Looking to build a 3s4p 18650 pack for ebike... need help

[DerekG]

I originally started this in the ebike forum, buried in another thread, however, it has to do more with batteries and as such really belongs here (plus it got lost rather quickly in the other forum)

I have a 36 volt battery pack for my bike right now that I'm really happy with, however, once a week for around 2 miles I need a higher top speed than I can currently achieve (even with pedal assist).

To do this I plan on adding 12 additional volts to my system by wiring the old and new pack in series for a total of 48 volts nominal.

I've been getting lucky opening "dead" laptop packs over the past week when I snagged a few from a friend and to date I have 14 cells which have been tested/ charged with another 12 or so charging now (well 2 at a time but I have 12 that need to be charged)

However, I need a way to connect these cells together in 3s 4p or 4s4p pack to add the volts I want (probably end up with 3s4p because 4s4p hot off charger with other pack is about 61 volts which is too close to the cap ratings).

I was thinking of using 4 lengths of pvc cut to the correct length (3 cells) with a spring in each endcap and having all the endcaps wired together to produce the volts I need at 9aH. The cells I'm using are high quality Sony ones rated at 2C continuous and 5C for 30 seconds, plus my controller is only 22 amps so I should be fine there.

The only thing is that I don't have the time to put this together right now... which is where hopefully one of you comes in

I'm willing to pay for parts, labor, shipping (within reason obviously) for someone to assemble a 3s4P holder for me. The PVC idea was only a thought although it would be easy to grab the materials and cheap. I need to focus on my undergrad work here, however, if anyone has some free time / wouldn't mind making a little extra money please let me know if the idea is even feasible

 

[tostino]

Lmao funny you should mention a 18650 pack. I am just finishing up my 2nd today.

Mine is a little larger scale though, at 7s14p.

I solder my cells, and am very careful about heat. My first pack lost no capacity from soldering them what so ever, and I am expecting the same with my 2nd one.

I'd be willing to build your pack if you want.

Here is a few pictures of what I did today at the bottom of this thread: http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=13447#p202575

I don't really trust a holder to make a good connection 100% of the time.

 

[DerekG]

First off thanks for the response

Here is where I am right now... I'm looking at 34 18650s which all hold a charge at 4.2 volts and appear to have reasonably similar capacities (from 2.2 to 2.4 aH stock)

I was hoping I could setup a cheap holder which would justify keeping these cells vs buying a new 12 volt pack in the future, however, for my idea to work I would need this project to come in under the price of a 10aH 12 volt lithium setup.

The reason I don't want to solder here is b/c I don't have a reliable way of measuring capacity/ internal resistance. If you could do this and then solder a 3s4p pack then that would be ideal as I would charge each parallel set individually to keep the charge current low and keep everything safe.

If you do solder a pack then I would need andersons on the terminating ends so I could easily hook up to my existing setup in series. Do you think you could also do this... the terminating wires with the andersons wouldn't need to be more than 1.5feet or so.

Also, what would I do... is it even legal to send 30 18650's via UPS or fedex b/c of the lithium?

Maybe it would be better if I just setup everything here / bought a new pack. I do need the extra voltage, however, I don't want to start something which is ultimately going to be more trouble than it's worth