Lithium Cell Balancing at Nowty Towers - Resurrections

[openspaceman]

Nowty this seems awfully complicated and makes me wonder if buying pylontech additional storage to parallel with my growatt battery may be ill advised?

Also looking at the SOC on the webserver for my battery should it directly correlate with the voltage of the battery?

[nowty]

Nowty this seems awfully complicated and makes me wonder if buying pylontech additional storage to parallel with my growatt battery may be ill advised?

Also looking at the SOC on the webserver for my battery should it directly correlate with the voltage of the battery?

If I'm reading your sig correctly, your Growatt battery model is not the same chemistry as my Growatt battery, yours is LIFEPO4 (LFP) like Pylontechs but it looks like its a 16 cell battery as the spec for your battery says nominal voltage 51.2V.

51.2V / 16 = 3.2V which is the nominal voltage of a LFP cell. The datasheet also says charging voltage up to 57.6V which would be 3.6V for 16 LFP cells, and that is near max for them.

So if your copying Joeboys thread, you would need to buy 16 cells and not 15 like Joeboy as Pylontech's have 15 cells. But you've got to be "game on" for this sort of tinkering.

EDIT - Sorry openspaceman, I didn't fully understand your query.

No, no, no, definitely DONT buy Pylontech to parallel up with your Growatt GBLI6532 as they appear to be a different number of LFP cells. You'd end up doing what I'm doing with an extra cell. But as I've said, making your own pack like Joeboy should work but you would need 16 cells.

[openspaceman]

Thanks as always Nowty, I have still a lot of learning to do.

I'm not competent to make my own battery unfortunately so may just have to fork out for another growatt.

[nowty]

My resistors arrived about half an hour ago and earlier I bled a bit more power off the new cell with my 1 ohm resistor.

I have fitted a 10W rated 20 ohm resistor across the cell, its pleasantly warm to touch burning about 0.5W.

At 12:20 today the voltages are,

Pylontech 51.12V, so each cell is 3.408V
New Cell 3.352V

I will monitor and note the voltages again tomorrow at around the same time.

Checked again today at the same time 12:20.

Pylontech 50.98V, so each cell is 3.411V
New Cell 3.405V

Therefore the new cell has increased its voltage relative to the Pylontech cells somewhat.

So 20 ohm resistor not low enough as I expected, so I dropped to a 15 ohm one but voltage still increased over the next couple of hours as it hit the top end, so I've dropped to a 10 ohm one which I think should do it as its the closest one to burning off 1W which I had previously approximately calculated. The 10 ohm resistor runs hot, but not too hot to touch.

The other good news is the first type of overvoltage protection definitely turns on on partially at 3.6V and full at 3.66V, so at a lower voltage than I had previously tested, it seems there is simply a few seconds of time delay before it switches on. So I am quite happy with it now as I don't mind it regularly hitting that voltage. The second type I tested does not switch on till higher so I wont be using the second type anymore.

I shall continue to monitor the relative cell voltages and my additional over voltage protectors are now in the UK so should arrive in a few days when I can build up a larger protection circuit with redundancy.

[nowty]

Checked again today at the same time 12:20 with my 10 ohm resistor on since yesterday with the new cell hitting the cell protector voltage yesterday afternoon so the new cell would have been slightly above the Pylontech cells, but only just.

Pylontech 51.77V, so each cell is 3.451V
New Cell 3.491V

So, I think the 10 ohm bleed resistor closely matches my Pylontechs pro-rata parasitic load. I will need to monitor it for a few more days to work out whether it under or over bleeds the new cell as I need it to slightly under bleed and then it will reset every time it gets a full charge and hits the 3.66V voltage protector(s).

[nowty]

Two more days and midday readings have been,

9th / Sep
Pylontech 50.58V, so each cell is 3.372V
New Cell 3.373V
---------------------------------
10th / Sep
Pylontech 50.41V, so each cell is 3.361V
New Cell 3.366V

The protection board has not even operated so it’s as near a fit and forget solution as I can get to.

My extra protection boards arrived today so I am made up a separate protection device so I’m not messing around on the live working cell.



All done and operational now.

[Joeboy]

Two more days and midday readings have been,

9th / Sep
Pylontech 50.58V, so each cell is 3.372V
New Cell 3.373V
---------------------------------
10th / Sep
Pylontech 50.41V, so each cell is 3.361V
New Cell 3.366V

The protection board has not even operated so it’s as near a fit and forget solution as I can get to.

My extra protection boards arrived today so I am made up a separate protection device so I’m not messing around on the live working cell.



All done and operational now.

You are using the blade fuse connection as a quick isolation switch so to speak?

[Stinsy]

Will you need to check the voltages every few weeks incase something changes and your 20 Ohm bleeds too fast?

[nowty]

You are using the blade fuse connection as a quick isolation switch so to speak?

The fuse is mainly in case of me dropping a spanner or something across it, but it could be used as a safety isolator if I have to replace one of the protector boards.

I decided it was a pointless exercise fusing each individual protector board as the worst case failure is current limited by the resistors so would not blow a fuse anyway.

I decide to connect the bleed resistor by a croc clip, so if I ever needed to take the Pylontech string out of service for a long time, I could easily disconnect the bleed resistor so it would not totally discharge the cell.

[nowty]

Will you need to check the voltages every few weeks incase something changes and your 20 Ohm bleeds too fast?

The final bleed resistor is 10 ohms, but I can check the voltage easily every day by looking at my remote monitor in my lounge. Simple maths lets me work out whether the cell is too high or too low. Blue is the whole string voltage and the red one is the new cell voltage but I also have to take into account that the blue one over reads by 0.2V and the red one under reads by 0.01V. If I'm away, I'm always asking SWMBO what's the blue display saying, then making a decision on whether she can plug her car in or do an ironing marathon.