But how many batteries in parallel do you need? It is crucial that you understand the Amp-draw of your system, or you will discharge your batteries too hard and permanently ruin them - on a hard acceleration in your car, or on a rapid charger.
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Note: Only adding batteries in parallel increases your maximum power. Adding them in series increases the available energy, (like, how far you can go), but not the power. You need power for hard accelerations or rapid charging.
You need to find out the Amp-draw or kW requirements of your load (motor in our example). Then you need to find out how much you can safely draw from your supply (batteries typically).
With a bit of napkin-maths, you can then quickly find out what you need to do.
Example: 100kW Electric car motor acceleration Let's try to figure out the Amp draw your batteries can support. You have a 100kW motor, lets allow 15% extra for inefficiency, so it draws 115kW. We know that we have a 400V car and that it is a 96s3p configuration (pouch cells).
You know that: Watts = Amps x Volts
So lets divide the 115kW by 400V to get Amps again, and then divide by our three pouch cells sharing the load in parallel.
Great: 115,000W / 400V / 3P = 96A (rounded)
96 Amps is what your individual pouch cell can support. Quite a lot actually.
In this estimate you are relying that the manufacturer did their homework - which they most certainly did, otherwise people would be returning their batteries left-right and centre. Now turns out this example was the Nissan Leaf setup. If you want to be able to draw more power from these cells, you need to put more of them in parallel.
Here you have worked backwards from an existing setup, assuming someone else did the hard work for you.
But if you build a system from scratch yourself, you need to find the maximum Amp draw your cells can support (If you don't know what the battery C-Rate is, take a look). Leave some room for error, maybe 20%, and work from the bottom up instead:
Example: How many 8000mAh 18650 2C batteries do you need in parallel for 100kW?
8000mAh = 8Ah, You found in the battery datasheet that they are 2C batteries.
8Ah x 2C = 16A is the absolute maximum you can draw per cell. Let's not go near that, and leave 20% room for error, so 13A per cell
Assuming you have the same motor as before, and you have a 400V car, you put 96 cells in series to get the voltage (to run within the manufacturers specification).
Similar to the first example: 115,000W / 400V = 285A (for the whole pack)
Ok: 285A / 13A = 22 cells in parallel.
That means you end up with a 96s22p setup to replace the pouch cells with cylindrical cells.
Thats also, 96 x 22 = 2112 connections! And you can't solder to the cells without precautions. Also don't forget to add a Battery Management System