Review my latest battery build? Molicel p42a 10s4p 21700

[Moecycle]

Hey Folks,

The first battery I build finally failed, so I got to make a new one!

Decided to go with 21700 cells this time. Last time I posted my battery I got a lot of great feedback on mistakes I could improve.

So if you feel like giving me some feedback on this build, would be very happy to hear it.

The p42a cells are from Nkon.nl
BMS is a cheap Chinese unit from ebay

 

[Bikes4two]

• Looks to be a nice build - I'm no expert so I'm as keen as you are to see what others say/suggest before I get to my second build.
• Dare I ask what failed in the first build?
• And what is the thick grey padding material you've applied on top of the fish paper?

Cheers,

 

[saneagle]

Hey Folks,

The first battery I build finally failed, so I got to make a new one!

Decided to go with 21700 cells this time. Last time I posted my battery I got a lot of great feedback on mistakes I could improve.

So if you feel like giving me some feedback on this build, would be very happy to hear it.

The p42a cells are from Nkon.nl
BMS is a cheap Chinese unit from ebay

Looks fine - nice and neat. The only thing I'd have done different is the way you've attached the sense wires. I'd have run the nickel strips longer and folded them over the edge so that you could attach all the sense wires in the same flat plane so that they don't bend over the sharp ends of the nickel strips. Before bending I put a piece of that sticky-backed battery card stuff or a sheet of that thin fibreglass over the whole side as extra insulation to keep the sense wires completely separate from the cell-pack. Like you did, I always attach the sense wires to the negative ends, so that if the insulation gets compromised at the bend and it touches the case, it doesn't do any harm.

 

[StuartsProjects]

The first battery I build finally failed

Failed in what way ?

 

[Nealh]

Not a bad effort and pretty good , use of cell spacers and cell insulator rings is commendable.
Good choice of cell & brand, Moli are one of the big six cell brands to buy and before the P45B came out the P42A was one (and still is) on of the best cells to buy.

If I was to be picky then I noted the following.

I see a 30a rated BMS, if the nickel is only 0.15mm then the 4 x nickel in series aren't capable of 30a without getting hot (though you may only be taking 20a max), I would have sandwiched a 0.05/0.08mm piece of cooper strip under each bit of nickel on the series bus.

The copper wire on the first & last cell group, taking off the current in this way one is doubling the current on the 2nd to last cell , tripling it on the last but one cell then quadrupling it on the last cell as it travels along the paralell group with the discharge wire attached.
For a more even current load one could have taken the current off equally via Y leads and cut the nickel strip in between the middle two cells , then one gets an even 25% from each cell.


And thirdly agree with d8veh , regards the sense wire attaching.
Extending the parallel bus to bend 90 degrees over the side would save any chance of a wire pinch. A bit of fish paper under the 90 degree tab or some of that capton inslating tape would be ideal.

 

[AntonyC]

I read once that the spot welds are arranged around the button top to avoid a heat sensitive feature under the centre of the button. Maybe it was about older battery tech? But if that's the case the solder connections should be better off-centre too and could be done before welding.

 

[Nealh]

When I plan my off take wire arrangement for the first and last group , I solder the wiring to the nickle/copper sandwich off the battery then spot weld on to the cell ends.

One is ok on the positive end as the top often sits on three or four legs so not central but on the negative the cell is flat and it is said to avoid directly the centre point.

 

[Moecycle]

Thanks for the feedback everyone!

• Dare I ask what failed in the first build?

It wasn’t a complete failure, the battery would clip when under heavier load (250W bike). It was my first battery build so there were a lot of points of failure. I tried my best to dissect it to diagnose. I found two of my welds on cell group 9 failed. Didn’t dig much after that. It was a very rough build. No cell spacers so it was under structural strain.

• And what is the thick grey padding material you've applied on top of the fish paper?

It’s just basic foam padding to protect the battery from light impacts. https://www.ebay.co.uk/itm/261530567751?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=FeISftz6Rwi&sssrc=4429486&ssuid=zHDEdvKzSRC&var=&widget_ver=artemis&media=COPY

The only thing I'd have done different is the way you've attached the sense wires. I'd have run the nickel strips longer and folded them over the edge

Great idea! Do you happen to have a photo?

I see a 30a rated BMS, if the nickel is only 0.15mm then the 4 x nickel in series aren't capable of 30a without getting hot (though you may only be taking 20a max)

Yeah I’m only taking 20A max. There is actually a 20A fuse in there (it’s powering a street legal 250w bike). Agreed that if I was going to draw 30A I would need to rethink my busses.

For a more even current load one could have taken the current off equally via Y leads

this was definitely the most frustrating part of the build. I love the idea of the Y leads. Do you happen to have a photo of how you arrange these on a 4p?

I read once that the spot welds are arranged around the button top to avoid a heat sensitive feature under the centre of the button.

Yeah I’m pretty concerned about this. I had to apply way more heat than I wanted to attach those copper wires on the first and last group. If anything fails it will be this. Not doing that again…

I solder the wiring to the nickle/copper sandwich off the battery then spot weld on to the cell ends.

Do you have a photo of this sandwich approach? I’m thinking of a higher powered bike for my best build.

Cheers everyone

 

[saneagle]

Thanks for the feedback everyone!



It wasn’t a complete failure, the battery would clip when under heavier load (250W bike). It was my first battery build so there were a lot of points of failure. I tried my best to dissect it to diagnose. I found two of my welds on cell group 9 failed. Didn’t dig much after that. It was a very rough build. No cell spacers so it was under structural strain.



It’s just basic foam padding to protect the battery from light impacts. https://www.ebay.co.uk/itm/261530567751?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=FeISftz6Rwi&sssrc=4429486&ssuid=zHDEdvKzSRC&var=&widget_ver=artemis&media=COPY





Great idea! Do you happen to have a photo?


Yeah I’m only taking 20A max. There is actually a 20A fuse in there (it’s powering a street legal 250w bike). Agreed that if I was going to draw 30A I would need to rethink my busses.



this was definitely the most frustrating part of the build. I love the idea of the Y leads. Do you happen to have a photo of how you arrange these on a 4p?


Yeah I’m pretty concerned about this. I had to apply way more heat than I wanted to attach those copper wires on the first and last group. If anything fails it will be this. Not doing that again…


Do you have a photo of this sandwich approach? I’m thinking of a higher powered bike for my best build.

Cheers everyone

You don't really need a fuse because over-current is handled by the BMS. If you must have one, I'd go higher. You're basically protecting against a dead short so the value doesn't matter.

I've had to replace many blown 20A fuses on bikes with 15A controllers, and not because there was a short. There are many circumstances where you can get current spikes. I always replace those 20A fuses with 25A or 30A ones. There was one model of bike the shop sold where every bike blew its 20A fuse in the first couple of weeks, so I replaced them all before the bikes were sold.

 

[Nealh]

@Moecycle see #5 where I have crudely painted a copy of your pic, one can see the imposed Y leads I have placed .

For the copper rod/ bar which you have on the first and last cell group, cut your busses to size then carry out the soldering off the battery . Once done one should (if using slightly wider buss material) have plenty of buss to spot weld on to the cells.

If you want to learn a bit more, then these follwing threads on Endless sphere in the USA are worthy of reading and book marking.
Common pack design mistakes, how to avoid? | Endless Sphere DIY EV Forum (endless-sphere.com)

18650 spot welding -how to- ULTIMATE REPOSITORY | Endless Sphere DIY EV Forum (endless-sphere.com)

"Copper/nickel sandwich" buses for series connections | Endless Sphere DIY EV Forum (endless-sphere.com)





Some of the ES battery makers glue cells and don't often use much cell to cell protection so now and then we hear of disaster stories and some have literally burnt their houses /businesses down.

With the copper sandwich method one doesn't need to use nickel , Ni coated steel is better as it has less resistance when welding so one can get better weld penetration so the strength of the weld is better. Typically the cell can is only made from thin tin/steel so ni nickel is a bit cheaper and give rthe mechanical strength , for the current flow one is simply using the thin 0.05mm - 0.1mm copper for this ability and thn Ni steel is just for rigidity.

 

[AntonyC]

A couple of questions but I've only used screw top NMC cells so be gentle

The P currents are much smaller than S currents. Imagine a 10S2P build. With weld penetration being an issue why are S and double P strips used forming a #, why not a single P strip forming an H, giving a single thickness over the cell top for easier welding? Or solder copper Ps in H style then weld the S's?

Also are Y lead end terminations worth it? Suppose the copper is ring main cable near enough 13AWG like an XT60 takes, it's about 5mOhm/m. So that's 0.1mOhm cell to cell, about 1/100 of a cell's internal resistance. If the battery lead is 0.5m long it'll consume a watt at 20A, again not a problem in heating or inefficiency. (Nervously checks his sums...)

* Corrections:
Battery lead is out and back, so 2 watts. In a badly unbalanced group the P current could be as much as half the load current (!), but nickel strip or copper wire as above would cope.

 

[AntonyC]

It turns out H strips are a thing:

from here:

Introduction to battery pack design and building, Part-2

If you are new to battery-pack building, but you are also a pretty capable fabricator, this article will define some of the common materials and methods that are popular so you can decide what woul…

www.electricbike.com

 

[guerney]

Are there any cheap welders capable of welding thick pure copper strips? Or should we club together to buy Jimmy's? (If he isn't going to resume trading). What we really need, is superconducting cheddar which melts at 60 degrees Centigrade.

 

[Nealh]

Jimmy's copper capable welder is about 30k to buy, for cheap diy option then the K-Weld at about 250 euros can do copper direct 0.2mm I believe . Though one also needs a good battery with high a discharge rate 3s/12v as a power source

 

[guerney]

Jimmy's copper capable welder is about 30k to buy, for cheap diy option then the K-Weld at about 250 euros can do copper direct 0.2mm I believe . Though one also needs a good battery with high a discharge rate 3s/12v as a power source

Do we know which welder Jimmy uses specifically? Also what are the dimensions of the copper interconnects he uses? I suppose one of our members could risk a peek at their Jimmy-constructed battery, if sufficiently encouraged to be curious.

Could stiffness be added to an interconnect structure (layered) made using 0.2mm copper strips, if welded together one at a time using the K-Weld? Perhaps the first 4 layers with holes drilled through and welded together, welded to a final 5th to be welded onto the battery cells?

https://ecyclerevolution.co.uk/product/kweld-complete-kit/

(I've given up hoping for the discovery of superconducting cheddar which spontaneously evaporates without increasing volume at 60 degrees Centigrade)

 

[saneagle]

Do we know which welder Jimmy uses specifically? Also what are the dimensions of the copper interconnects he uses? I suppose one of our members could risk a peek at their Jimmy-constructed battery, if sufficiently encouraged to be curious.

Could stiffness be added to an interconnect structure (layered) made using 0.2mm copper strips, if welded together one at a time using the K-Weld? Perhaps the first 4 layers with holes drilled through and welded together, welded to a final 5th to be welded onto the battery cells?

View attachment 53613

https://ecyclerevolution.co.uk/product/kweld-complete-kit/

(I've given up hoping for the discovery of superconducting cheddar which spontaneously evaporates without increasing volume at 60 degrees Centigrade)

I have reservations about pure copper connections. Copper oxidises in air, which makes it unsuitable for many electrical applications. it needs to be coated with something to stop that, like plating, tin-lead dip, lacquer or whatever.

 

[peterjd]

Hi, I've been reading this thread with great interest since I have half a battery's worth of recovered batteries from my original battery failure (from a Swytch kit) reported some time ago. I've got all the necessary gear to do a reduced capacity rebuild but have been seduced by summer riding and my new bike. Since I now have access to 5 Bosch type lawn mower 36V batteries (4 from lawnmowers and one compatible - see previous posts), my current thinking for an Autumn build of the recovered batteries is a minimal 36V (10S,1P) lightweight backup battery along the lines of a Bosch build and without a BMS as such but a monitor of battery condition such as used by model builders which I also have. Anyone done anything like this?

 

[guerney]

Several years ago a friend (who was considering making a gigantic battery for his Mercedes van, converting to electric instead of running on home-made biofuel, made using waste oil from chip shops [smelt of chips wherever his van went]) mentioned that his friend had made a ebike battery using reclaimed cells - this was before I got my ebike so I didn't enquire for details after I was told that "It went very badly".

 

[peterjd]

Even more reason for a minimal design used occasionally and monitored constantly?

 

[StuartsProjects]

Since I now have access to 5 Bosch type lawn mower 36V batteries (4 from lawnmowers and one compatible - see previous posts), my current thinking for an Autumn build of the recovered batteries is a minimal 36V (10S,1P) lightweight backup battery along the lines of a Bosch build and without a BMS as such but a monitor of battery condition such as used by model builders which I also have. Anyone done anything like this?

There is a thread on Bosch batteries here ......

https://www.pedelecs.co.uk/forum/threads/36v-bosch-tool-battery-for-e-bike-use.45789/