Building a battery with 18650 cells

[Arbol]

I want to build my own battery, with a specific geometry (quasi triangular, to fit inside a standard bike frame) and capacity & discharge rate requirements.

AFAIK, only em3ev "officially" builds quasi triangular batteries. I believe Schwibsi, a member here, can do that, too. But I want to build my own.

I have been asking around who could spot weld in Barcelona, but I cannot find anybody. Even, an organization devoted to e-vehicles, they have told me this is a recurrent issue, but nobody really knows how to do it.

A member of this forum, cwah, kindly suggested he could maybe do that. Definitely I want to explore how this could go. But I have found another way:

supowerbattery111 sells packages of cells already spot-welded, either in series on in parallel. For example, if I wanted a 10s5p of these nice Sony VTC5, I could ask "brics" in 10s or 5p, I have been told. Then, connecting these brics is to be done with soldering, not spot welding, which I assume I could get it done locally.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=221396528935&ssPageName=ADME:X:RTQ:US:1123

Question:

The "normal" thing would be to have 10 brics of 5p, in order to connect them in series to build a 10s5p, right? It would be as if I had 10 cells, and each cell would be connected in series, and also connected to the BMS. Is that right?

 

[KirstinS]

I bought a 7.4v booster pack from these seller about a year ago

The cells were supposed to be Panasonic cgr18650ch

They appeared to be so as well. All I can say is they packed up in a couple of weeks. They were treated well and had a lipo alarm etc

I suspect they were fakes

Since then I got into harvesting free cells from laptop batteire

These have outperformed supposedly superior panasonics every-time


Maybe I was just unlucky but worth letting you know my experience


The seller wasn't helpful in resolving the issue and just ignored me after a while

 

[KirstinS]

Btw I solder my packs

Not as neat by miles though !

 

[Arbol]

Thanks, KirstinS, having direct experience is invaluable!

When you say that you solder your packs, may I ask how do you do that? Do you solder instead of spot welding? I have heard this may damage the cells.

Neatness does not matter, what matters is efficiency and security.

 

[KirstinS]

That's right - I solder the cells. Quite often I'm soldering pairs of 18650 which still have spot welded strips on them - easier than soldering directly

My understanding is that prolonged heat exposure damages them. A higher wattage soldering iron reaches temperature much faster and so you need to hold it to the cell for less time - the heat doesn't go though the whole cell but is on/off quickly

I use loads of solder and thick , high amp rated wire. My controller is 17amp only

That's my understanding at least but I'm no expert - enthusiastic amateur at best

All my cells are free so I don't need to worry so much about famage

Laptop cells have a low c rating so I have to put 6 in parallel to help mitigate this. They are not as good as high drain cells. You can get them from old drill packs, like matika but they are low ah rated - maybe 1500mah


If cwah has offered to spot weld it for you then I'd take him up on it unless postal costs make it unviable