haven't charged the cells to full capacity yet as I only have a bulk charger and thought that once the BMS was connected that it would take care of this for me.
There are no setting on the BMS, they must be per-programmed in. I'm going to buy an IMAX B6 V2 charger/discharger today to check the cells as I only have a bulk charger. Do you think it will perform the cell checking/balancing sufficiently? Or is there anouther piece of equipment of procedure I should follow as well?

Last night I contacted the BMS manufacture in China and they had me do a resistance test across the P- to B- wires and my meter recorded 0.L which they said means the BMS is defective and they will send me a new one.

So, going forward I will check the cells and their capacity and individually charger them up before I assemble them into the pack and hopefully the new BMS works!
Thanks.

 

[electrominer]

Put a load (like your controller) on the BMS output, and it will probably drop to nothing, because that symptom usually means the BMS has shutdown to protect the pack due to low voltage cells or some other problem.

If the voltage does drop to nothing under load, then you will want to recheck your connections to the BMS, and verify that you do have cell voltages at the actual BMS board.

If you do, then you should check to make sure it's the right kind of BMS (meant for LiFePO4, rather than other chemistries).

If it is, then there may simply be something wrong with the BMS.


If the load doesn't cause the BMS output to drop any further in voltage, then there's something else going on inside the BMS; either something might be hooked up wrong or it might be defective.

Thanks for the reply.

Last night I contacted the BMS manufacture in China and they had me do a resistance test across the P- to B- wires and my meter recorded 0.L which they said means the BMS is defective and they will send me a new one.

I'll test the new BMS when I receive it as you outlined above. Other than it says LiFePo4 16S on the BMS exterior, I don't know how else I'd know if it was for a different battery type or not.

So, going forward I will check the cells and their capacity and individually charger them up before I assemble them into the pack and hopefully the new BMS works!
Thanks.