4Springs' Brumby

Post up a thread for your EV. Progress pics, description and assorted alliteration
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Re: 4Springs' Brumby

Post by francisco.shi »

This is looking good.
Are you making the BMS? I assume you are because it bolts to the end so you would have had to make the board to match the hole spacing.

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Re: 4Springs' Brumby

Post by 4Springs »

francisco.shi wrote:
Wed, 01 Jan 2020, 05:51
Are you making the BMS? I assume you are because it bolts to the end so you would have had to make the board to match the hole spacing.
Yes, this is the second version of this BMS that I've made this year. The other one was for a home storage battery.
That hole spacing is very tight! Looks like I'll have to trim the tab on each cell. The tabs look like they're lined up, but fractions of a mm difference mean that the tabs get chewed up by the bolts.

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Re: 4Springs' Brumby

Post by francisco.shi »

The spacing was a bit tight. I didn't want to end up with a weird spacing.
The screws are generally less than 5mm so they should just clear.
What I found was that getting the tabs to line up perfectly was the problem.
What I did to get around that problem was to make a plastic U shape with the inside of the U at 45mm. Then clamp the cells while the tabs are held by the U. Once I did that it solved the problem.

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Re: 4Springs' Brumby

Post by 4Springs »

I'm just starting to assemble these modules.
I'm used to LFP cells, which have a tinned copper cathode, and an aluminium anode. I polish these immediately before assembly to remove any oxide coating.
I went to polish one of these aluminium tabs, and it got duller instead of more shiny. Perhaps these aluminium anodes are tinned? If so it will make assembly much easier, just give them a wipe down with metho.

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Re: 4Springs' Brumby

Post by francisco.shi »

You just need to polish them to remoce the oxide layer and quickly apply some electrical contact grease and clamp them. That is what I did and it seemed fine the contact resistance was as low as the contact on the copper side.

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Re: 4Springs' Brumby

Post by jonescg »

Yes they will probably be nickel plated so won't need much preparation. The contact grease is always a good move.
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Re: 4Springs' Brumby

Post by francisco.shi »

Only the copper tabs are nickel plated. I had a lot of trouble with contact overheating on the aluminium tabs until I removed the oxide and applied contact grease.

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Re: 4Springs' Brumby

Post by 4Springs »

francisco.shi wrote:
Thu, 16 Jan 2020, 09:20
Only the copper tabs are nickel plated. I had a lot of trouble with contact overheating on the aluminium tabs until I removed the oxide and applied contact grease.
Ah, thanks for the clarification. I'd started assembling without polishing.
IMG_0364.jpg
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This stuff really does look like a plating. If you found it better to remove it though, then that's what I'll do. All 200 cells worth (double sided).

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Re: 4Springs' Brumby

Post by francisco.shi »

I think you are being too aggressive. I think I used some 600 sand paper and did it by hand. Only a few strokes and then quickly applied the contact grease. Then they could sit for a few min while I did the other tabs. From memory I think I folded the sandpaper and did both sides of the tab at the same time.

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Re: 4Springs' Brumby

Post by T1 Terry »

A 3M scotchbrite type pad https://www.ebay.com.au/itm/31Pcs-2-Rol ... SwU1hc20GI and some kero does a great job of cleaning the cell terminals and links. I prefer to use the thin copper links with the hump in the middle because it allows for some movement between the cells without loosening the bolts. I use Alminox to stop the oxidization build up returning and it seems to be good for 3 to 4 yrs before the whole process needs to be repeated.

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Re: 4Springs' Brumby

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So I read Francisco's comment and tested 2000 grit sandpaper. It takes off a bit of the layer, but not the whole lot:
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This was very similar to the scourer that I started out with:
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So I switched back to the scourer. The things in T1 Terry's link look quite similar, but onna stick. The scourer is what I'm using for the aluminium interconnectors. Then clean them off with metho and apply terminal grease. Then wipe 'off' the terminal grease because I really don't want much on there.
I've assembled 15 of my 200 cells in a day Good thing we bought another car to use while the Brumby is out of action!

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Re: 4Springs' Brumby

Post by 4Springs »

Finally finished a battery module!
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So this is 30 cells, roughly 22 V, 7 kW, 30 kg.
5 cells in parallel, 6 in series.

The main terminals are at the top. Other things you can see are:
  • Black wires going to red/black Anderson connectors connect to the heaters. There are three heaters sandwiched between the cells, connected in series, each with 3 ohms of resistance wire. Connected to a 20V supply (such as a laptop power supply), this provides about 50 W of heat into the cells.
  • Tan-colouored Teflon sheets between some cells. I sandwiched the heaters between two of these Teflon sheets, to provide some heat-resistant electrical insulation. But I noted that where they came out over the terminals at the end, they provided some level of whoops-proofing against shorting terminals when doing up the bolts. So I inserted some even when there was no heater there.
  • Blue/Black wires are BMS communication connections. These run from one BMS board to the next, daisychaining through the six boards.
  • Copper-coloured wires with red terminals are from the BMS (mounted on the negative end) to the positive end of the cell. The two wires are one for current and one for voltage sense.
  • Perspex cover over each end, held on with Velcro. Not exactly IP rated, but enough to reduce the likelihood of accidentally shorting out on something.
The threaded rod means that I can compress the cells somewhat. Nothing like the 15 psi recommended by the link that Francisco gave - that would be 1000 kg worth of compression! But fairly compressed.

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Re: 4Springs' Brumby

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One down 7 to go?
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Re: 4Springs' Brumby

Post by T1 Terry »

4Springs wrote:
Thu, 16 Jan 2020, 17:14
So I read Francisco's comment and tested 2000 grit sandpaper. It takes off a bit of the layer, but not the whole lot:
IMG_0366.JPG

This was very similar to the scourer that I started out with:
IMG_0367.JPG

So I switched back to the scourer. The things in T1 Terry's link look quite similar, but onna stick. The scourer is what I'm using for the aluminium interconnectors. Then clean them off with metho and apply terminal grease. Then wipe 'off' the terminal grease because I really don't want much on there.
I've assembled 15 of my 200 cells in a day Good thing we bought another car to use while the Brumby is out of action!
The stick bit is to put in the drill, then you can polish the plates in a wash tub or in a container part way full of kero or metho, or just dip the pad in the kero/metho every so often to wash away the oxide build up and add a bit more fluid to wash away the rubbish as you polish. The scourer pad is probably ok if you don't have a lot to do each time, but I can't imagine how my arms would feel after doing the number we have to do on each install :lol:

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Re: 4Springs' Brumby

Post by 4Springs »

I have been planning (and doing) something that I really should run past the educated folk here.
I've made up heaters for my cells consisting of resistance wire. So like Nichrome, but a different metal. The idea is that I run wire up and back in a pattern, then sandwich this between cells. I'll hook this up to a power source (probably 20 V laptop power supplies), to heat up the wire between the cells.
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I've been assembling the cell modules with this wire sandwiched between. Actually it is a cell, then a sheet of Teflon, then the wire, then another sheet of Teflon, then another cell. The cells are softish, and so the wire makes a small indentation on the surface. I've assumed that this will not be any kind of problem, but should ask before I go too much further! If it was going to be a problem then I'd use a sheet of aluminium between the wire and the cells. This would be that much more assembly, and that much more thickness to the module.

When I put the current on, the wire heats up to perhaps 30-40°C. The intention is to heat them from the mains, so only heated when charging or sitting, not while discharging. I've been putting three of these heaters in every module (modules are 20 or 30 cells).

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Re: 4Springs' Brumby

Post by jonescg »

I'd be reluctant to intentionally deform the surface of the cells. Is there not a flat tape version of the same stuff?
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Re: 4Springs' Brumby

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jonescg wrote:
Thu, 30 Jan 2020, 08:30
I'd be reluctant to intentionally deform the surface of the cells. Is there not a flat tape version of the same stuff?
Haven't found any, but that doesn't mean much.

I've dismantled a module that has been assembled compressed for 5 days.
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I can feel the lines with my fingertips.
The wire is 0.6 mm diameter. I've put the joins (to regular wire) inside the zone that is compressed between cells, to provide strain relief. These are about 2 mm in diameter. Wire + solder + heatshrink. I could move the joins outside of this space, in which case each of the cells should have indentations of about 0.3 mm.

I have some 0.8 mm aluminium sheet that I could put either side of the PTFE sheet. These would stop the indentations, and would give some thermal conductivity which would be nice to spread the heat out. But they would add about 15 mm to the module height: 0.8 + 0.8 + 2 mm x 3 heaters = 10.8 mm, plus a bit because bits always happen. Would be a bit less if I moved those joins out: 0.8 + 0.8 + 0.6 mm x 3 = 6.6 mm, or perhaps 9 mm.

I also have some plastic sheet - the stuff that the cells were packed in. I'm guessing PET. I think this would be as effective at stopping the indentations, and it is much thinner (too thin for my vernier rule to measure), easier to cut and is otherwise a waste material. But thermally insulating, which means that the heat will not transfer into the cells as well. I've just looked up the melting point of PET, and Google reckons 260°C (if indeed this is PET). Way higher than the 30-40°C that my Nichrome is anticipated to reach.

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Re: 4Springs' Brumby

Post by francisco.shi »

Could you use a heater element from a bag sealer?
They are a thin strip about the thickness of the tape. I would expect the thickness to be thin enough that you will not mark the cells.

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Re: 4Springs' Brumby

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francisco.shi wrote:
Thu, 30 Jan 2020, 14:06
Could you use a heater element from a bag sealer?
They are a thin strip about the thickness of the tape. I would expect the thickness to be thin enough that you will not mark the cells.
They are pretty thin, I hadn't thought of them. Even though I got the idea of the PTFE sheet from my experience with bag sealers. They'd need to be the correct resistance, and my initial thought is that they would be quite expensive.
- goes off to Google, finds that a 300 mm one is about $15 -
Yes, this would be expensive. Would need probably 3 per row, so 72 of them all up.

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Re: 4Springs' Brumby

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Ok, so I've done an experiment.
It looks like I should put something between my Nichrome and my cells. The two options are plastic or aluminium sheet.
Plastic is thinner, cheaper, an electrical insulator, easier to cut and less likely to damage the cells. But it is a relatively poor conductor of heat. How bad a conductor of heat really? We're talking pretty thin stuff.

I set up two cell sandwiches.
Plastic sandwich (on the right in the picture) is:
Cell, Plastic Sheet, Teflon Sheet, Nichrome Heater, Teflon Sheet, Plastic Sheet, Cell.
Aluminium sandwich is:
Cell, Aluminium Sheet, Teflon Sheet, Nichrome Heater, Teflon Sheet, Aluminium Sheet, Cell.
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I put a thermocouple in the middle of each Nichrome heater, and I have a non-contact thermometer to measure the surface temperature of the top cell.
IMG_0388.JPG
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Both elements are 3.5 Ohms. I've set them up in series, with the power supply set to 12 V and 3 A.

In theory, the heat generated from the Nichrome wire in the plastic sandwich will have a harder time getting out to the cells. So the surface of the cell will be cooler, and the internals warmer than the aluminium sandwich.

Here are my results:
I had a control cell (on the left in the picture), it was 27.4°C at time 6, and at time 22 it was 28.3°C. It had a bit of dappled sun for the final few minutes, unlike the others. That was one reason to stop the experiment when I did, since the sun was starting to come through the window.

Al = Aluminium
PET = Plastic
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The results were not compelling.
In general the plastic sandwich behaved as expected, but the difference was not great. The internals were generally warmer and the external surface cooler.

Something that was decidedly different than the final setup is the level of compression. There were air gaps in this setup that will not be in the compressed module.
I've set up for a repeat, but with a brick on each one. Not quite the same, but gives more compression than the weight of the single cell.

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Re: 4Springs' Brumby

Post by wovenrovings »

I wondered if you could use something like the carbon based paint used in this video to paint resistance heaters on to the cells.
https://www.youtube.com/watch?v=df58TiEhFn0

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Re: 4Springs' Brumby

Post by T1 Terry »

I'd go with the PET sheet because it gives you electrical isolation as well. The aim is the keep the cells above 0*C when charging and I believe the PET sheet will not conduct the cold from outside the way the aluminium would and therefore provide a more even heat.

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Re: 4Springs' Brumby

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T1 Terry wrote:
Fri, 07 Feb 2020, 12:11
I'd go with the PET sheet because it gives you electrical isolation as well. The aim is the keep the cells above 0*C when charging and I believe the PET sheet will not conduct the cold from outside the way the aluminium would and therefore provide a more even heat.
I've gone with the PET - mainly because it is easier than cutting up aluminium. I had the PTFE sheet for electrical insulation, but I've taken that out now I'm using PET. Reduces the size of the battery module by a few more microns.
The datasheet for these cells calls anything below 10°C an 'excursion' condition. Normal operation is between 10° and 45°C. Without heating they wouldn't get above 10°C during the night time for about 6 months of the year here. This might still be the case even with heating, but it should take a bit of the chill off.

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