MS16 Cell Selection

Max Energy Capacity <= [FILL IN AFTER REG DROP]

Our maximum energy capacity is limited by ASC2026 regulations. We want our pack configuration to allow our energy capacity to be as close to this number as possible (if not exactly on this number). [1]

Max Voltage < 165V

Constraint based on motor controller’s max cont. bus voltage [2]. Higher voltage is always better because we want to minimize current to reduce power loss from high current (P=I^2R).

The max voltage is more-so a constraint on our series count. It’ll stay at 36 (37 is a prime number, 38 has 1, 2, 19, 38 as factors, so neither are very good for us when splitting the pack into modules)

Minimize Weight

The battery is a big contributor to the car’s weight. The lighter it is, the more efficient our car will run. Additionally, a lighter pack is easier to remove, making serviceability and overall quality of life better.

Minimizing weight relates to the cell’s gravimetric energy density and size (larger sizes are often more energy dense than smaller size, though this is not at all always the case).

Minimize Volume

The battery takes up a considerable amount of space. As we aim for a slimmer car with MS16, minimizing volume is something to keep in mind.

Minimizing volume relates to the cell’s volumetric energy density and size (larger sizes often have higher mAh capacity and so will allow us to use less cells for the same amount of energy → less volume taken up).

Minimize Heat Generated

One area of improvement (or deletion) for the MS16 battery system is the removal of forced air cooling using fans. Minimizing heat generated from the pack may allow us to either move to passive cooling (cooling with no fans) or have no cooling at all. This can allow us to simplify design and also have less parts.

Minimizing heat generated relates to the cell’s DC internal resistance and mAh capacity (because lower cell capacity means higher parallel count without decreasing series count → current draw is split across more cells → P=I^2R, less heat generated).

Manufacturer

We’re going to be buying hundreds of cells. In keeping with our goal of making MS16’s battery pack reliable, it would serve us better to buy cells from manufacturers that are already well-known (LG, Samsung, Molicel, Panasonic, Sony, Sanyo).

Size (eg. 18650, 21700, 26500)

The physical size of the battery determines the volume of the pack. Larger sizes also correlate positively with higher gravimetric and volumetric energy density (not always true). For MS16 we will be sticking to cylindrical cells.

Chemistry

Different chemistries have different properties. For MS16 we will be sticking to Li-Ion NMC/NCA cells. Good reading for common Li-Ion chemistries: BU-205: Types of Lithium-ion

Open

Gravimetric (Wh/kg)/Volumetric Energy Density (Wh/L)

Gravimetric energy density shows how much energy a cell can hold per unit mass (usually Wh/kg). Volumetric energy density shows how much energy a cell can hold per unit volume (usually Wh/L).

We generally want to maximize this or at least keep it reasonably high such that it doesn’t affect the car’s performance (too heavy) or design (batteries take too much space) to any large extent.

Capacity (Ah)

There isn’t much wiggle room for the pack’s series count due to the max voltage constraint given by the motor controller, we’re also keeping the motor the same. The parallel count will depend on the cell capacity.

Note: Most manufacturers will give their nominal capacity at a 0.2C rating, which is standard. It would be ideal to also look at the cell’s capacity at higher discharge currents/C-ratings and see how close it remains to the nominal capacity.

DCIR/ACIR (mOhms)

DCIR and ACIR technically measure different things, DCIR measures internal resistance and ACIR measures internal impedance [3]. DCIR is more useful to us since it’s more reflective of the actual use case of the cell under DC load, but ACIR is generally more standardized and is often what you’ll see in datasheets [4]. The two values do correlate though, so if there aren’t any DCIR values of the cell available from the datasheet or the internet, the ACIR still gives a good ballpark idea. [5]

DCIR matters to us because the higher the internal resistance the more power will be lost to heat. This means more energy in the cell is wasted and the cell will get hotter easily.

Max Charge/Discharge (A)

Max charge isn’t incredibly important because the only charging source for the pack is from the solar array, and it won’t be supplying very much current. [6]

Max discharge current will be what determines our cell’s max power output capability. The max power for our motor is 5000W [7], but we’d benefit more from power consumption vs. speed driving test data from MS15 [I’ll include it here after we get that data].