If BMS for Lithium Battery Packs work, then why the much concern about Overcharging them?

[RotEvery]

I notice that my Felicitysolar Lithium Battery packs' BMS's stop the charging process each time they reach 100% SoC irrespective of increase in the absorption voltage above 54.4V. They have no communication connection with my Sorotec inverter/charger.

If battery pack BMS's work well like this, why the so much concern about these type of battery packs being overcharged? Why all the extra expenses on shunt trips, et cetera? Shouldn't efforts be more focused on having more robust BMS within each battery pack?

 

[DIYrich]

IMHO, 54.4v is not 100% charged.

At this time BMS are single products designed to protect each cell in the battery, and a separate charge source controls the overall pack. With UL9540, I don't think there will be a market for an AIO BMS/charger.

 

[OM617YOTA]

I'm ridiculously new at this, but my system-design philosophy is that if the inverter and charger settings are right, the BMS basically never has to step in. The BMS is the last line of defense to save your batteries, not something you should be bouncing off of on a regular basis.

My BMS is set to disconnect at 2.5v/cell and 3.65v/cell. My AIO's are set to the Victron numbers in the recommended charge profile sticky, which is 3v/cell and 3.525v/cell.

 

[RotEvery]

IMHO, 54.4v is not 100% charged.

At this time BMS are single products designed to protect each cell in the battery, and a separate charge source controls the overall pack. With UL9540, I don't think there will be a market for an AIO BMS/charger.

One of the battery packs consistently shows 100% at 54.5V while the other shows 100% at 55.3V and remains 100% when the voltage goes back to 54.4V. Both packs were bought the same day. Brand new.

 

[DIYrich]

I'm ridiculously new at this, but my system-design philosophy is that if the inverter and charger settings are right, the BMS basically never has to step in. The BMS is the last line of defense to save your batteries, not something you should be bouncing off of on a regular basis.

The bms keeps the cells balanced inside the pack. The charger can only manage the pack as a whole.

 

[DIYrich]

One of the battery packs consistently shows 100% at 54.5V while the other shows 100% at 55.3V and remains 100% when the voltage goes back to 54.4V. Both packs were bought the same day. Brand new.

You need to see what is happening at the cell level.

 

[Will Prowse]

Because BMS can fail. And certain situations can stress the MOSFETs in the BMS. For long term use, it's better to control cycling thresholds with the inverter and solar charge controller absorption voltage than to have the BMS do it. It will just prolong the life of your system.

 

[Will Prowse]

But you need to get the battery absorption voltage higher than the balancing threshold on the BMS. If not, cell drift will become excessive over time and will reduce the life of your battery.

Which is why you should set the absorption to whatever the battery recommends. You will not hit OVD if you follow their recommendations. You can see this in my videos on 12V batteries when they disconnect at 15V. if you set the absorption to 14.5V you will be fine.

 

[RotEvery]

With UL9540, I don't think there will be a market for an AIO BMS/charger.

interesting

 

[Will Prowse]

IMHO, 54.4v is not 100% charged.

At this time BMS are single products designed to protect each cell in the battery, and a separate charge source controls the overall pack. With UL9540, I don't think there will be a market for an AIO BMS/charger.

You are right. 3.4V per cell is not enough.

If system is being permitted, you're right. Comms will eliminate aio/cheap BMS combo. But off-grid market will buy them regardless. I think both markets will grow based on sales volume for last ten years and increasing utility prices.

 

[Will Prowse]

As higher voltage ess batteries are used, I assume fets will no longer be present at the pack level. There will be passive dissipative resistors and voltage sensing leads to connect to a main BMS, but that's it. This is what Tesla EV packs are currently doing, and I assume everyone else in the market.

I think moving forward, most off-grid packs will still have fet based BMS with OVP so individuals can still use a lower voltage system voltage and mismatched batteries.