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Hello this is my first time on this site and will see how this goes. I'm a high school student and have a 2002 subaru wrx impreza and want to convert it. I want to use the tesla motor from EV west (the motor is rated at 400 kwh, 275-400 volts, and 1,000 amps) and the LG chem 16 cell 2.5kw batteries. I have been researching for some time but still can’t find answers on some things. One being how to wire the batteries together. Should they be put in series or parallel or a combination of both? If both how should the be wired together? I’m also confused on how to charge the batteries once they are wired. I know very little about BMS other than they manage the batteries. What kind of BMS should I get? How do I install them? And what do they do?
And the last question is if anyone sees any problems that won’t work with this kind of build. I know some of these questions are general but it’s just I don’t know much about it is all:) thank you for any information you can give and it will be greatly appreciated.
 

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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.
 

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Thank you so much for the reply. This was super helpful but now I have more questions from what you said. I have a budget and its about 20,000 and when thinking about it it’ll be more like 25,000 but I am planning on selling the subaru parts to make up for that. And I have thought of many different ways of doing this but the tesla motor (bought through EV West) and LG chem batteries make the most sense. The LG chem will be cheaper as well instead of going with the tesla batteries. The next issue that was slightly answered is how many of these battery modules I need and how to wire them. (the battery is 2.5kwh, and rated at 60 volts, 45 amp hour) I was thinking of using 12 of them but I don’t know if I can run the big motor with that many or if I need more. I know you explained how EV batteries are wired together but I still don’t get it enough to do my own. I understand that you put them together in series to add up voltage and parallel to add up the amps. But how would I do this with my set up. Would it work if I had 6 pairs of 2 batteries in parallel and put those 6 pairs in series? That would be 360 volts but only 540 amp hours?(I don’t think I’m doing this right). The amps are confusing me.

And do you have any names of chargers, and BMS systems that might work for this build or do you need more info? Lastly thank you again for the info this was really helpful.
 

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The next issue that was slightly answered is how many of these battery modules I need and how to wire them. (the battery is 2.5kwh, and rated at 60 volts, 45 amp hour) I was thinking of using 12 of them but I don’t know if I can run the big motor with that many or if I need more. I know you explained how EV batteries are wired together but I still don’t get it enough to do my own. I understand that you put them together in series to add up voltage and parallel to add up the amps. But how would I do this with my set up. Would it work if I had 6 pairs of 2 batteries in parallel and put those 6 pairs in series? That would be 360 volts but only 540 amp hours?(I don’t think I’m doing this right). The amps are confusing me.
6 x 60 volts = 360 volts for the combination, so you have that right
2 x 45 amp-hours = 90 amp-hours... not 540 amp-hours

You are still confusing storage capacity (in watt-hours for energy, or amp-hours for charge) with power (in watts) or current (in amps). Although this combination has a capacity of only 90 amp-hours, you can use that capacity at a high rate, even hundreds of amps. If you take the energy out of the battery at 450 amps, for instance, the 90 amp-hours would be used up in only 12 minutes (because 450 amps x 0.2 hours = 90 amp-hours). 360 volts multiplied by 450 amps would be 162 kW, and you're not going to do that for more than a few seconds at a time anyway.
 
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