[di11ard]

Apologies in advance as I am trying to understand battery capacity vs motor output concepts.

I am in the early stages of building/converting a vehicle to electric. A build log will be created in the proper forum in the coming weeks. But first, there are a few basics I need to understand & review before jumping into this project.

Ideally, I would like to use Tesla battery modules as the size & modularity of these untis will work perfect for my needs.

Additionally, I am would like to use a 400kW Tesla large rear drive unit in my build

I do not seek long range capabilities... Something between 50 & 80 miles will be more than acceptable (at crusing speeds, not full tilt).

If I use 4 Tesla battery packs, will I have the ability to run a Tesla motor at full output capabilities or is there a lack of discharge to fully power the motor? How do I calculate the output capacity based on the number of Tesla battery modules?

Thank you in advance.

 

[brian_]

... I am would like to use a 400kW Tesla large rear drive unit in my build
...
If I use 4 Tesla battery packs, will I have the ability to run a Tesla motor at full output capabilities or is there a lack of discharge to fully power the motor? How do I calculate the output capacity based on the number of Tesla battery modules?

I assume by "4 Tesla battery packs", you mean four modules from a Tesla pack.
No, there is no chance of being able to operate the motor at its full capability.

Batteries are limited in their discharge rate, so with one-quarter of a full Tesla Model S or X pack, you can only get about one-quarter of the full pack's power.

The other problem is that you will have only one-quarter of the full pack's voltage. That won't matter at low speed, but as speed increases the voltage required to drive the motor increases, so at high speed even if you were willing to destroy the cells in the modules with excessive discharge rate, they wouldn't produce enough voltage to drive the motor to full power.

It's not by coincidence that the most powerful EVs have the largest battery capacity.

 

[john61ct]

The load (controller/motor) "pulls" demands the current in amps.

The batteries' Ah capacity just "stands by", has the ability to deliver, doesn't "push" current.

Peak amps are (should be) very short term.

Range varies enormously by weight hills and driving style.

 

[brian_]

The load (controller/motor) "pulls" demands the current in amps.

The batteries' Ah capacity just "stands by", has the ability to deliver, doesn't "push" current.

Peak amps are (should be) very short term.

Sure, but any battery is limited in its ability to withstand discharge current without damage... even short term. If it were as easy as just ignoring peak current, the electric drag racers would just use any random salvaged EV modules, since they only need a few seconds of output... but in the real world that doesn't work.

Note that in this comment I was referring to discharge rate (not amp-hour capacity) and power (not energy):

Batteries are limited in their discharge rate, so with one-quarter of a full Tesla Model S or X pack, you can only get about one-quarter of the full pack's power.

 

[di11ard]

Thank you both for your insights.

I think I am making a bit of progress in understanding how these components relate.

If we could remove the range from the equation to tackle my understanding of battery capacity vs power.

Based on the specs provided on HSR's website

Per battery unit:
Volts --> 22.2
3s Amps --> 1,520
10s Amps --> 1,000

With that in mind... Can you tell me if my math & understanding of these concepts are correct? Am I calculating & scaling the values correctly?

1 Battery Unit * 22.2 Volts * 1,500 Amps = 33.3kW (for 3 seconds) OR ~45HP
1 Battery Unit * 22.2 Volts * 1,000 Amps = 22.2kW (for 10 seconds) OR ~30 HP

2 Battery Units * 22.2 Volts * 1,500 Amps = 66.6kW (for 3 seconds) OR ~90HP
2 Battery Units * 22.2 Volts * 1,000 Amps = 44.4kW (for 10 seconds) OR ~60HP

3 Battery Units * 22.2 Volts * 1,500 Amps = 99.9kW (for 3 seconds) OR ~134HP
3 Battery Units * 22.2 Volts * 1,000 Amps = 66.6kW (for 10 seconds) OR ~90HP

4 Battery Units * 22.2 Volts * 1,500 Amps = 133.2kW (for 3 seconds) OR ~179HP
4 Battery Units * 22.2 Volts * 1,000 Amps = 88.8kW (for 10 seconds) OR ~120HP

5 Battery Units * 22.2 Volts * 1,500 Amps = 166.5kW (for 3 seconds) OR ~223HP
5 Battery Units * 22.2 Volts * 1,000 Amps = 111kW (for 10 seconds) OR ~149HP

6 Battery Units * 22.2 Volts * 1,500 Amps = 199.8kW (for 3 seconds) OR ~268HP
6 Battery Units * 22.2 Volts * 1,000 Amps = 133.2kW (for 10 seconds) OR ~179HP

 

[john61ct]

Re: Newbie Question --> Understanding Battery Power Output

Sure, but any battery is limited in its ability to withstand discharge current without damage... even short term.
…
Note that in this comment I was referring to discharge rate (not amp-hour capacity) and power (not energy):

So determine which cells comprise your pack and search for **user** test results and discussions of how high a C-rate they handle without overheating.

Since the TMS is absent you want to be very conservative, since overheating does not just damage the cells but can create a nasty huge fireball impossible to extinguish.

As part of your DIY TMS, be sure to allow current to be capped - limp-home mode - based on pack temps rising at all quickly.

 

[brian_]

Re: Newbie Question --> Understanding Battery Power Output

So determine which cells comprise your pack and search for **user** test results and discussions of how high a C-rate they handle without overheating.

Yes, that makes sense. Of course Tesla puts a lot of effort into seeing how hard they can push these cells and updating the cars to allow higher and higher rates, under limited conditions and for a limited time period.

Since the TMS is absent you want to be very conservative, since overheating does not just damage the cells but can create a nasty huge fireball impossible to extinguish.

di11ard, is the intention here to run Tesla modules at extreme levels without the circulating fluid thermal management system? That makes no sense, and I don't see any mention of plans to omit (or include) thermal management in the earlier posts. The HSR page which you linked says that for discharge rates greater than 1 C (meaning more than 233 amps for these 233 Ah modules), coolant flow is required. 233 amps at 22.2 volts is only 5 kW (per module).

 

[brian_]

Based on the specs provided on HSR's website

Per battery unit:
Volts --> 22.2
3s Amps --> 1,520
10s Amps --> 1,000
...

It would probably be easier for everyone to use the industry standard terminology, and call this "battery unit" a "module", as HSR does.

 

[brian_]

... Can you tell me if my math & understanding of these concepts are correct? Am I calculating & scaling the values correctly?

1 Battery Unit * 22.2 Volts * 1,500 Amps = 33.3kW (for 3 seconds) OR ~45HP
1 Battery Unit * 22.2 Volts * 1,000 Amps = 22.2kW (for 10 seconds) OR ~30 HP

2 Battery Units * 22.2 Volts * 1,500 Amps = 66.6kW (for 3 seconds) OR ~90HP
2 Battery Units * 22.2 Volts * 1,000 Amps = 44.4kW (for 10 seconds) OR ~60HP
...

The problem is that this current will only flow if the voltage provided by the battery (and regulated by the controller) is high enough to drive that current through the motor at whatever speed the motor is turning. The reason that modern EVs use the voltage that they do (nominally 360 V is typical) is so that they can get the desired power even at the top of the speed range.

 

[di11ard]

Re: Newbie Question --> Understanding Battery Power Output

di11ard, is the intention here to run Tesla modules at extreme levels without the circulating fluid thermal management system? That makes no sense, and I don't see any mention of plans to omit (or include) thermal management in the earlier posts. The HSR page which you linked says that for discharge rates greater than 1 C (meaning more than 233 amps for these 233 Ah modules), coolant flow is required. 233 amps at 22.2 volts is only 5 kW (per module).

I will absolutely be running liquid cooling.

 

[di11ard]

It would probably be easier for everyone to use the industry standard terminology, and call this "battery unit" a "module", as HSR does.

Noted! Will do... MODULE, not battery unit. My apologies.

 

[di11ard]

The problem is that this current will only flow if the voltage provided by the battery (and regulated by the controller) is high enough to drive that current through the motor at whatever speed the motor is turning. The reason that modern EVs use the voltage that they do (nominally 360 V is typical) is so that they can get the desired power even at the top of the speed range.

Ugh. I am not a smart man.

I would like to understand how to calculate how much power can be made based on the number of battery modules used. Power for 4 modules, or 6, or 8, etc.

 

[kennybobby]

Your calculation is correct for determining the electrical power for the various number of modules, which determines the voltage, times the maximum current available for 3 seconds and 10 seconds intervals. You have correctly converted that to units of HP. Power = Volts x Amps

So your results somewhat match the data on the HSR site, they have already made these calculations for one module and you can add the modules. They list max power for 3 sec as 30kW, so 6 modules is 180kW. This is close enough to your calculation for back of the envelope estimates.

They list the energy of a module as 5kWh from 22V x 233 A-hr (voltage x capacity). So 6 modules would give you a 30kWh pack.

For comparison, the early Nissan Leaf had a 24kWh pack and could travel 80 miles.

So with proper selection of motor and drivetrain it would be possible to build a car to go 60 miles. This requires sizing the motor torque and speed range to fit the weight of your target vehicle, and selecting or building a motor controller, aka inverter, to drive the motor from your battery pack.

There are 3 major components which all must be sized to play nice together: the motor, the inverter and the battery pack.

First you must select your vehicle to know the weight and tire size and aerodynamic drag, then you can calculate the power needed to travel at a desired speed and to accelerate as desired.

Once you know this mechanical power, then you can select a motor and gearbox that can provide the torque and speed needed to propel the car, in conjunction with selection of an inverter that can control the motor.

If you already have a battery pack, then the wire size and number of turns for the windings of the motor can be selected based upon the available voltage and the current needed to meet the torque requirement. Then it's just a matter of finding such a motor already available, or having it made. Generally the speed of a motor is related to the voltage, and the torque of a motor is related to the current. Then an inverter will need to be found or designed to operate the motor.

OR

you find an existing motor and inverter that can provide the torque and speed needed, and then build a pack to provide the voltage and current per the motor label and inverter specs.


So my point is that just looking at batteries is only 1/3 of the problem, and it is the last third of the solution. If you buy 6 modules and a large tesla motor, then you have already picked 2/3 of the components, but they may not be the correct solution unless you can build a custom inverter to work within those constraints. And not knowing the vehicle power requirements is a big question that should be answered early on in the design process.

 

[brian_]

I would like to understand how to calculate how much power can be made based on the number of battery modules used. Power for 4 modules, or 6, or 8, etc.

What the combination of modules can provide is what you have calculated. As mentioned earlier, what power will come out of them depends on the motor (and controller), and how it matches the battery. The Tesla motors are designed to work with a 360 volt (nominal) battery pack.

 

[john61ct]

Determine your target peak C-rate

Ah capacity at your target voltage times that is your peak amps.

So e.g 3C means a 200Ah pack yields 600A, but of course only briefly.

Most likely range will demand more Ah, but that does not give licence to exceed the safe C rate.

Ugh. I am not a smart man.



I would like to understand how to calculate how much power can be made based on the number of battery modules used. Power for 4 modules, or 6, or 8, etc.

 

[brian_]

There are 3 major components which all must be sized to play nice together: the motor, the inverter and the battery pack.
...
So my point is that just looking at batteries is only 1/3 of the problem, and it is the last third of the solution. If you buy 6 modules and a large tesla motor, then you have already picked 2/3 of the components, but they may not be the correct solution unless you can build a custom inverter to work within those constraints.

This is all great background and explanation.
Even if building a custom inverter, unless you are willing to include voltage multiplication in it (which is almost never done, with Toyota hybrids the exception), you still can't fix the problem if you have chosen a battery configuration without enough voltage for the chosen motor.

Suppliers of salvaged EV components who recommend small numbers of Tesla Model S/X modules are typically recommending them for lower-voltage motors, not for use with Tesla motors. Some builders do use a less than full set of Tesla modules with a Tesla motor, because they don't need full rated power at high speed (at or beyond typical highway speed limit), but not just four modules.

 

[di11ard]

Your calculation is correct for determining the electrical power for the various number of modules, which determines the voltage, times the maximum current available for 3 seconds and 10 seconds intervals. You have correctly converted that to units of HP. Power = Volts x Amps

So your results somewhat match the data on the HSR site, they have already made these calculations for one module and you can add the modules. They list max power for 3 sec as 30kW, so 6 modules is 180kW. This is close enough to your calculation for back of the envelope estimates.

They list the energy of a module as 5kWh from 22V x 233 A-hr (voltage x capacity). So 6 modules would give you a 30kWh pack.

For comparison, the early Nissan Leaf had a 24kWh pack and could travel 80 miles.

So with proper selection of motor and drivetrain it would be possible to build a car to go 60 miles. This requires sizing the motor torque and speed range to fit the weight of your target vehicle, and selecting or building a motor controller, aka inverter, to drive the motor from your battery pack.

There are 3 major components which all must be sized to play nice together: the motor, the inverter and the battery pack.

First you must select your vehicle to know the weight and tire size and aerodynamic drag, then you can calculate the power needed to travel at a desired speed and to accelerate as desired.

Once you know this mechanical power, then you can select a motor and gearbox that can provide the torque and speed needed to propel the car, in conjunction with selection of an inverter that can control the motor.

If you already have a battery pack, then the wire size and number of turns for the windings of the motor can be selected based upon the available voltage and the current needed to meet the torque requirement. Then it's just a matter of finding such a motor already available, or having it made. Generally the speed of a motor is related to the voltage, and the torque of a motor is related to the current. Then an inverter will need to be found or designed to operate the motor.

OR

you find an existing motor and inverter that can provide the torque and speed needed, and then build a pack to provide the voltage and current per the motor label and inverter specs.


So my point is that just looking at batteries is only 1/3 of the problem, and it is the last third of the solution. If you buy 6 modules and a large tesla motor, then you have already picked 2/3 of the components, but they may not be the correct solution unless you can build a custom inverter to work within those constraints. And not knowing the vehicle power requirements is a big question that should be answered early on in the design process.

Completely understood and thank you so much for the great feedback.

The vehicle is a sand rail that has been setup for pavement use only. I have been building this over the last few years. Currently, she has a 2.3L Turbo Ford engine that I am ready to replace.

I would like to be at 400kW if possible (even if only for brief periods)... The #1 goal is a quick 0-60. Ideally, as close to 3 seconds as possible.

I am more inclined to use Tesla battery modules due to their size, form factor, integrated tech, BMS, etc.

I am wanting to use a Tesla motor "kits" from EV West or HSR Motors for this conversion.

I will be adding a build log (in the proper forum) and further info/pictures about the vehicle. Sadly, photobucket sh!t the bed and my photos/account have not been restored yet...

In the meantime, here is a walkaround video of the street rail in her current form:

 

[di11ard]

What the combination of modules can provide is what you have calculated. As mentioned earlier, what power will come out of them depends on the motor (and controller), and how it matches the battery. The Tesla motors are designed to work with a 360 volt (nominal) battery pack.

Awesome. Thank you!

 

[brian_]

In the meantime, here is a walkaround video of the street rail in her current form...

That YouTube player doesn't display properly in this forum, at least in some browsers - all I see is a big blank, because the player uses Adobe Flash and that is blocked. Here's the link, to be clicked:
https://youtu.be/6zJbAAdES98

 

[di11ard]

YouTube links like that (using the YOUTUBE forum command) don't display properly in this forum, at least in some browsers - all I see is a big blank. Here's the link, to be clicked:
https://youtu.be/6zJbAAdES98

Thank you and my apologies. I had to play with the youtube embedding option to get the video to display for me... Sorry to hear it doesn't work on your end.

I will attach a photo so people (that are logged in) can see what we are working with.