3. Battery Pack

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Battery Pack

The batteries store the energy for powering the motor. We recommend Lithium Iron Phosphate (LiFePO4) chemistry cells. The more cells in line, the bigger change you will have for a fault in the cycle. For now, 32 cells seems a good optimum. So, for most cars we recommend 96V battery packs, comprised of 32 cells in series.

Note: The information below the picture is essential when you are deciding about the capacity you'll need.

Making an educated guess for the optimum capacity of the battery pack


When you want to use a battery 3000 times (some claim to be able to be used 5000 times), we advise you, in common use, to unload untill 70% of the capacity. Dependable on some factors you’ll need 100-200 Wh to drive 1 km.

 

 

A few estimates should be made to calculate the educated guess for the number of Ah you’ll need:

1. How many kilometers is your route most driven? Take this amount and equate this as 70% from the needed autonomy (distance between 2 charges). Of course it’s not only good for your batteries to not unload them too much, it is also good to know you have some extra capacity when you meet a road block and have to drive an extra 5 km;

2. Estimate how your car will perform on the spectrum between 100 and 200 Wh, depending on:
- the weight of your car (the heavier, the higher the value will be);
- the topspeed you want to drive (driving 70 km/h is much less energy consuming than driving 100 km/h);
- the air resistance; and
- the rolling resistance.

3. Estimate the amount of energy needed for heating.

An example

You want to drive every day up and down to work with a 2CV, this will be 2 x 40 km. The route will partly go by highway and mostly there are traffic jams. This will take you 30 minutes each way. In wintertime of course you’ll need heating.
Assumption: you use a 32 cells 96V battery pack.

ad 1) The autonomy will be 80/70*100 = about 115 km.

ad 2) Experience with the 2CVolt learned us a 2CV with an easygoing driver uses about 110 Wh per km. 

ad 3) We have mounted an electric heater of 2000 W. In wintertime this will operate for 2 x 30 min.
The amount of energy you’ll need is: 115 km x 110 Wh/km + 2000 Wh = 14.650 Wh
Think about Ohm’s law: 14.650 Wh / 96 V (pack voltage, 32 cells) = +/- 153 Ah.
So the (Thundersky) battery of 160 Ah is the best choice. In this example it would be a waste of money to pick a 200Ah battery.
Note: the ‘commercial’ autonomy of the 160 Ah battery in this example is about 154 km, because in these calculations the assumptions include a total unload of the batteries, a low speed, no heater, etc.

Placing the batteries

It is advisable to keep all the cells in one physical location if at all possible. Cable resistances, temperature and moisture differences in different locations of the car can result in unforseen problems and shortened battery life. This has nothing to do with the BMS, it is about design and common sense. Also placing batteries in car crumple zones SIGNIFICANTLY COMPROMISES SAFETY in the event of an accident. Remember that batteries are the fuel tanks of an EV and due consideration should be given to locating them, even at the expense of load space. Note: It is also important to maintain weight distribution between the front and rear axles of a car.


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