21700 10 Ah Battery Build: Peer Review

[carlbikekits]

Hi,

I have completed a 10 Ah battery and, as well as my own learning, would appreciate thoughts and improvement suggestions relating to safety, performance and electrical maths.

Key points:
10S2P using branded 5mAh cells
Daly 20A BMS Waterproof Common Port
30A Internal Fuse
0.15mm nickel strips
17A 200W Cables
XT90 connector
7A KT controller


BMS wiring *will need to lengthen wiring to house fuse outside heatshrink battery wrap



Spot welds of 0.15mm nickel using SEQURE SQ-SW2 welder *ok but could upgrade welder*



BMS soldering



Covered all points with heat resistant tape



Hot glued BMS. *Far too much length on BMS wiring. Thinking will cut to size?



Surrounded with battery insulation paper



Wrapped in heatshrink and will house in LIPO bag.
* Thinking upgrade to 14A controller with XT connectors

Thank you for your time

 

[Nealh]

SIngle nickel strip 0.15mm x ? ( 8 mm?) might get a warm if drawing constant power so one would lose watts power to heat wastage form the battery.
0.15mm Ni x 8 = 1.2mm2 , typically 1mm2 Ni is approx. 4.6 ampacity optimally though one will find 7a to be acceptable.
For your 14a max controller you are on the limit for ampacity, one won't gain more by using a controller with more current draw. If the cells were capable of more current draw one would have to double up the Ni Buss or use 0.08mm copper beneath the Ni Buss.

Besides being 21700 cells which brand /model spec ?

 

[Nealh]

Looks ok to me , cell end insulators used for short protection and cell holders used for mechanical strength are std procedure for a safe build.
Nice to see the off take wiring presoldered to the nickel buss before spot welding, saves unwarranted added heat to the cells which can cause internal damage.

Not really a criticism but for future builds it is worth cutting H strip buss (or spot weld a small strip of Ni so that it forms a 90 degree bend over the side of the cells) the sense wire solder can then be along the side of the battery rather then the wire having to bend over the top edge possibly causing a chaffe or short.
One can use a bit or barley paper or capton tape under the right angle bit of nickel for insulation.

 

[Nealh]

The only real criticism from me is some of the spot welds on the negative can are near to dead centre , this should be avoided as the cathode attachment of a cell is typically at the centre of the bottom of a can. Heat damage/penetration is a risk to the cathode attachment.
One is better cutting the H strip so it is a proper H cut and the ends reach all the way over to the cell wrap, then one can spot weld at the edge away frorm the central area.
The positive end matters not as the raised cap sits on three or four legs and the anode is attached to the sides.

 

[Nealh]

Ni ampacity is approx. 4.6a per 1mm2 for optimal current flow or 7a for acceptable current flow.
Acceptable current flow is that which may incur some warming of the buss leading to some performance loss from the battery, some watts are lost to heat
For mm2 it is material thickness x material width. Mutliply the mm2 x 4.6 or 7 to get a rating .

Cu Copper ampacity is some 3.5x greater so 1mm2 optimal is approx. 16a and acceptable is 25a.
Though with my copper I use a full width single piece copper (of a pprox 30mm) for my builds, 0.08cu x 30mm = 2.4mm2 so approx. 38a capable.
For strength I use cheap Ni plated steel over the copper to prevent the copper tearing.

One is better to over spec the buss ampacity rating then under rate it.
So if you have to think about thicker Ni or doubling the Ni buss, it is a no brainer just to buy thin copper and cheap Ni plated H Buss..

 

[Nealh]

Craft knife snap blades are pretty similar to material to the tin/steel uses in the cell cans, so test your spot weld strength on the blades . That way one can see how well the spot weld adheres by the plier rip off test and also one can look on the opposite blade face to see if any burn thru occurs , this would indicate to much power and that will be transfered in to the cell can.

 

[Nealh]

So adding paint to your battery pic.
The added Grey to the LHS of the Buss is where one should cut the buss shape to so it forms a H, this then allows one to spot weld well away from the center of the cells negative end.
On the RHS either cut the Buss H strip so an extended right angle leg bends over the edge (approx .1cm -1.5cm and solder the sense wire indcated in Red on the side Or spot weld a seperate right angle piece over the Buss. All of the right angle legs can sit on the same side of the cell , just use a whole peice of the barley paper (for insulation) then bend the tabs over.

 

[Nealh]

Your welder specs look fairly reasonable and the fact one has adjustment over power in put means it may well be Cu capable with Ni plate capable with the sadwich method . Worth a practice as it can do 65ms , one may be able to do 0.08 copper with a 0.1mm or 0.15mm Ni plate on top at about 40 - 50ms power.
Ni plate offers less resistance then pure Ni and is the reason why cell cans are made of steel/tin and why it is good to use over thin copper.

Having the manual foot control is also a good feature and allows one to place and adjust the probes position before welding , both the Malelectrics and K-weld offer this feature as well.

 

[Nealh]

The Daly common port appears to be better then the seperate port Daly's for reliability.
Should you at some stage find no power then likely if not the fuse , the BMS has failed.

 

[carlbikekits]

This is great feedback. Thank you.

I'm going to look more into the copper and nickel thing. Any reason not to use pure nickel (rather than coated), other than price?

Many thanks

 

[Nealh]

Over all a very good first effort Carl and just a few tweeks for the next build as we all do once we scrutinise that which can be done a bit better or nicer.

The only reason for using Ni plate over pure Ni is the resistance, with Ni plate one needs less current/ power for the weld to take and may get a better result.
The less current/power needed for a weld means less heat /energy to the cell, plus less stress/warming up of the welder mosfets.

 

[Nealh]

Have read of this thread Carl.
"Copper/nickel sandwich" buses for series connections | Endless Sphere DIY EV Forum (endless-sphere.com)

In #150 it shows why insulator washers are a very valuable item to use , if in that post they weren't used then a fire could well have been started.

One will also see with a lot of the builds there are quite few that don't use cell separation as the shapes don't allow it, but one assumes the better builds will use barley paper between opposing groups.

 

[carlbikekits]

Have read of this thread Carl.
"Copper/nickel sandwich" buses for series connections | Endless Sphere DIY EV Forum (endless-sphere.com)

Thanks. Had a read. A lot to get my head around there.

In your 21700 build
- are you completing a single weld of both copper and NP combined? Or a weld of copper plus a weld of NP?
- you've cut copper squares rather than strips/H? All fine?

Thank you again

 

[Nealh]

The copper & NP is one single weld , it is harder to do copper on it's own so the NP facilitates the process.
It is far simpler to have a one piece Buss then fiddly bits all over the place that look untidy, the NP H strip is one piece and both make for a neater job.

The NP isn't be used for ampacity/current flow , it is purely adding rigidity /strength & means to facilitate an easier weld wth less resistance.

 

[carlbikekits]

SIngle nickel strip 0.15mm x ? ( 8 mm?) might get a warm if drawing constant power so one would lose watts power to heat wastage form the battery.

Besides being 21700 cells which brand /model spec ?

These are LG M50LT 21700 with 5Ah and max discharge 10A

I've attempted the copper/ nickel plated sandwich and will need a more expensive welder. Also, have read through the forum thread (thanks) and it seems that, even with a KWeld, many are having hit and miss with the strength of the weld (my main concern).

So, I'm wondering if 0.15 nickel is just fine for my 10S2P 10Ah? I don't know the maths of it all. But I get good welds and my battery is working so far.

Also, my controller is continuous 7A but max 14A. So assuming that's sufficient?

Any sticky thread on the maths of battery making would be appreciated. Thanks

 

[carlbikekits]

I've been testing the battery on my commute. Range and reliability are my two main needs.

Running through the miles so far with over 30 miles and half battery remaining. This is on 4 as standard and 5 on hills. I'm one who doesn't see the point in using pedal power if you've got a battery. So I want to test properly on real world usage.

I chose LCD3 as I anecdotally believed that this would tell me some power info that my trusty little LCD4 won't. I'm yet to see it so am I missing something? I can see the watts drawn (typically 220W and higher at start) and that it is 36V. I was hoping it would tell me something else, say how efficient the battery build is?

Sandwich method aside, am I safe with my 0.15 nickel 10S2P 10A little build?

Thank you

 

[saneagle]

Short press of the power button tells you the battery voltage.

Can I ask which welder you've been using and was there any special technique, and did it work OK.

You only need copper for high current. With 2P and a 14A controller, that's only 7 amps per cell maximum. You have a double path from each cell pair, so each strip will carry a max of 7 amps. That isn't very much, and your controller would overheat if it ran at 14A continuous anyway. Your actual current rarely gets to 14A, as you can see by the watts on your LCD, so I reckon that a single strip of 0.15 nickel would be enough.

Obviously, if you decided to use the battery on a different application with a more powerful controller, you should think again.

If you want to be a pedant, here's a chart that shows what to use:

https://endless-sphere.com/sphere/attachments/ampacity-powestream-extrapolation-jpg.248217/

 

[carlbikekits]

Short press of the power button tells you the battery voltage.

Can I ask which welder you've been using and was there any special technique, and did it work OK.

Sure, SEQURE SQ-SW2 welder. Just worked fine.

 

[lenny]

 

[Solarbake]

This is my new 21700 battery built for my swytch gen 3 which I tapped into the speed controller so any 36V Battery can be utilized .
I used the samsung 50 E from fogstar in a 10s2p Format. Whole pack cost me less than a £100.
The powers great and so is the range.They are rated at 10A discharge .Heres a few pic of my pack.
Fogstar by the way are great genuine cells and next day delivery just make sure you buy from the wholesale website.Also I can run on two Makita LXT 5amp/h battery's if I want with the adpater