Idiot's guide to a self build battery

[Caesium]

Yeah I certainly wouldn't want hose clamps or cable ties to be in direct contact with the battery. That'd be a heck of a pressure point.. can see it easily damaging the internals.

Your idea of some threaded rods sounds pretty good, probably a lot easier to get it all nice and snug. Screwfix seem to do a decent selection of 1m long ones too.

[Joeboy]

The individual packs are 72mm thick, so 360mm plus the end plates and insulating paper means I will need 12 of 500mm lengths to make up three banks. Probably stick at M5 or even M4, decent step up in washer sizes to spread the load on the timber and just use plain nuts, two of on each to spin up easy and lock in place. Done once, never thought of again!

[Caesium]

Yep exactly my thoughts, though I have a bag of M6 nuts somewhere so I'll probably just use those and get rods to match.

Next line of thought... top balancing these cells before putting them into use. Apparently this is important to get them all at same state of SOC. I would have thought the balancer would help here but perhaps if they're too far out of sync then the balancer won't be able to sink enough current to correct it, which leads to things like runaway voltages on cells that are too high above the rest of the pack.

So to that end, considering a buck converter that can do 3.65v at up to 20A, something like https://www.aliexpress.com/item/4001231282413.html

.. this spending money thing is addictive

[Joeboy]

Yep exactly my thoughts, though I have a bag of M6 nuts somewhere so I'll probably just use those and get rods to match.

Next line of thought... top balancing these cells before putting them into use. Apparently this is important to get them all at same state of SOC. I would have thought the balancer would help here but perhaps if they're too far out of sync then the balancer won't be able to sink enough current to correct it, which leads to things like runaway voltages on cells that are too high above the rest of the pack.

So to that end, considering a buck converter that can do 3.65v at up to 20A, something like https://www.aliexpress.com/item/4001231282413.html

.. this spending money thing is addictive

Nice one! I will have a read and a look, thank you. I hear that the cells can vary be a lot so I guess I'd voltage match in banks of 5 then charge, any anomalies would be dealt with as a single?

I am roof hanging the expansion pack beside the Pylontechs so went for aluminium 20mm angle and mild steel rods and zinc heated nuts n washers at M6.

£1,687- 15 of LF280K (A) batteries incl del, bus bars n bolts
£60.72 - 1 of 15s 5A balance board.
£60 framework and bolts

£1,808 total

[Caesium]

Nice one! I will have a read and a look, thank you. I hear that the cells can vary be a lot so I guess I'd voltage match in banks of 5 then charge, any anomalies would be dealt with as a single?

I think there's a few schools of thought here. The main problem is top-balancing these things can take a very long time, if you consider that at 3.65v@20A it would take 14 hours to charge one cell assuming it took 20A all the way to full, which they don't, they drop off the amps the fuller they get. And we've 15 to deal with, so over 210 hours.. more than a week solid! To go any faster you need a lot more amps to fill these things up and good luck finding anything that can produce 3.65v at much more than 20A.. it gets expensive fast.

So I have seen some suggestions (I'm reading another forum as well which seems to have a lot of USA-based offgrid folks using these batteries, I won't name it here tho) to just charge them in series, which you can do a lot faster. ie basically just hook them up to our Pylontechs and turn on the inverter. So now you can get a good few kW into them and fill them up in a day. I'm unsure whether we should have the balancer attached at this point, or just monitor individual cell voltages, or both? I think possibly the balancer just might not be able to cope with the imbalance at this stage.

After they're mostly full from doing series charging, then detach and reconfigure in parallel to do the top-end balancing which now hopefully only takes a couple of hours if that, because they're already nearly full.

At least that's my current understanding, still reading and learning

[Oldgreybeard]

When building packs for electric bikes years ago I just used to parallel up all the new cells to form one big cell and leave them all on charge for a day or so. Doesn't matter if they are absolutely fully charged or not after that time, near enough is good enough as they will all be in near-perfect balance with each other by then. The BMS doesn't then need to do much once the pack is rearranged so the cells are in series as the pack starts off in good shape.

[Joeboy]

When building packs for electric bikes years ago I just used to parallel up all the new cells to form one big cell and leave them all on charge for a day or so. Doesn't matter if they are absolutely fully charged or not after that time, near enough is good enough as they will all be in near-perfect balance with each other by then. The BMS doesn't then need to do much once the pack is rearranged so the cells are in series as the pack starts off in good shape.

That's great OGB, I plan to build in place on the wall then bring them up to the Pylontechs top voltage. I am mentally ok for a couple of days doing this but not much beyond. Once back home and off the smartphone nd access to big screen I'll do myself an idiot sheet of the Pylontechs true top voltage and tape it up right where I can see it. I like the idea of linking the LF280k's all together and letting them find their own passive offline balance before feeding them the current. To match the Pylontechs or as I said earlier bring the pylontechs down to the LF280k's then bring the LF280k's online.

Caesium, you are truly running with this! I'm away to have a look at your buckboard link. I plan to add the balancing board side after I've got it all up on the wall, settled to its as delivered equilibrium and then power matched to the Pylontechs. I'm sure a day or so off no balancer wont do any harm? Hopefully nothing too wildly out of whack on arrival!

[Caesium]

Caesium, you are truly running with this! I'm away to have a look at your buckboard link. I plan to add add the balancing board side once I've got it all up on the wall and settling to its as delivered equilibrium. Hopefully nothing too wildly out of whack on arrival!

Yeah once I get started there's no stopping me

I'm thinking about some breakers now, just to avoid too many accidents.. probably a few of https://www.ebay.co.uk/itm/155047696936 inline with my 20mm cable connects.

[Joeboy]

Caesium, you are truly running with this! I'm away to have a look at your buckboard link. I plan to add add the balancing board side once I've got it all up on the wall and settling to its as delivered equilibrium. Hopefully nothing too wildly out of whack on arrival!

Yeah once I get started there's no stopping me

I'm thinking about some breakers now, just to avoid too many accidents.. probably a few of https://www.ebay.co.uk/itm/155047696936 inline with my 20mm cable connects.

I had thought about rotary isolators but that just what I'm used to. Not that far ahead yet! They look fine though.

[Joeboy]

Caesium, you are truly running with this! I'm away to have a look at your buckboard link. I plan to add add the balancing board side once I've got it all up on the wall and settling to its as delivered equilibrium. Hopefully nothing too wildly out of whack on arrival!

Yeah once I get started there's no stopping me

I'm thinking about some breakers now, just to avoid too many accidents.. probably a few of https://www.ebay.co.uk/itm/155047696936 inline with my 20mm cable connects.

I had thought about rotary isolators but that just what I'm used to. Not that far ahead yet! They look fine though.

As it's also a thermal trip switch and depending on where you fit them, are you not exposing yourself to a potential massive voltage imbalance if one switch trips?

I would be looking for a double pole rotary isolator to switch off both legs at the same instant to isolate the LF280k bank completely, leaving the Pylontechs doing their thing.

How and if I'd need to have further rotary isolators on each bank I'm not sure. There shouldn't be any maintenance and I can't actually kill power at battery level so likely I'd just go with the one big on/off switch to isolate the 15 LF280's from the Pylontechs. I am hoping to have a good look at Nowtys setup when I get back to a big screen. His ideas always have merit.