The battery calculator (found here) is a great tool to help you imagine, plan and create the battery pack for your next electric vehicle. First I'll cover just a bit of battery configuration review, then I'll move onto the calculator explanation, using an example.
As an oversimplification; higher pack voltage leads to increased top speed, while higher amp hours effects torque and range.
Linking cells together in series (connection + to -) adds the Voltage together, while the Amp Hours remain constant.
Linking cells together in parallel (+ to + and - to -) adds the Amp Hour rating, while keeping the Voltage constant.
All the LiFePO4 cells we carry are rated for 3.2 volts, whereas the different Amp Hour ratings denote the price. For this reason most packs are made of many cells linked together in series because the voltage for a single cell is very small.
The first thing you will see is the question of which variable you want to calculate. By default it is marked on Range, but by and large I use it to calculate Pack Voltage. So if you're following along the example go ahead and switch the variable to calculate Pack Voltage.
The first box on the top left is the vehicle weight. After gasoline extraction and electric replacement, the vehicle usually weigh in at the same ballpark. So I use the original curb weight of the vehicle to make estimations. We'll say I have a 1995 Civic DX. Curb weight is 2,450lbs. This step is pretty simple.
The next box down from that is the Desired Range. Simply enter in how many miles you need to drive into this box. I need to drive 35 miles for my daily commute.
After Desired Range, we see the Pack Voltage box. The max voltage has a great deal to do with maximum speed, but we won't cover that now. For now, you'll need to know that each cell carries 3.2 volts, and therefore the pack voltage will tell you how many cells you will need. Since this is the variable I'm calculating, I'll leave the pack votlage box empty.
The last of the four boxes on the left is the Battery Amp Hour (Ah) Rating. This is where you start to realistically analize your project. Batteries are available in many different Ah ratings, so you will want to experiment quite a bit with this variable. I'll shoot for 100Ah cells in my Civic.
Now I'm ready to hit the calculate button. It shows that to get a 35.53 range, I will need a pack voltage of 108.8, using 100Ah cells. So how many is cells is that? Since each cells has 3.2 volts, I simply take my pack voltage and divide that by 3.2. This tells me I need 34 of the 100Ah cells to get the performance I need.
Now you're ready to start calculating your own project! Jump to the calculator found here. If you have any questions or recommendations about this guide, feel free to email firstname.lastname@example.org
1. Don't feel limited by the Ah selections available. They can be "cutomized" using dilligent math. By linking cells together in series, and in parallel, you can get closer to the Ah rating you want. Example: If I need a 140 Ah pack, I will need to link one 100Ah cell to one 40Ah cell in parallel, this will give me a 3.2v 140Ah cell. By carefully duplicating this again and again, I can get an entire battery pack with 140Ah.
2. The progression of cells to range is not linear. A 2500lb car may go 35 miles with 35 cells, but it doesn't mean that it will go 350 miles with 350 cells. The battery calculator is not designed to chase the added weight of extra cells. Since a 350 cell pack will add quite a bit of extra weight, manual weight calculations should be done, then considered for the calculator input.
3. This tool is a rough estimator, not an absolute determinant. Many other factors (wind, incline, aerodynamics, air in the tires, gear ratio, wheel diameter etc. etc.) contribute to the actual performance of the vehicle.
4. If you're making a small car with very little storage space for batteries, you may hit this limit first. After calculating a battery pack, check out the dimensions of the actual cells, design for a while, and determine if this pack can actually fit into your vehicle.