Pack planning: range, efficiency, cable, and fuse

Do the pack on paper before you buy cells. The site already has the arithmetic. This note is the order of operations, not a second calculator.

1. Efficiency, then range

Range is usable energy divided by consumption. Consumption is Wh/mile at the speed you actually drive, plus auxiliaries.

The range calculator takes nameplate kWh, Wh/mile, aux watts, and speed. It treats 90% of nameplate as usable, adds aux_W / speed_mph to Wh/mile, then estimates highway a bit worse and city a bit better. That 90% is a planning allowance, not a chemistry law. BMS reserves, cold, and age will eat more. Do not plan a trip to 0%.

The toolbox hint is the right starting bracket: about 250–350 Wh/mile for cars, 400–600 for trucks. A brick of a classic sits toward the high end of “car.” Highway at 70+ will not match a 45 mph number. If you do not have logged Wh/mile yet, pick a conservative number and move on. You can refine it after the first instrumented drive.

2. Capacity from the range you want

The battery calculator inverts the same idea: miles × Wh/mile, then applies a margin and the same 90% usable assumption. Read both outputs. “Minimum required” is the raw trip energy. “Target capacity” is what you should be shopping toward.

Margin is not optional. You will lose capacity to temperature, to not running the bottom of the SOC window, and to the fact that the first estimate of Wh/mile is always optimistic.

The calculator will also list cell-count sketches from the existing component table. Those rows are already in the toolbox. This article is not going to invent new cell prices or add parts. If a row looks stale, treat it as a starting geometry (series/parallel, mass), not a quote.

3. Current from power and pack voltage

Conductors and fuses care about current, not kWh.

I ≈ P / V on the DC bus. Same motor kW at 96 V is a very different cable than at 350 V. Peak current (acceleration, grade) is what the short run from pack to controller has to survive. Continuous current is what the cable and fuse have to live with without cooking.

Get a motor power ballpark from the motor calculator if you do not already have a controller current limit. Then size the DC cable for the current you will actually command, not the brochure peak unless you plan to use it.

4. Cable, then fuse

Fuse protects the cable. Cable has to be able to carry the current. Do those in that mental order even if you click the tabs the other way.

The cable calculator uses ampacity and voltage drop for a given length. Short pack-to-controller runs still need the ampacity. Long runs (rear pack, front motor) fail on drop as often as on heat. The tool’s “suitable” flag is a voltage-drop check, not a blessing to undersize.

The fuse calculator aims for roughly 125–150% of continuous current, and it will not recommend a fuse above the cable’s ampacity. That is the point: a fuse larger than the cable is decoration. Pack fuses live as close to the battery as you can put them. The cable downstream is what they are saving.

ATO/ATC in the tool are ~32 V blade fuses. ANL is a low-voltage class only (≤ 80 V DC). Above that it asks for a DC-rated Class T / aR, not a part number. If 125% of current is already above the cable’s table ampacity, it will not invent a fuse — it says the cable is too small. Use a DC rating at or above the pack. DC is not AC. Do not grab a random AC breaker and call it a traction fuse.

A sane sequence

1. Range — pick Wh/mile and a range that is honest for how the car will be used.
2. Battery — turn that into a target kWh with margin.
3. Motor — if the donor still needs a drivetrain size.
4. Compute pack current from power and voltage.
5. Cable and fuse — current and length, then protection that matches the conductor.
6. Weight if the donor is light and the pack is not.
7. Charge time once you know kWh and the onboard charger. Inlet behavior is a separate note: pilot / PWM and why a random J1772 board may be the wrong side.

If the numbers only work by assuming 180 Wh/mile in a cabin-on highway commuter, the pack is too small. Change the pack or change the mission. Do not change the physics in the spreadsheet.