40Ah to 60Ah 20S 20P 18650 pack......thoughts?

[Spacey]

So I pondering building a rather large 18650 pack for a little fun go kart idea I have for shits and giggles.

Looking to push between 4,000w and 7,000w with 20 Parallel and 20 Series 18650 packs. Any thoughts on the cells I should use.

I do like the nice steady volt drop of the LG 18650's but have been looking into the Samsung and Panasonic cells as well, would really like better than 500 cycles.

 

[mistercrash]

Sony US18650NC1 2900mah, 58Ah at 20P and from the spec sheet, good for a little more than 3C before breaking a sweat. Or Sony US18650VTC4 2100mah, 42Ah at 20P but a lot of punch since they show to deliver 2000mah at something like a 14C discharge. Or maybe I don't know what I'm saying.

 

[Wheazel]

I would look into powertool cells for such a build if the plan is to get many cycles.

 

[Nuts&Volts]

From the research I've done I think the Sony cells offer the best cycle life. The NC1 (230wh/kg, 3C), V3 (193wh/kg, 5C), or VTC4 (176 wh/kg, 14C) are your best bets. I've also been impressed by the Samsung, but the cycle life doesn't seem to be quite as good as the Sony cells. I've done various capacity test on cells, but no cycle test. I've only seen data from spec sheets on cycle life

 

[Hillhater]

So I pondering building a rather large 18650 pack for a little fun go kart idea I have for shits and giggles.

Looking to push between 4,000w and 7,000w with 20 Parallel and 20 Series 18650 packs. Any thoughts on the cells I should use..

Thats going to be a very expensive pack with 400 of any decent 18650...$2k + at least before you start assembly.
.. a lot of $$$'s just for S & G's ??
You will need to take extra care of connections, insulation and vibrations, since a kart is very rough on everything on it.
Lots of connections to worry about
Also do you need a 40+Ahr (3-4kWhr) pack, or is that just to get the Amps for the motor ?
A pack that big will keep you on track much longer than your body can hold up !
..and recharge time ???

It would be much simpler, and more compact to use HK Nanotech lipo.
20s 2p of 5Ahr, 5s, 90c packs would easy do the job for half the cost and weight.
But shorter run time and pack life !

 

[ecross]

.

 

[circuit]

Let's see what we get. Use LG C2 cells as example (because they are cheap). Declared capacity is 2800mAh, real will be like 2600. So them we get:
20p*20s = 20*2.6*20*3.7=3848 Wh / 20kg/44lbs
If you will be pulling 4-7kW, that is only 1-2C. LG cells can handle 1.5C long term just fine (and 2.5C short peaks). And even 2C if you are OK with elevated temp.

BTW, if you have calculated "4kW" based on motor amps*battery voltage, then your real consumption will be almost twice less.

 

[mistercrash]

In a go-kart with no suspension other than the tires, vibration will be a problem like mentioned earlier by Hillhater. Spacers/holders for 18650 cells would help construct a strong pack, there's some on ebay that are quite cheap and they are very well made. They keep the cells well aligned, well supported and leave a 1 mm gap for ventilation and to keep the cells from rubbing against each other. On top of the holders, you will need a sturdy box or enclosure to hold all those cells together tightly, with some sort of cushioning inside the enclosure. Any wires for series connections or balance tabs should be routed neatly and supported every couple inches, again to cope with vibration. You don't want wires rubbing against each other. Hundreds of soldered joints may not be up to the task either, in a go-kart, I would think it is better to spot weld using thin Nickel strips as bus bars, they should allow more movement compared to solder joints and hold up better to vibrations. If you have to solder maybe use tinned copper braid instead of a solid copper bus bar. Also consider breaking up the pack in four 5S20P batteries, I'm thinking that four lighter bricks will cope better with vibrations and shock then a huge, stiff and heavy single brick. It would be easier to service also and very easy to balance charge when needed using a single BC168 charger. Just some thoughts I had, hope it helps.

 

[Spacey]

Sorry for the lack of input on my part guys...have been away from a computer for a couple of days.

Thank you so much for all your thoughts, always good to bang heads with like minded people, here is where I am at with this:

1: Have just bought and received the 600WS Capacitor Discharge Welder PCB's and a huge 7 Faraday capacitor with which to build an awesome dual pulse resistance welder.

2: Ordered a load of 18650 cell holders

3: Getting ready to pull the plug on about $1,600 of 18650 cells...yikes.

This cart will be a full suspension with big balloon tires which will be used on the vast (vast for UK anyway...back yard size for other countries) army practise plains. Will be powering it with a Crystalyte TC65 Massive motor I have kicking around unused and hoping to power it with an Alltrax controller for 7 to 8k peak power.

Range is important but not as important as not killing the cells with heat and excessive strain from power, 2.4Ah to 2.8Ah would be nice from each cell. Any higher capacity would sacrifice some output ability.....but this is why I was thinking about the 20 cell Parallel set up to ease the strain on the cells.

The pack will be built into 4.2v 20P module packs for easy disassembly but still all probably spot welded together....looking forward to building this welder. Will get the proper cell tabs rather than just plain nickel strips.
Pics of welder kit to follow at some point today.

Thanks again for all your thoughts on the subject

 

[Punx0r]

Looking forward to seeing this project built

 

[Allex]

Spacey
This is one is an interesting cell
http://endless-sphere.com/forums/viewtopic.php?f=14&t=56854

 

[circuit]

Maybe you will be interested in this assembly method?

http://endless-sphere.com/forums/viewtopic.php?f=14&t=55870