use small cells to bulid battery

[wertert]

Hi

Looking around at the various battery building FAQs most people seem to use LG MG1 or equiv 18650 which are about 65mm x 18mm in size.

I was wondering if anyone knew of a smaller cell that would be suitable to build a 36v battery for an ebike conversion. I'm probably mad but i'm thinking about a stealth install with the battery physically located in the frame. Ideally I want to build a long thin battery about 32mm wide. Can this be done ??

Thanks all.

 

[sjpt]

3x18mm should fit in a triangle into about 32mm, so you could possibly build a 3 parallel 10 series battery, somewhat over 650mm long. You need to consider heat dispersal if it is really enclosed. Also 3sp will only give you a low capacity battery with limited power delivery. Smaller cells would probably have lower capacity so you wouldn't be much better off; manufacturers have put a lot of effort into good 18650 batteries.

I don't know if it would be necessary to solder the 10 series batteries or whether you could get away with them just being pushed together. I'm sure there are lots of extra practicalities that others will bring up too.

 

[Nealh]

Enercig 32600 /5500mah & 16.5a, these stats alone mean it's a poor choice.

 

[Nealh]

3x18mm should fit in a triangle into about 32mm, so you could possibly build a 3 parallel 10 series battery, somewhat over 650mm long. You need to consider heat dispersal if it is really enclosed. Also 3sp will only give you a low capacity battery with limited power delivery. Smaller cells would probably have lower capacity so you wouldn't be much better off; manufacturers have put a lot of effort into good 18650 batteries.

I don't know if it would be necessary to solder the 10 series batteries or whether you could get away with them just being pushed together. I'm sure there are lots of extra practicalities that others will bring up too.

3 x 18650 will need at least 37/38mm for a tri configuration though more like 40mm or more to allow for when shrink wrapped together and wiring.
On top of that you will then need a small BMS.

 

[wertert]

Hi All

Thanks for the replies. Yes 3 side-by-side together won't fit and I only have 450mm of length to play with :-(
I was thinking more narrow battery ? - same voltage but lower mah ? Doesn't exist I think.

 

[KirstinS]

Hi All

Thanks for the replies. Yes 3 side-by-side together won't fit and I only have 450mm of length to play with :-(
I was thinking more narrow battery ? - same voltage but lower mah ? Doesn't exist I think.

You might find lipo options. But they come with some serious safety disadvantages and , theses days, don't have the energy density advantages they had a few years back. 18650 have really come along and lipo doesn't really make sense

Unless you have really specific needs such as tiny battery with a high drain in a odd shape. And are prepared to take all the safety disadvantages that go with

 

[vfr400]

You can put a 20 cell battery in one of those tool bags that go under the saddle if you want stealth or get a bottle battery. There are a few nice bottle batteries on Aliexpress.

 

[wertert]

Thanks again all for the advice. I think it's back to the drawing board. The bike I'm working on is a 2019 Brompton M6L folder and i've always felt the main horizontal tube would be large enough for a battery ( permanently installed ). The outside diameter is ~45mm and approx 45cm of space inside. I reckon I could fit 10 18650 cells in there. I would need to make up 2 'tubes' of 5 cells and thread them in one at a time because of the restricted entrance ( clamping mechanism gets in the way). These tubes would have to be able to deal with the slight bend in the tube about a 3rd of the way in.

The original idea of this was to just help up the last 3km of hill near my house which is really nasty at the end of the day. I wanted to go for a stealth install with a simple battery, hub motor, controller & peddle sensor config. No screen/controls etc.

 

[Nealh]

Any battery inserted will need added protection and rigidity to prevent rubbing and shorting of cells so clearance has to be considered, flexing/movement will lead to cells becoming disconnected from series connections.

 

[vfr400]

That's right. It's never a good idea to put lithium cells in a metal container. Ideally, you'd want to put them in a plastic or fibreglass tube because heatshrink soon wears through if there's any vibration, and each cell is at a different voltage. At least the fire would be contained in your metal frame!

 

[wertert]

That's right. It's never a good idea to put lithium cells in a metal container. Ideally, you'd want to put them in a plastic or fibreglass tube because heatshrink soon wears through if there's any vibration, and each cell is at a different voltage. At least the fire would be contained in your metal frame!

ok I was thinking each 5 cell 'tube' would be shrink-wrapped with cables running up the side of the tube ( inside the shrink-wrap. So the heat-shrink wouldn't be suitable ? The 5 cells in a tube would also simply be butted up against each other. If the heat-shrink broke down the cells would lose connection.

 

[Nealh]

Cells need mechanical contact and not just physical contact to prevent intermittent contact.

 

[Woosh]

The 5 cells in a tube would also simply be butted up against each other.

the high current between one cell to the next inside the tube cannot be sustained by just simple contact.
The metal (nickel) will eventually get passivated and increase the junction's resistance.
You need to weld one cell to the next.

 

[wertert]

is that possible with cells in a tube design ? How about spot welding a section of nickel onto each end of the cells, then welding the sections together. Once cool bend them flat. wrap the lot.

I'll attach an awful picture.

 

[Woosh]

some people do that but you would need the tabs to have round corners not to damage the insulation.

 

[Nealh]

Spot welding is necessary.
Your idea is correct but not the right way unless you want them to short out. The side of the can is a v- under the thin shrink wrap, shrink wrap easily melts so doesn't offer much insulation.
Lay 5 cells next to each other v- to v + /top and tailing them, spot the nickel across each pair of contacts then simply bend them so cells end up as a longer sausage.
You have to ensure nickel doesn't protrude and use an insulator as always on the v+ of each cell.
Creasing the correct cut piece of nickel first will help later on.

 

[anotherkiwi]

The problem with 0.15 mm nickel strip is current carrying capacity. https://endless-sphere.com/forums/viewtopic.php?t=68005

1P10S you will want to use cells with real 15 Amp constant discharge so you would be better using copper strips (0.15mm or 0.2mm x 5 mm). You could get away with using thicker nickel strip as you can see from the table but copper bends easier from memory and is optimal not acceptable.

nkon sells cells with nickel strip pre-welded so a low tech solution would be to solder those nickel strips together. My Sony laptop battery is made that way BUT!!! a laptop battery isn't being asked for 15 Amps of current...

 

[Nealh]

Spot welding copper 0.1 or 0.2mm is near impossible for the diyer though it appears that you can use spot nickel over thin copper and the process works like a sandwich method. I haven't tried it out as I have no 0.1mm copper, ES member rojitor in Spain has tried it using his Korean mini welder and has managed to spot weld a ni/cu/ni sandwich as well as a cu/ni/cu sandwich but copper only no go.

 

[vfr400]

Copper oxidises in air. It might be a good conductor, but if you want to use it for a battery, you need to protect it somehow, like nickel plating or tin/lead plating. I guess sealing it with heatshrink would work, but it has to be properly sealed against air getting in.

 

[anotherkiwi]

I think potting it in silicone paste might do the trick as well as the usual shrink coating.

IIRC from chemistry (or was that physics?) in high school, welding copper and nickel together will cause galvanic corrosion at the spot welds too.