advice on evaluating lithium cells for ev battery pack use

[captainjack]

could I get some advice on evaluating lithium cells for use in building an ev battery pack...what are the key parameters one must know in order to select the best cell for use in an ev pack, or to know if a cell is suitable for use under ev battery pack conditions? I am considering an 18650-20 2000mah cell manufactured by samsung...this is a lithium ion cell, maximum discharge current listed at 2CmA(4,000mA)...is lithium ion not preferred because of cost, thermal instability or ????
my goal is a 144 volt 100amp hour pack.
thanksaq

[MalcolmB]

Hi Jack

It depends partly on what you plan to do with the cells. If you're looking at spec sheets you'll want to be sure that the cells can deliver the current you need. Look for the C rate. This tells you what current the cells can deliver as a multiple of the cell capacity.

For example, if a 2 Ah cell has a continuous discharge rate of 5C it can deliver 10 amps continuously. The spec sheet should also tell you how fast the cells can be charged, again often in the form of a C value.

If you're talking about testing cells then you need to measure the cell resistance, although spec sheets will often give this as well. The lower the cell resistance the higher the current a cell can deliver without overheating.

Those cells you mention sound rather small. You would need around 2500 of them for the pack you quote. That's a lot of connections.

There are three main types of lithium ion around: lithium cobalt, lithium manganese and lithium iron (not ion). The last one is the one that most EV builders seem to be choosing, mainly because it's safer than the other two and should be less of a problem environmentally. Having said that, quite a few members here use the Thundersky lithium cobalt cells with good results.

What have you got planned?

[captainjack]

malcolm:

thanks for you reply...I am a real electric newbie. These cells were made by samsung, as a battery pack, 16+volts, 8amphour power source, with a bms in the hard case...don't have any specifics on the bms circuit....from lurking on the electric scooter and other forums, I understand that bms circuits can inhibit high amp discharges...going back to a very basic question that has been in my mind,...ev motors draw massive amps...clearly people have made battery packs in ev applications using small 3.6 volt cells, connected in series to deliver sufficient voltage, and these strings then in parallel for sufficient amphour capacity...my question is this...is a string of 3.6 volt cells, whether it is 2 cells or 50 cells in length, limited in the amps it can discharge to the discharge rate of the individual cell, or is amps deliverable in series additive like voltage? what is an optimum discharge rate for an individual cell in an ev battery pack?..again forgive me if this is a ridiculous question.

[MalcolmB]

Not daft questions at all, although it would help if you could give an idea of the application you're thinking about. The pack you mention sounds as if it's for an ebike.

If you want to make large packs out of small cells it's usual to connect cells in parallel first. Say for example you're using 10Ah 3V cells, you might connect 10 in parallel. This effectively gives you a single 100Ah 3V cell that can deliver ten times as much current. By connecting in parallel first you simplify battery management, since you effectively have fewer cells to monitor.
You would then connect as many of these larger cells in series as you need to supply the voltage you want, so for a 144V pack you would connect 48 in series.

For practical reasons it makes sense to choose cells with an Amp hour capacity that is close to the final capacity you want, so in the case above you would choose 100Ah cells if you could find them (they do exist). This reduces the number of connections needed and so reduces the risk of faults. Likely reasons for choosing smaller cells are that they let you tailor the shape of the pack to fit the available space, or because the cells you want are only available in small sizes, such as A123 cells.

The optimum discharge rate for any cell is the lowest possible. The faster you drain a cell the lower its real-life capacity and the fewer useful cycles you get out of it. Having said that, some cells can handle higher discharge rates much more comfortably than others. Usually the manufacturer will specify the maximum continuous discharge rate as a C value. This varies widely from chemistry to chemistry and from maker to maker. Taking a 10Ah cell as an example, a "cheap" cell might have a continuous rating of 0.5C (often written C/2), in other words 5 Amps. A top of the range cell could have a continuous rating of 10C, in other words 100A.

Again you should match the continuous discharge rate to your application, so it depends largely on the size of the pack you can afford and your required maximum current.

[captainjack]

my project is a 1965 series IIA land rover...using a GE motor, zilla controller...the issue has been deciding on a battery pack...I am not wild about welding all the battery boxes in for lead, and still have limited range...I am planning on a 144volt pack, would like to commute, need at least a 50 mile range and be able to keep up with traffic.

the samsung cells are only rated at 2C max discharge,,,4amps...how does the "C" discharge rate of a cell relate to a pack composed of multiple cells performance?...is there a minimum "C" rate of an individual cell below which it is not suitable for use in constructing a pack?

[GregsGarage]

There isn't a minimum C rate as such, it will depend on the ah capacity of your pack. The ah capacity required will be determined by the range you require. For example lets say that a 100ah pack would give you 25 miles range (note: I am just making up numbers to illustrate a point. I will leave voltage out of this for simplicity, but assume the same voltage for all examples).

The zilla can handle 1000 amps so your cells would have to be rated for 10C discharge (100ah x 10 = 1000 amps). It doesn't matter if you use a single 100ah cell or 100 1ah cells, the cells have to be rated for 10C.

If you wanted more range and built a 500ah pack for 125 miles range, you now only require cells that can handle 2C (500ah x 2 = 1000 amps). Again it does not matter how you get to the 500ah capacity, the individual cells must be rated for 2C

Try and get the largest cells you can to make building your pack simpler.

Hope that helps,

Greg

[captainjack]

greg:

thanks for the great insight...and thanks to malcom too...your assistance will help the ev world grow...

[captainjack]

greg:

thanks for the great insight...and thanks to malcom too...your assistance will help the ev world grow...

[aminorjourney]

Just for information.

I have a Lithium pack for sale right now, with included BMS.

14 * 100AH 3.7V Thundersky LCP100s Four cells are almost brand new. two will need replacement shortly (about £120 per cell). The other six are okay at the moment, but do have a fairly high internal resistance.


£900

Nikki.