Battery capabilities question

[Poker]

So my Sherman S has Samsung 50E 21700 batteries setup in 24s8p (correct me if I'm wrong)

According to the spec sheet from Samsung they should be able to provide 9,8A continuous discharge.

Does that mean I could be pulling (9,8*8=78,4) 78,4 amps from the batteries constantly (unrealistic, I know) until I reach something like 72V (or even 60V as the spec says cut of at 2,5V per cell)  under load with the sag and it would not degrade the cells more than in the spec?

If you consider the control board to be 85% efficient (how close is that to reality?) it does seem possible to power a stronger motor than the standard 3500W motor it comes with.

[alcatraz]

The max discharge current relates to the temperature of uncooled cells. I believe that max current assumes a 20C ambient temperature which means that on a hot summer day the max current should be lower.

The max current can be maybe twice as high if you have cooling.

Heat/cold degrades the cells (when in operation). So indirectly you're right. You'll degrade the cells by pulling more power because they'll run hotter.

Also, don't forget that there's a lot of sag in high capacity cells like 50E compared to high current. It means that you'll go below 72V a lot earlier than you think if you intend to ride hard. It depends on the size of your pack.

8P is a lot of cells sharing the load so you'll get a good performance even with high capacity cells. 

 24S8P with 5000mAh cells make a 3552Ah pack. 

[alcatraz]

And just to be clear. Continuous max current is not really what is going on when riding. Maybe the average current, yes, but the peaks can be a lot higher than the max discharge current and you'll probably be fine.

[RagingGrandpa]

On 8/26/2023 at 9:11 AM, Poker said:

Does that mean I could be pulling (9,8*8=78,4) 78,4 amps

Briefly, yes; sustained, no. 
There are fuses rated at 60 amps. Expect them to melt for sustained currents that exceed 125% the fuse rating. 
 

On 8/26/2023 at 9:11 AM, Poker said:

According to the spec sheet from Samsung they should be able to provide 9,8A continuous discharge.

From user testing, you can see that a full discharge of 50E at 10A will give a 45°C temperature rise for a single cell resting in still air. 

Let's assume Sherm-S uses BMS temperature sensors with an alarm threshold of 60°C; and we know the cells are packed tightly in a sealed enclosure, plastic on one side, alloy on the other. Cooling of the inner rows of cells will be slower than the single-cell test, so expect even higher temperature rise than what is shown above. 

Starting from normal room temperatures ~23°C, the pack would overheat and trigger the BMS alarm, if such a load were sustained for more than about 60% of the ride.

For short bursts, the temperature rise is insignificant; cell temperature will become a problem only after about 15 minutes of sustained load. 
 

On 8/26/2023 at 9:11 AM, Poker said:

If you consider the control board to be 85% efficient (how close is that to reality?)

The controller is >95% efficient during high-speed high-load operation. We're losing a maximum of 200W to controller heating. 

Your "85%" is a more typical efficiency of the motor, representing the difference between electrical input power and mechanical output power. But the cells don't care if your motor melts, so this 85% doesn't matter to your question regarding "could the pack do >7kW"?
 

On 8/26/2023 at 9:11 AM, Poker said:

it does seem possible to power a stronger motor than the standard 3500W

Ah, the old "motor rating" dilemma...

3500W is a continuous rating, and is just an estimate based on a set of assumed conditions regarding speeds, currents, starting temperature, and cooling. 

Sherm-S has been demonstrated to send up to 300A into its motor, easily exceeding 10kW.
But it cannot sustain this load: the motor, controller, or both, would melt if such a thing lasted for more than a minute. It's a heating problem: the longer a heavy load is sustained, the more the temperature rises.

 

Conclusion:
Changing to a heavier motor with a higher power rating would not help, because Sherm-S's existing motor is not overheating during its maximum output.  
To further increase system power, the controller would need to be up-sized first, because the controller is the component limiting the power level. Not the motor. 

[Poker]

Thank you both for the thorough replies.

Regarding the temperature increase graph, it was way more than I thought. But when you have 24 cells in series surely the heat would be (significantly?) less if you draw 10amps in total from them?

I understand that they have internal resistance and since the same current is passing through all of them it will generate heat in all of them, but the discharge of each cell would be 1/24th of 1C. Dissipating the heat generated from the cells in the middle is ofc a real problem with continuous high drain.

[RagingGrandpa]

30 minutes ago, Poker said:

But when you have 24 cells in series surely the heat would be (significantly?) less if you draw 10amps in total from them?

If you had a small pack with 24s 1p, then that 10 amps is seen by every cell. So all 24 cells will rise about 45°C before they're empty. 

And then if you have the Sherm-S pack with 24s 8p, it's as you said:

On 8/26/2023 at 9:11 AM, Poker said:

pulling (9,8*8=78,4) 78,4 amps from the batteries

Like this, every individual cell is exposed to 9.8 Amps. They'll get warm! 

[Poker]

Alright, that does make sense.