It's good that you're doing research. I wouldn't go planning or spending money on anything yet, as many things might not work out the way you're first thinking.
One thing you're mixing up is power and energy. Power is the flow rate of energy. Energy is an amount. If you think about energy as water in a bucket, if you have 5 gallons of water, that's an amount, that's energy. If you have a spigot on the bucket, so that when you open it you're pouring out 1 gallon per minute, that's power, the flow rate of energy, or, how fast you're going through the amount of energy you've stored.
Things that use energy use them at a certain rate (depending on what they're required to do). A 100 watt light bulb has 100 watts of power flowing. If you had a battery with enough energy to run that light bulb for 1 hour, you would have 100 watt-hours of energy. If you had a battery big enough to run that bulb for 10 hours, it would be 1,000 watt-hours of energy.
Batteries are measured in watt-hours (or kilowatt-hours). It's how much energy they can store.
Motors and devices are measured (among other things) in the max amount of watts (not watt-hours) they can handle without overheating. Note that that's max. If you're crawling in a parking lot, you might be using 400 watts, not 400,000 watts like you would be at max acceleration.
Watts are ( Volts * Amps ). Sometimes you will see batteries rated in amp-hours instead of watt-hours. That's because for the same chemistry, each cell, regardless of size will have the same voltage and only differ in amp-hour capacity. To find out watt-hours from amp-hours just multiply it by the battery's nominal voltage.
First figure out your budget.
Then figure out some basic costs for different ways of doing things.
Then when you start thinking about specifics, start taking measurements and seeing if you can actually fit the components you want into the places that you want.
When you have most of that planned, then maybe start buying components.
...
On batteries, first, there is no free lunch. You do not increase capacity by arranging them differently.
Batteries in series will have their voltages add up, but their amp-hours stay the same.
Batteries in parallel will have their amp-hours add up, but their voltage stay the same.
Supposing all your batteries are the same size, you chain up enough in series to reach the target voltage you want. Typically you want this as high as your other components will allow (motor and controller), and then you buy your charger last to suite the voltage you'll be using.
Then, if you want to double (triple, etc) the size of your pack, you'd have more cells in parallel.
Most people would connect their cells in however many parallel they want, and then connect those groups in series to other groups.
For those buying large-format cells, you might only have one string of cells in series. You'd just buy single cells the right size and be done.
On something like a Tesla that uses many smaller cells, they'll have 30-some cells in parallel, and then connect many 30-some cell bundles in series with each other to reach the target voltage.
If you only have a certain bundle of cells (you salvaged a vehicle or that was the bulk amount someone was selling as a set), sometimes you might go to a slightly lower voltage just so you don't have a bunch of extra cells you can't use. I.E. If you had 112 cells, and you wanted 40 in series for ~160 volts, you could only use 80 cells (40 pairs in series = 80, and 32 cells left over you can't use). But you could put only 37 in series you'd have nearly the same voltage (148v), but now you'd be using 111 cells (37x3) and only 1 left over. That means you get to use the energy of 111 cells instead of 80, even though the voltage is a little lower.
A BMS monitors the battery voltages of each parallel group of cells. So if you have 40 groups of cells in series, it will have 40 wires going to those intersections (well, 39 intersections, plus 2 more on the ends). If for example, a few cells got disconnected or died in the middle of the pack, you need to know that so you can replace them and not reverse charge or overcharge them.
When energy flows out of the pack, it does so at an equal amp rate from each cell in series. When energy flows back in when charging, ditto. But over time, some with lower capacity (just happens with age, temperature, etc) will fill up before their neighbors and the charger wouldn't know that and would overfill them and underfill others (it just knows "160v", it doesn't know exactly how that gets bucked up inside the pack). A BMS would alert you to that and stop early.
Some (most) BMSs will do cell balancing for you too. So if a cell reaches it's max voltage before its neighbors, the BMS bleeds that extra voltage off as heat, so that the others can continue to fill. This doesn't help the capacities equalize, a weak or damaged cell is still weak or damaged, but it does help the problem from becoming worse from being over charged or overdischarged (which will harm that cell's capacity further). Sometimes this is during charging, sometimes this is passive in the background. Usually a balancer isn't anywhere near powerful enough to keep pace with the charger, but it only corrects imbalances, which, if it's left on the pack all the time, should never become extreme.
You don't *need* a BMS, you can function just fine without one, guys ran whole seasons without them and then just manually balanced every couple months (charge pack in bulk, then top up any weak cells one at a time). But BMS are so cheap and easy to wire, if you're buying Tesla components it would be a foolish place to cut corners for a beginner.
thank you for any information you can give and it will be greatly appreciated.
