Nowty Towers DIY Battery Project

[AGT]

Getting withdrawal symptoms…

[nowty]

Getting withdrawal symptoms…

The new battery is just working nicely, not a huge amount to add.

I have reduced my main charger max voltage to 55.44V (about 98% SOC) and my supplementary charger to 54V (about 96% SOC). I had noticed the voltage rise at the top end was a little too aggressive for my balancer to cope, its absolutely fine now.

I've been busy extracting the Pylontech's out of the battery cave and selling on to make room for the next DIY battery bank, it has resulted in some bad backache which I'm only just starting to get over.

A new toy has arrived,
1 to 6 NM torque screwdriver, it was on offer from Amazon at £33 at the time, it seems to have gone up to £36 now. A tip when buying from Amazon, whatever the total comes to, go to Cardyard and buy an Amazon voucher for 3% off the face value, https://www.cardyard.co.uk/shops/amazon , then pay with that, it only takes a minute or so before you get it by email.
https://www.amazon.co.uk/dp/B0CL4L452F

The only annoying thing was that the set did not come with a 10mm socket which is needed for a 6mm nut , but it turned out I already had one and it fits in the case under the screwdriver.



The busbars are currently only torqued up to 1NM which is hardly much more than hand tight and even at 100A I can barely feel any warmth using the back of my fingers on the busbars as a temperature sensor. I have read that aluminium welded studs should not be torqued to more than 4NM. I tried that on a steel nut and bolt on a piece of copper strip and that seems very tight to me, more than I dare on the cells, 3NM seems more than adequate.

The springs have not moved at all so I'm soon going to torque up the busbars to 3NM. I did not buy the flexi busbars but with my pre tensioned sprung force I don't see much point as there has been no measurable movement. I have read that cells may swell more as they age over the years, but I will measure the spring length now and then. I can always slacken and tighten if necessary.

[AGT]

Does it tie into an existing BMS?

[nowty]

Does it tie into an existing BMS?

I don't use a specific BMS, but one could easily be added, either on its own or in addition to the active cell balancer.

But what is a BMS exactly ?, there are many types but the best definition I have seen is on the Fogstar website and gives it in a single concise sentence.

BMS (Battery Management System)
The BMS ensures that all of the individual cells in your battery are operating and balancing cohesively, it also ensures that your cells are protected from issues such as under and over-discharge, temperature instability, cell and current imbalances.

I tend to take an overall system view, so lets take each component separately,

Cell balancing
They generally come in two types, top balancing and active balancing. Top balancing requires regular full charges or else it will never function and they usually burn off power from the high cells through resistors so are inefficient. I prefer active balancing as it operates all the time irrespective of the SOC and they transfer power from high cells to their adjacent lower cells in proportion to the cell voltage difference. The one I use only uses about 16mA in quiescent conditions.

Short circuit and Overload Protection
I use appropriate fuses cascaded to protect against a short circuit event or a prolonged overload of a single battery bank. In reality I could only overload a single battery bank if I were only running on a single battery bank, say if the others were out for maintenance.

Under and Over Discharge
I use a max charge voltage and a min discharge setting on my inverters. I also use a max and min protection voltage setting in one of my inverters if I am going to be away for some time. This will allow the inverter to force charge from the grid or dump battery power to the grid in extreme cases.

Temperature Instability
My batteries are in a place which is cool and stable typically no less than 10 degrees in Winter and no more than 15 degrees in Summer. So that in itself prevents most temperature issues. The main temperature issues are with cold temperatures and the management of the max C charge rate which can be about 0.1C even at 5 degrees or so above zero. When I get my final battery bank in, I can still do 200A (10kW) and not even exceed 0.1C.

Monitoring
Having a number of parallel connected battery banks of different capacity and chemistry has its challenges, I have a permanent voltage readout in my lounge. I then read off a complicated spreadsheet giving me a blended voltage vs SOC. But this has now been superseded by my Victron Smart Shunt which counts Ah's thus giving me an accurate view of battery voltage, current and SOC from anywhere using their Victron Remote Monitoring (VRM).

What I don't have is individual live cell voltage and temp monitoring and active control of the inverter(s), but you pays your money and you takes your chances.

[AGT]

Cheers Nowty,

[nowty]

The final power off take solution, cables fixed in position giving support and no stress on the bus bars.



And I replaced the mid battery solid bus bar separating the decks, for a 500A forklift fuse just in case I accidently weld a spanner or screwdriver to the busbars and avoid a Predator film like event.

Luck would have it that the fuse fits perfectly between the two battery decks.

All busbar screws now tightened up to 3Nm with my torque screwdriver, in fact I may go to 4Nm as it was really easy.



I checked on my now redundant 16th pylontech cell, these had not had any compression at all and have done about 400 full cycles over the past 2.5 years.



They have expanded about 2mm each.



They still work fine, but I think compression is needed if you want a long life out of LFP prismatic cells, so I am happy that I decided to go with compression for the new battery banks. Not sure what to do with them now, I might turn them into a 12V battery.

[AGT]

Hope you don’t mind me asking, as I’ve just splashed some cash due to yours and others inspiration
I know you don’t have a BMS but if you did how would a 2p16s battery like this work with a BMS, wouldn’t the 2 cells in parallel fool the BMS if there was an issue with one cell?

[Stinsy]

Hope you don’t mind me asking, as I’ve just splashed some cash due to yours and others inspiration
I know you don’t have a BMS but if you did how would a 2p16s battery like this work with a BMS, wouldn’t the 2 cells in parallel fool the BMS if there was an issue with one cell?

BMS is a horrible and somewhat meaningless term used frequently by people who don’t know what one does!

Nowty has a really great active balancer. This is one very important part of a BMS. The active balancer takes power from the highest cells and sends it to the lowest ones. Many balancers bundled into a “BMS” only bleed power off of the highest cells at the top end and do so very very slowly. Nowty has conservative min an max voltages set in his inverter/charger(s) rather than having the “BMS” cut the circuit when voltages are too high or too low. Many modern BMSs allow sophisticated cell-by-cell voltage logging. Nowty checks this periodically with a multimeter.

The main problem with lithium cells arranged in large serial batteries is that the voltage across the whole pack can be within spec but a single cell could be out of spec and therefore become damaged. The above steps mitigate this problem.

[nowty]

Hope you don’t mind me asking, as I’ve just splashed some cash due to yours and others inspiration
I know you don’t have a BMS but if you did how would a 2p16s battery like this work with a BMS, wouldn’t the 2 cells in parallel fool the BMS if there was an issue with one cell?

A full cell monitoring BMS in my system would simply "see" 16 cells of double the Ah capacity and two unequal capacity cells in parallel have the same voltage but different currents. If one of the cells in a pair was to degrade in capacity so much that the voltage of that pair would rise or fall outside of the BMS cell limits, the BMS would still shut the system down.

People have to make their own decisions about what they are willing to do with DIY systems, if your concerned about two cells in parallel, don't do it and just make the frame half the width for 16 cells.

I see this thread more about the frame, adding the preload force with springs and illustrating exactly how strong a frame has be be in order to get the 300kg preload force.

[nowty]

Hope you don’t mind me asking, as I’ve just splashed some cash due to yours and others inspiration
I know you don’t have a BMS but if you did how would a 2p16s battery like this work with a BMS, wouldn’t the 2 cells in parallel fool the BMS if there was an issue with one cell?

BMS is a horrible and somewhat meaningless term used frequently by people who don’t know what one does!

Exactly Stinsy, folk might want to back up a few posts to read my explanation about what exactly a BMS is, it is frequently misunderstood.
https://camelot-forum.co.uk/phpBB3/view ... 291#p58134