Pylontech x Sofar - battery in standby mode far too early

[Tinbum]

Because the BMS is monitoring the battery and all it's cells. The inverter can't do that.

It will shut the invert down should their be any problem. eg Over current, Cell over voltage, restrict current depending on battery temperature. It also measures the voltage at the battery and allows for any voltage losses in cables or errors in measurement by the inverter.

[openspaceman]

Okay I understand the BMS contains the cell balancing circuits for all the cells and I can see that it could also also prevent the cells from overcharging by cutting off the charge and over discharge by stopping output. All this can be built into a battery of prism cells also. What about over current?

So what I don't understand is what data is exchanged by the BMS and inverter and why a master battery and slaves.

[Oldgreybeard]

Okay I understand the BMS contains the cell balancing circuits for all the cells and I can see that it could also also prevent the cells from overcharging by cutting off the charge and over discharge by stopping output. All this can be built into a battery of prism cells also. What about over current?

So what I don't understand is what data is exchanged by the BMS and inverter and why a master battery and slaves.

Me too.

My theory is (god, I sound like Anne Elk . . .) that each Pylontech battery pack is independent in terms of cell management. I think (but really do not know) that the BMS data transmission is just about transmitting status, not enabling one pack to control another, or any central comms unit, like an inverter, to exert anything other than very top level control over the battery packs.

I think (again, happy to be proved wrong) that each pack operates more or less independently in terms of individual cell management. I can understand the need for Pylontech to keep the way their technology works confidential, but being curious and inquiring by nature I can't help but wonder just how their systems really work, in detail.

[Tinbum]

Yes each pack is independent but transmits its data to the master battery. This then combines the data and sends whats required to the inverter. To the see what is transmitted you need to look at the pylontech CAN protocol readily available online. This data can be different depending on the make of battery. Hence why the inverter has to know the battery type. Ive not sniffed the data between battery packs so am not sure if the master can control the slave packs.

[openspaceman]

Of course this is all a bit of a revelation to me but I do understand the management of LiPo batteries is somewhat more technical than charging Lead Acid.

My need to understand more about this communication between battery and inverter is because I am still waiting for a local electrician to come and make the connections between my new panels, charger and battery. Which will bypass this data exchange. As I have said I have been advised against doing this by my supplier but my alternative is to wait for an approval, G99, from the DNO and simply buy an inverter, this latter bit would definitely need a qualified electrician.

[Oldgreybeard]

Yes each pack is independent but transmits its data to the master battery. This then combines the data and sends whats required to the inverter. To the see what is transmitted you need to look at the pylontech CAN protocol readily available online. This data can be different depending on the make of battery. Hence why the inverter has to know the battery type. Ive not sniffed the data between battery packs so am not sure if the master can control the slave packs.

Thanks very much, it would be useful to see the raw data from each pack, individually, even if it's just for reassurance that all is well when something unexpected happens. I'll have a look around at what might be available to do a "health check" on every pack, individually, as I'm curious to understand a bit more about the way they work.

As an aside, I've decided to charge my six packs to 100% again tonght, in the hope that two or three hours sat at 100% with no discharge might held to resolve any invisible cell balance issues that there might be.

In summer this happens regularly, from PV charging, but the last time we had enough sun to hold the packs at 100% for more than an hour or so was around 6 to 8 weeks ago, and my gut feeling is that holding at 100% for an hour or two may be useful in terms of balancing the packs.

I'll see what happens and report back, in the hope that it may add to the knowledge of the collective (thought it best to balance the earlier Monty Python reference with a Star Trek reference).

[Tinbum]

I intercept the CAN data between the battery and inverter as I alter the max charging voltage slightly lower than that the battery asks for after various reports. I also transmit this data to my elteks chargers which also use CAN but at a different baud rate and in a different protocol.
You can see all the battery packs data using battery view software and the RS232 consol port.

[Tinbum]

You can tell when the cells are balancing as the battery pack temperature reported by the battery rises significantly.
Cells have to be over 0.030v different for the cells to start to balance (if I remember correctly). You can see the settings using a serial consol to the consol port. The current is very small and the battery had to be right near to full as that's when you'll get that voltage difference.
In my set up I can also hold the voltage high when I think its needed to give the battery cells more time to balance, bypassing the bms shut off signal to the inverter.

[Oldgreybeard]

I intercept the CAN data between the battery and inverter as I alter the max charging voltage slightly lower than that the battery asks for after various reports. I also transmit this data to my elteks chargers which also use CAN but at a different baud rate and in a different protocol.
You can see all the battery packs data using battery view software and the RS232 consol port.

Thanks again, I'll have a go at doing something similar, just by way of reassurance that each pack is OK.

[Tinbum]

I do find that the cell balancing is one thing that could be done better by the BMS.

The currents involved are so small that it could take a long time. I find that the cells are still balancing when the BMS tells the inverter to stop charging. As soon as the inverter gets that signal the voltage drops and so the cell differential drops and so does the balancing.

Because I can hold the voltage high using a setting I have on node red I can continue the balancing until I see it has stopped by watching the individual battery pack temperatures in batteryview. This is normally less than 10 minutes. If you leave it longer the packs just individually do into idle. I can't say that this is an improvement in the balancing over what the BMS actually does though. If the battery cells are way out, which mine haven't been other than on one brand new battery, the cell balancing will start earlier.

With the battery that did have cells way out I had to use my way of balancing at a constant voltage (52.5v) to stop a cell over voltage error.