Lithium Cell Balancing at Nowty Towers - Resurrections

[nowty]

Not sure this is a good idea, but still thinking about it.

I see the new batteries are 120Ah but your Pylontechs are 50Ah. Are you proposing not to fully charge them and / or how are you going to keep them in balance with the pylontech cells?

Edit- i think i would have been inclined to remove one growatt cell.

I'm not 100% sure of this myself !, Its something I thought about several years back and discounted it for the reasons you mentioned.

Then I went down the route of utilising an overvoltage DC auto disconnect for the Pylontechs so I could use the higher charging voltages. This worked fine for a few years but a few months ago the voltage sensing relay contacts froze because of DC going through them to energise the DC disconnect relay so I decided to abandoned that and go back to the lower charging voltage. Because this has effectively reduced the total capacity of my battery banks, this might cause me issues in winter, especially with my new air to air heatpumps which I am expecting to use on the battery banks outside of the cheap rate period.

So, I had another think about the original plan and I now reckon its workable, but will need to closely monitor the extra cell voltage.

You are correct that the Pylontech's I have are 50Ah, but I have a stack of 8 of them in parallel so to match the cell size I need a 8 x 50Ah = 400Ah one. Then thinking about it a bit more, if I had a smaller Ah cell the voltage would increase and decrease faster and very easily go overvoltage or undervoltage. But if I used a larger cell, then the voltage would rise and fall slower so if I can balance the new cell at the midpoint it would very unlikely to either go over or under voltage. Balancing at the midpoint is easier said than done, but I can get it initially in the ball park, then alter it manually as needed whilst monitoring the voltage of the cell verses the whole battery bank voltage.

I have bought 4 x 120Ah cells which will be set up in parallel to give a single 480Ah cell and it is this cell which will be placed in series between my Pylontch's and one of the main DC bus bars of my battery banks. This cell Because of the increased size, assuming its balanced correctly, I am expecting the voltage of the cell to remain within a small voltage range, say 3.2v to 3.5v.

That should allow me to increase my charge voltage back up to 56v or so with the Pylontechs remaining happy at around 52.5v. The added battery will add direct useable capacity of 1kWh and indirectly an additional capacity of 7kWh from the Growatt batteries due to the increased charging voltage.

There is another couple of advantages too,

• The typical DC operating voltage will remain circa 3v higher so typically less current flow, so less cable losses.

• In Summer the Pylontech's hardly get exercised because the voltage does not typically fall low enough to get them operating, but the LFP chemistry of them should have double the cycle life of the LMCO chemistry, so this should get them into the operating zone and extend the operating life of the Growatts.

With regards to your idea of the removal of a Growatt cell, yes I had thought of that too, but discounted for,

• I'd still lose about 3kWh to 4kWh so not really worth it.

• The work involved would be horrendous and I would have to replace two of my active balancing boards, whereas the new cells just go into the existing space where my now obsolete overvoltage disconnect relay was. I don't even need any additional cables.

[Tinbum]

The advantage with the removing one Growatt cell is that's its fit (or remove in this case) and forget. You'd be able to keep the cells for spares or sell. (I'd be interested in a couple to replace 2 duff ones I have )

The cell balancers should still work for 13 cells though shouldn't they. I have a Seplos BMS that I bought for 14 cells and when you look at it it's actually a 16 cell with wires cut off and a blob of solder shorting 2 of the pins on the sensing chip and that's way more complicated a BMS. Your balancers are even more simple and i've seen some sold to cover a range of cell numbers.

The Pylontech are expensive batteries and for me I'd not really want to experiment, though I think it probably would work. The other thing is what happens to us- my wife would be lost and doesn't have a clue about the tinkering I do daily to keep everything running at optimum. I'm preparing to put spares everywhere so they can just be a quick swap if something fails. For example my Teensy that does the CAN communication. Another spare teensy (pre loaded) and PCB will be in the rack next to the working one and also I plan on putting in a spare SD card with the Arduino sketch (program) on it.

[nowty]

Actually my cell balancers would work as they are 7/8S boards and I use two per 14 cell string, but I had forgotten you have to overlap a cell, so they could be reconfigured for 7+7-1 = 13 cells. But still, I'd be more inclined to sell off the Pylontechs at todays crazy prices and build up either a new third LFP bank of 16 cells or try and find another 5 Growatt batteries to build up a third bank to replace the Pylontechs.

Alternatively, I could remove ALL Growatt and go 100% Pylontech for an easier life but that would cost £20k (minus what I could sell them for) to keep the same capacity.

One of my problems is that the batteries are in a very confined and hard to access space so any major changes would involve a LOT of backache, time and effort.

My only issues with what I am doing are,

• How often will I need to manually re-balance the large single cell.

• Are the Pylontechs going to behave somehow differently and unexpectedly with this extra cell in its string. I cannot think why they would but you can never rule out the unexpected.

[Joeboy]

Not sure this is a good idea, but still thinking about it.

I see the new batteries are 120Ah but your Pylontechs are 50Ah. Are you proposing not to fully charge them and / or how are you going to keep them in balance with the pylontech cells?

Edit- i think i would have been inclined to remove one growatt cell.

I'm not 100% sure of this myself !, Its something I thought about several years back and discounted it for the reasons you mentioned.

Then I went down the route of utilising an overvoltage DC auto disconnect for the Pylontechs so I could use the higher charging voltages. This worked fine for a few years but a few months ago the voltage sensing relay contacts froze because of DC going through them to energise the DC disconnect relay so I decided to abandoned that and go back to the lower charging voltage. Because this has effectively reduced the total capacity of my battery banks, this might cause me issues in winter, especially with my new air to air heatpumps which I am expecting to use on the battery banks outside of the cheap rate period.

So, I had another think about the original plan and I now reckon its workable, but will need to closely monitor the extra cell voltage.

You are correct that the Pylontech's I have are 50Ah, but I have a stack of 8 of them in parallel so to match the cell size I need a 8 x 50Ah = 400Ah one. Then thinking about it a bit more, if I had a smaller Ah cell the voltage would increase and decrease faster and very easily go overvoltage or undervoltage. But if I used a larger cell, then the voltage would rise and fall slower so if I can balance the new cell at the midpoint it would very unlikely to either go over or under voltage. Balancing at the midpoint is easier said than done, but I can get it initially in the ball park, then alter it manually as needed whilst monitoring the voltage of the cell verses the whole battery bank voltage.

I have bought 4 x 120Ah cells which will be set up in parallel to give a single 480Ah cell and it is this cell which will be placed in series between my Pylontch's and one of the main DC bus bars of my battery banks. This cell Because of the increased size, assuming its balanced correctly, I am expecting the voltage of the cell to remain within a small voltage range, say 3.2v to 3.5v.

That should allow me to increase my charge voltage back up to 56v or so with the Pylontechs remaining happy at around 52.5v. The added battery will add direct useable capacity of 1kWh and indirectly an additional capacity of 7kWh from the Growatt batteries due to the increased charging voltage.

There is another couple of advantages too,

• The typical DC operating voltage will remain circa 3v higher so typically less current flow, so less cable losses.

• In Summer the Pylontech's hardly get exercised because the voltage does not typically fall low enough to get them operating, but the LFP chemistry of them should have double the cycle life of the LMCO chemistry, so this should get them into the operating zone and extend the operating life of the Growatts.

With regards to your idea of the removal of a Growatt cell, yes I had thought of that too, but discounted for,

• I'd still lose about 3kWh to 4kWh so not really worth it.

• The work involved would be horrendous and I would have to replace two of my active balancing boards, whereas the new cells just go into the existing space where my now obsolete overvoltage disconnect relay was. I don't even need any additional cables.

I am looking forward to getting back home to read this in detail (notepad) at side and seeing the pics. SWMBO has put the kibosh on more batteries for the moment. I'm sure she'll change her mind once the induction hob goes in. Until then looking forward to seeing your progress Nowty!

[Joeboy]

Actually my cell balancers would work as they are 7/8S boards and I use two per 14 cell string, but I had forgotten you have to overlap a cell, so they could be reconfigured for 7+7-1 = 13 cells. But still, I'd be more inclined to sell off the Pylontechs at todays crazy prices and build up either a new third LFP bank of 16 cells or try and find another 5 Growatt batteries to build up a third bank to replace the Pylontechs.

Alternatively, I could remove ALL Growatt and go 100% Pylontech for an easier life but that would cost £20k (minus what I could sell them for) to keep the same capacity.

One of my problems is that the batteries are in a very confined and hard to access space so any major changes would involve a LOT of backache, time and effort.

My only issues with what I am doing are,

• How often will I need to manually re-balance the large single cell.

• Are the Pylontechs going to behave somehow differently and unexpectedly with this extra cell in its string. I cannot think why they would but you can never rule out the unexpected.

I'd happily buy a couple pylontechs off of ye at mates rates!

[Stinsy]

Going for silly prices at the moment!

https://www.ebay.co.uk/itm/185521031636

[Joeboy]

Going for silly prices at the moment!

https://www.ebay.co.uk/itm/185521031636

[Tinbum]

I was very tempted to put mine on as I hadn't opened a few of the boxes.

[nowty]

I was very tempted to put mine on as I hadn't opened a few of the boxes.

The problem is you'll have to buy them back at some point and Winter is coming.

[Tinbum]

Yes that was a couple of months ago, when delivery of the new ones was about next month so would have been ok.