[sifawangiaEV]

Hi all,

I'm planning on using 4 Tesla battery modules (5.2kwh, 24V, 250AH each, wired in series) for my build but first I need to purchase the right fuses, cables, and circuit breaker to make sure my system is protected.

In order to find this out, I calculated the amps flowing through my system by dividing the total watts by the total voltage (20800/96 = 217 amps). With this information I decided to use 4/0 AWG cables, a 250A 160VDC circuit breaker, and 300A 700V semiconductor fuse to protect the cables from overheating and the other parts in the EV from blowing.

I was looking at some Tesla battery modules to buy and came across this in one seller's description, "[The Tesla batteries] are rated at 500 amps, 750 amps peak...Maximum Discharging Current (10 sec.):750 Amps." This got me double thinking my calculation and if the wire gauge, circuit breaker, and fuse were appropriate for this application.

Does anyone have any insight to give me on this? Will I have 500 amps or 217 amps flowing through the cables? And most important of all, will the wire gauge, circuit breaker, and fuse I chose be appropriate for the protection of my system?

 

[roach374]

(for legal reasons, I am not an electrician, so this is just my uninformed opinion) If you've chosen a fuse value lower than the maximum/peak discharge rate of the cells, it just means that the fuse will blow before the cells do, which is what you want. And a 4/0 cable is more than capable of handling that current. I would also take a look at your motor, though, and what current you expect the motor to draw when you floor it. If the motor "asks" for 1000A every time you hit the gas, you're going to be replacing a lot of fuses...

 

[brian_]

In order to find this out, I calculated the amps flowing through my system by dividing the total watts by the total voltage (20800/96 = 217 amps).

Why did you choose 20.8 kW? That might be appropriate for your motor and controller, but it entirely determines the current. If that power is correct, the current is correct.

I was looking at some Tesla battery modules to buy and came across this in one seller's description, "[The Tesla batteries] are rated at 500 amps, 750 amps peak...Maximum Discharging Current (10 sec.):750 Amps." This got me double thinking my calculation and if the wire gauge, circuit breaker, and fuse were appropriate for this application.

Does anyone have any insight to give me on this? Will I have 500 amps or 217 amps flowing through the cables?

Why do you think that the maximum capability of the battery modules would be relevant to your expected current?

 

[sifawangiaEV]

Ok, for some more context: I'm using a Warp9 DC motor which has a max amp draw of 500A. I'm going to set my max amp draw to 300A for a month or two on my Zilla controller, and then go up to 400 or 450A after that depending on the amp draw of my other appliances.

I know that fuses and breakers are supposed to be sized to the wire used, not really the application. I was planning on using 4/0 awg cables but their ampacity rating doesn't seem enough for my application seeing that it can hold 260 amps at 90C (167F). I was thinking about using a kcmil size wire but I've never heard of anyone using that size wire so it seems wrong. I would assume that if I floored it, my cables would be carrying 400/450 amps, way more than their rated ampacity. Can someone please help me with this?

Why did you choose 20.8 kW? That might be appropriate for your motor and controller, but it entirely determines the current. If that power is correct, the current is correct.

I incorrectly got 20.8 kW by multiplying the Tesla battery's 5.2kWh by 4 getting 20.8kWh. I don't know what my total power is because I don't know how to get kW from kWh.

Why do you think that the maximum capability of the battery modules would be relevant to your expected current?

I incorrectly thought that 500 amps would be flowing through the cables, rather than being the amount that the battery had in store.

 

[brian_]

Ok, for some more context: I'm using a Warp9 DC motor which has a max amp draw of 500A. I'm going to set my max amp draw to 300A for a month or two on my Zilla controller, and then go up to 400 or 450A after that depending on the amp draw of my other appliances.

That makes more sense. Except that I don't know what "other appliances" might be relevant.

I incorrectly got 20.8 kW by multiplying the Tesla battery's 5.2kWh by 4 getting 20.8kWh. I don't know what my total power is because I don't know how to get kW from kWh.

I don't know what you mean by "total power".

Kilowatt-hours (kWh) is a measure of energy. Each Tesla Model S module can hold 5.2 kWh, so four of them hold 20.8 kWh.

Kilowatts (kW) is a measure of power, or the rate of energy transfer. You could use 20 kWh of energy by using 20 kW for one hour, or 10 kW for 2 hours, or any other combination of power (in kW) and time (in hours) that multiplies out to 20.

You "don't know how to get kW from kWh" because you don't get kW from kWh; or, the energy stored in the battery does not determine the power that you will use. There is an output current limit for a module, but that's a different specification from the energy content. Resellers of used Tesla Model S 5.2 kWh modules typically claim that they can handle up to about 750 amps; with the modules wired in series (so their voltages add up), the current limit of the combination would still be 750 amps. You don't need to wire for 750 amps unless you fuse for 750 amps, and you don't need to fuse that high if your controller will never take that much current.

I incorrectly thought that 500 amps would be flowing through the cables, rather than being the amount that the battery had in store.

But you were right about that: if the controller has a 500 amp limit, or the motor uses up to 500 amps, those are the current in the cables between the controller and the motor. It has nothing to do with what the battery stores.

Because the controller converts some current at the battery voltage to a higher current at a lower motor voltage, the current from controller to motor does not, in general, equal the current from the battery to the controller. The current from battery to controller will always be no higher than the current from controller to motor.

 

[sifawangiaEV]

But you were right about that: if the controller has a 500 amp limit, or the motor uses up to 500 amps, those are the current in the cables between the controller and the motor. It has nothing to do with what the battery stores.

Ok, that makes sense. So what's the max current my appliances (motor [400 or 450A], DC converter [~40A], and accessories from 12V battery [amp draw unknown]) can draw from the battery at the same time for continuous amounts of time? Would it be 500A? I just don't want to draw more than my battery can deliver.

Do you think 4/0 awg cables would be enough to handle the max current draw of my motor, dc converter, and 12v accessories?