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#### fjfonline

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I have just started learning about EV conversion and would want to start my own Conversion soon. I have been studying the info but I can't seem to find why for example with some builds I have seen the Tesla or other battery modules are connected in a combination of series and parallel. Why do I rarely see a series only setup when there are a lot of modules connected together?

I would like to fully understand why they are combined in series and parallel configuration.

#### oudevolvo

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It depends on:
• The system voltage you want
• The power (current) you want to have available
• The amount of space or weight capacity you have
So let's say you're doing a relatively low voltage conversion of around 96V.
Then 4 Tesla modules could be a nice match (4x 22V nominal = 88V).
But every module only is around 5 kWh, usable around 4,5 kWh
So that means a pack of 20 kWh. Depending on the weight of your vehicle that gives you a certain range.
And these 4 modules can provide 230 A continous at 88V that is 20 kW. For a small vehicle that is OK perhaps, but for a bigger one not. So if you want to have more range and more power, you need more batteries.
But if you add them in series the voltage goes up (and that does not match the system voltage you had chosen).
So by adding another batch of 4 modules in parallel:
• You keep the nominal voltage of 88V
• Double the pack to 40 kWh
• Double the continuous current
With higher system voltages you often see setups using batteries in series only. The currents can be much lower for the same amount of power.

#### fjfonline

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Thanks for explaining. I think I understand now.

So to test my understanding of this:

I was planning on using the large Tesla drive unit and 20 battery modules (22V 232ah) and if I place them in series I would end up with 102Kw (440V and 232ah) right? But if I take the same 20 modules and put 16 in series (352V/232ah) and 4 parallel (22v/928ah) and combine them then I would end up with 231V/1160ah??

🤔

#### OR-Carl

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But if I take the same 20 modules and put 16 in series (352V/232ah) and 4 parallel (22v/928ah) and combine them then I would end up with 231V/1160ah??
This does not make any sense - you seem to have confused something. When the modules go in series, you add the voltages, and when in parallel you add the capacity, so that part is correct.

When you put modules in series, those are then called strings. So putting 10 modules in series makes a string with 220v and 232ah. Putting two 10-modules-strings in parallel would get you 220v and 464ah. The strings would have to be the same voltage in order to be paralleled (or all you will get is a fire). A very hot fire.

If you are set on using 20 modules, you could do 10s2p, 5s4p, 4s5p or 2s10p, depending on which voltage makes the most sense for your project.

#### fjfonline

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This does not make any sense - you seem to have confused something. When the modules go in series, you add the voltages, and when in parallel you add the capacity, so that part is correct.

When you put modules in series, those are then called strings. So putting 10 modules in series makes a string with 220v and 232ah. Putting two 10-modules-strings in parallel would get you 220v and 464ah. The strings would have to be the same voltage in order to be paralleled (or all you will get is a fire). A very hot fire.

If you are set on using 20 modules, you could do 10s2p, 5s4p, 4s5p or 2s10p, depending on which voltage makes the most sense for your project.
Now I get it. I did not know this rule: "The strings would have to be the same voltage in order to be paralleled".
That explains the available options: 10s2p, 5s4p, 4s5p or 2s10p.

Thank you all for your input. It is becoming very clear now.

#### jpitz31

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Get a basic electronics book and take a look at series and parallel circuits. As indicated above adding batteries in series adds voltage, while leaving amperage the same as one battery. Adding batteries in parallel increases amperage capacity while leaving the voltage the same as one battery. So by combining batteries in series and parallel you can build battery packs so that voltage and amperage increases as the batteries are combined. So if a basic battery cell is 4.2 volts and 3000 mamps. you can increase voltage in increments of 4.2 volts by wiring batteries in series, Then by taking two groups of the series wired batteries and then wiring them both in parallel you can increase amperage in multiples of 3000 mamps. Check out ebikeschool.com for some video examples

#### fjfonline

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Get a basic electronics book and take a look at series and parallel circuits. As indicated above adding batteries in series adds voltage, while leaving amperage the same as one battery. Adding batteries in parallel increases amperage capacity while leaving the voltage the same as one battery. So by combining batteries in series and parallel you can build battery packs so that voltage and amperage increases as the batteries are combined. So if a basic battery cell is 4.2 volts and 3000 mamps. you can increase voltage in increments of 4.2 volts by wiring batteries in series, Then by taking two groups of the series wired batteries and then wiring them both in parallel you can increase amperage in multiples of 3000 mamps. Check out ebikeschool.com for some video examples
Thanks for the great explanation @jpitz31 . I have been watching some of these basic lessons on Youtube and combined with the input from you guys I can now understand these basics.

#### JeffAU

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Another simple answer regaring the Tesla question: Because some tesla packs at least use around 7000 18650 3.2v cells, puting them all in series you would end up with around 25000 volts. Great if you get your hands on a train motor (here in Quensland Australia at least, the electric trains run on 25KV - a single phase of the 33KV lines in star configuration obviously) Another interesting fact about this is our first generation electric trains have 135KW motors, similar in size to most production EVs but 4 to 6 motors in each 6 car set as there were several varients. I'm not sure if they are AC (BLDC) or DC motors but they were designed and built late 70s - early 80s. I've got no plans on using an ex train motor, and don't know of anyone else who is either, although Queensland Rail are in the process of decommisioning and scrapping these first generation EMU (electric multiple unit) trains. 25KV is way higher than the maximum you would want in any EV build really. The railways just use it for overhead lines to prevent losses over these huge distances they cover - so stay away from them in case you didn't see any of the signs!

I am using Damien/EVBMW Prius Gen3 dual motor inverter controller and just recieved partially built board, completing it myself as I am electronics technician by trade.

#### Duncan

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Another simple answer regaring the Tesla question: Because some tesla packs at least use around 7000 18650 3.2v cells, puting them all in series you would end up with around 25000 volts. Great if you get your hands on a train motor (here in Quensland Australia at least, the electric trains run on 25KV - a single phase of the 33KV lines in star configuration obviously) Another interesting fact about this is our first generation electric trains have 135KW motors, similar in size to most production EVs but 4 to 6 motors in each 6 car set as there were several varients. I'm not sure if they are AC (BLDC) or DC motors but they were designed and built late 70s - early 80s. I've got no plans on using an ex train motor, and don't know of anyone else who is either, although Queensland Rail are in the process of decommisioning and scrapping these first generation EMU (electric multiple unit) trains. 25KV is way higher than the maximum you would want in any EV build really. The railways just use it for overhead lines to prevent losses over these huge distances they cover - so stay away from them in case you didn't see any of the signs!

I am using Damien/EVBMW Prius Gen3 dual motor inverter controller and just recieved partially built board, completing it myself as I am electronics technician by trade.
The normal procedure is to
Parallel FIRST - then connect the paralleled groups in series
This means that you only use one BMS - it monitors the "groups"

However if you are buying individual cells the overall experience is that the quality is dubious - the cells used in production EV's appear to be very good but "new" cells seem to have a 5% or so failure rate

The problem is that often a cell will fail and proceed to kill the cells paralleled up to it - kill its entire group

When I had that problem I went to using separate strings with a primitive BMS on each one - so if a cell failed I could remove that string from my system

I am now using cells/modules from a production EV - and no longer have that sort of problem

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