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MattsAwesomeStuff

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can you mix different size batteries? I would like to put 24 100 amp cells in rear pack and12 180 amp in the front pack I am doing a 63 Beetle with an AC50.
any reason why this combination will not work?
I don't think you're thinking about it correctly. So, let me slow down and explain the basics a bit.

1 - You will be chaining cells together in series, to create a higher voltage. That is, if one cells is 4.2v max, and you want an 84v pack, you'll need 20 cells in series to add up to that voltage.

2 - If you want to create multiple packs, that's okay too, but their end voltages have to match else when you connect them one pack will instantly dump all it's power into overcharging the other, blowing it up (voltages in parallel will equalize).

3 - Of any series chain of cells, the current (amps) that flow through them when discharging will be the same. That is, if you're drawing a 100 amp load through your 20 cells in series, every single cell will be discharging at a rate of 100 amps, regardless of the size or capacity of each cell.

4 - Let's say you start with 20 identical cells. If you swap out, say, 2 of those 20 cells and replace them with larger capacity cells, the voltage won't change, but when the other 18 cells are empty, those 2 will still have extra energy left in them that you can never get to, because all cells in series charge and discharge at the same rate.

5 - Same as above, but say you replace 2 with smaller capacity cells. When those 2 cells are empty, the other 18 will all still have energy that you can never get to.

6 - As you discharge cells, their voltage drops. The more depleted they get, the more their voltage drops. When the voltage reaches a minimum allowable level for that cell, you should stop discharging them or you will damage them. This is when your "tank" is empty. If you have 20 identical cells, they will all reach empty at the same time, that's perfect, no wasted energy and no damage. For example if you have 20 cells that are at 4.2v max when full, and should stop being discharged below 3.0v each, that pack will be an 84v pack when charged and only 60v when empty.

7 - Most controllers will be programmed/configured (by you or factory) to stop discharging below a certain voltage level, to not damage your pack. (In the example above, at 60v). This is the total voltage of the entire pack. Fancier controllers will be monitoring the voltage of each and every cell, or will communicate with a BMS board that does. These controllers will know if one cell drops too low, and regardless of the voltages of the other cells, will shut down/limp/flag a warning/etc.

8 - If you don't have one of those fancy controllers, and you have mixed cell capacities, your controller may operate in a way different than you expect and this may cause damage. For example, if you have half your cells at one capacity, and half of them at double that capacity, when the smaller cells are empty, the bigger cells will only be half empty. Suppose 10 cells are at 3.0v, but the other 10 cells are still at 3.6 volts. That's 10x 3.0 = 30v for the small ones, and 10x 3.6 = 36v for the big ones, 30+36 = 66v total. So if your controller is expecting 60v minimum, and your pack is at 66v, it's going to let you continue discharging when it will damage your cells. When it reaches 60v, your smaller cells are going to be over-discharged, and your bigger cells will still have energy left in them.

...

In your case you have 24 cells at 100ah. And 12 cells at 180ah.

- You can chain them all together in series to make a 36-cell pack, but with the above limitations and complications.

- You cannot join 24 in series to make one pack, and then connect the other 12 in series to make a second pack and then join the two. They will have different voltages and will instantly be ruined or explode, even though the capacity of each pack is pretty close (one is 2x as many cells at 1/2 the capacity).

- You could make 3 packs. 12 small, 12 more small, and 12 large, and then join all these together... but you'd join them in parallel and make a very beefy 12-cell pack, not a 36-cell pack. That's only going to be like, 42 volts, which I'm guessing is way too low for what you want.

In short, if you had to kludge it, you could, but the extra capacity of the big cells will be wasted and your controller will have to be told a custom cutoff amount.

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