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[Doug S]

+1 to being extremely careful. Either in parallel or in series, there are pitfalls that can bite you very fast. In series, the weakest cell always runs out of power first, its cell voltage drops through zero volts, then it actually winds up being REVERSE CHARGED by the current being pushed through it by the stronger cells; there's no quicker way to kill a battery and even make it catch on fire. In parallel, if one of the batteries fails short (whisker formation between anode and cathode is a pretty common way for a battery to fail), it can wind up drawing large currents from all the other batteries it's paralleled with, which again can cause a lot of heat and eventual pain for the owner.

Proper battery handling with a well-designed BMS is really one of the secret sauces the manufacturers of EVs have created. Unless you think you can do as good a job as the guys at Zero, Brammo or Tesla, let them handle that end...or at least see if you can find a commercial BMS that suits your usage. Remember that an 11kWh battery pack has as much energy in it as 1/3 gallon of gasoline, and it is possible to get them to release it all at once.

[Kocho]

Yup, not a good idea, generally speaking. But it's not the capacity difference, it's other factors that matter most. Best to work with good, known quality, matched cells (other than perhaps for quick prototyping on the cheap and with great caution).

Kocho is right you can use cells in series that are different capacities just as he said, but one cell will be at 3 volts when others might be at 3.6 volts, so this can be very tricky when looking at your pack voltage and  trying to divide by the number of cells.  Because that won't work in this case.  Just be careful.  This can work, but one mistake can burn your whole project down and perhaps your house.

Exactly. you end up with different cell voltages.. and since they have different IR they will drift more and more.. they would have to be individually monitored to consider it safe.

[xmjsilverx]

I understand the risks of this project and I will be as careful as I can.  I am not going out and buying a zero or other bike/battery because I want to learn about this.  These batteries can be had cheaply and I do realize they are not all going to be good batteries.  That's why I am looking for proper ways to test batteries.  I am not sure on the size of the pack I am going to build yet.  I think that all depends on the voltage I need.  I guess it's not technically an EV but I am leaning towards converting my commercial lawn mower to electric.  I do appreciate all the tips.

[LeftieBiker]

   Leaf cells are cheap and easily available, but they will slowly degrade. That might not matter as much in a (riding?) mower application, if you can build in lots of extra capacity. If you go that route, look for model year '14 and higher. They changed the batteries a bit for the better in '13, but Leafs built through March of that year seem to suffer the same high degradation rate as previous generation cars.

[Ndm]

Here's an article Ted Dillard wrote on building an electric lawn mower, http://www.homepower.com/articles/vehicles/project-profiles/diy-electric-tractor

[siccycle]

Lunacycle.com is a great wholesale source for 18650 cells. Quality cells from Samsung and Panasonic. Located in USA. Great tech articles too

[Lipo423]

I understand the risks of this project and I will be as careful as I can.  I am not going out and buying a zero or other bike/battery because I want to learn about this.  These batteries can be had cheaply and I do realize they are not all going to be good batteries.  That's why I am looking for proper ways to test batteries.  I am not sure on the size of the pack I am going to build yet.  I think that all depends on the voltage I need.  I guess it's not technically an EV but I am leaning towards converting my commercial lawn mower to electric.  I do appreciate all the tips.

As long is this is 100% clear in your mind + you get the proper measuring devices you will be fine. There are a few guys out there building packs and EV, and it is a lot of fun...just put t safety first on your list (I have myself  left "risky" battery packs outside home in a proper protected area to avoid potential fire risk, and in Europe we do not own wooded made houses as much as you do in the US)