Did it seem like a Ground Hog type thing where tomorrow never seemed to happenTomorrow, what size cells builds a better battery and what needs to be done to prepare the cells for use as a house battery.
The capacity requirement of the battery isn't the only thing that dictates the size of the cells used, more often it's the available space. For house battery use, the smallest practical sized cells you can use, as long as they are commonly available, the better.... within limits. A 200Ah or 400Ah battery is far easier to get along with if built from 100Ah cells, than if it was built from 200Ah or 400Ah cells, yet it would not be practical to build a 1,000Ah battery from 100Ah cells, but it would be better to use 200Ah cells.
Why? Why not use the cells that match the capacity you want, 200Ah cells for a 200Ah battery, 700Ah cells for a 700Ah battery etc........ It all comes down to averages, accessibility and mass production. But first an explanation of what's happening inside the cell.
Each capacity cell is made up of a lot of plates of lithium ferrous yttrium phosphate coated aluminium plate and graphite coated copper plates with a separator between them, each plate is less than a mm thick and the coating sin micros and sprayed on to the plates. All this is soaked in an electrolyte of salts and lithium and other stuff that you really don't need to know about, maybe a table spoon in each cells, not ltrs of the stuff like in you car battery.
An ion exchange through the separator from the lithium compound to the graphite compound stores the charge, the ions return to the lithium compound out of the graphite as that stored charge is released. The ions in the electrolyte act as an exchange, one ion leaves the graphite on one side of the separator and an ion close the surface of the lithium compound enters it, the ion doesn't have to travel all the way from one plate to the other, just one in and one out, that is how they can accept and release the electrical energy so fast. This electrolyte is a very limited amount so the plates need to be very close to each other so there is enough electrolyte to cover all of each plate. I've already mentioned about how laying the cell on it large flat side allows the electrolyte to drain from the upper cells, there are other precautions that also need to be taken to ensure the plates have enough electrolyte in close contact, but I'll save that for a later chapter.
So now you know roughly how the plates are arranged, now, each of these paper thin plates are linked to the copper or aluminium block that you see on the top of the cell. They just bolt them on, 2 little bolts through all the paper thin copper plates at one end and all the aluminium plates at the other end.
Now we get to the mass production part... hope you are still awake.
Each of these plates are made by the millions, they would all be similar, yet not exactly the same. Depending on the combination of those plates on the upper end of the std and those on the lower end of the std as to the exact capacity of that particular cell. The thickness of the material sprayed onto the plates effects how quickly they can absorb the ions, the thicker the material the longer it takes to get the ion in or out of the deepest part of the material.
Now for the averages part.
if you look at the cells as either side of the std design as a number it might help to explain this bit. If one cell is say plus 1 so 201Ah capacity and the next cell is minus 1 so 199Ah capacity, then these 2 cells connected in series, positive to neg, then the usable capacity of this battery is limited to the cell with only 199Ah capacity, but if cell 1 was 101Ah capacity and cell 2 was 99Ah capacity and these 2 cells were joined in parallel, positive to positive and negative to negative, you would have a cell with 200Ah capacity. What does that matter, these cells only really change voltage at the full state or very empty state, if one cell is even 1Ah less capacity than the next in series then it will be full before the others and it's voltage will rapidly climb to a point where damage occurs, yet the voltage on the other cell that has 1 more Ah capacity is still much lower, it isn't full yet. The same thing happens at the empty end, the cell with 1 Ah less is empty before the others so it's voltage will drop low enough to be damaged while the other cells still have plenty of energy to feed the load.
Next look at the possible difference in resistance within the cells, one high and one low cell joined together in parallel will balance each other out, but on their own in series they can't share out the differences so the voltage while charging will be higher in the high resistance cell yet lower while under load the same cell voltage will be lower. So averaging out the differences between the cells by linking 2 or more together in parallel means a far better balanced battery.
Now for the accessibility part...... tomorrow, I can hear the snoring in the background from here
T1 Terry
