Expected Battery Draw?

2 years 7 months ago #29533409 by taakinsi
I've gone through the documentation on Motor Amps vs Battery Amps and understand that the average battery amps will usually be lower than motor amps because battery amps are drawn in spikes. So when picking a BMS and battery fuse for protecting the battery from being overdrawn, how does one go about spec'ing out the max expected current draw? Should it be based on battery amps or motor amps? We have tried a few things with FBL2360 and seeing some unexpected results
Battery rating: Continuous - 30A, Peak - 60A
Fuse between battery and motor controller - 25A
From experiments, max total battery amps - 10A
From experiments, max total motor amps - 100A

In this experiment, the BMS got damaged from maybe drawing at 100A peak? So how come the fuse did not blow? It is a fast acting fuse. Or have I misunderstood what to expect in this scenario?

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2 years 7 months ago #29533413 by Gabriel_Isko
Replied by Gabriel_Isko on topic Expected Battery Draw?
The max current limit per motor channel of our controllers is given in their datasheet. I believe it should be around 60A for the FBL2360.

I was wondering if you could clarify some of your testing methodology. 100A is also over the maximum current limit of the FBL2360, so I'm not sure how you were able to provide that much current to your motor without damaging the FBL2360. Perhaps you are using an FBL2360S?

If you are simply looking for a Battery Management System, we are currently working on one. It is designed to be compatible with our controllers. It should be available soon.

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2 years 7 months ago #29533414 by taakinsi
Replied by taakinsi on topic Expected Battery Draw?
I said 100A total...so it comes to roughly 50A per motor for this test. I'm aware of the current limits of the controller and not looking for a BMS.
I am wondering how the 25A fuse is not getting blown when the FBL2360 is saying 100A is being drawn (~50A per motor).
For the situation mentioned above, what is the actual amps draw from the battery? If it is truly spiking at 100A (for motor current, 10A battery average) in this case like your docs suggest, shouldn't the 25A fuse have blown?

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2 years 7 months ago #29533415 by Gabriel_Isko
Replied by Gabriel_Isko on topic Expected Battery Draw?
Thank you for the clarification. It looks like your fuse isn't blowing because the motor controller is only drawing 10A from your battery maximum. The current going to your motors reaches 100A because of the PWM output of the motor controller. But this current flow never occurs in the connection from your battery to VMOT on the controller. Therefore, the current flowing through your fuse never exceeds 10A.

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2 years 7 months ago #29533416 by taakinsi
Replied by taakinsi on topic Expected Battery Draw?
Gabriel, your last reply seems to contradict the tech note found here: www.roboteq.com/index.php/applications/1...th-about-amps-rating under the heading "One last twist: Motor Amps vs. Battery Amps".

Since the motor current was able to reach 100A, doesn't that mean at some point, there was a draw equal to that from the battery? Yes, the battery *average* draw is less but if the controller is at 10% PWM for example, doesnt that mean the full 100A is getting drawn from the battery 10% of the time?

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2 years 7 months ago #29533419 by LROBBINS
Replied by LROBBINS on topic Expected Battery Draw?
A fuse is a filament that heats up and eventually opens - in a short time for fast blow, a longer time for slow blow, but even the fastest fuse is much slower than the PWM frequency. If you have a PWM of 10% and peak current of 100 Amps, that filament will cool down 90% of the time. In other words, it averages the current, so it will simply not blow. Also do note that a fuse is rated to PASS its rated current indefinitely, and that the time to blow depends on how much the actual current exceeds the rated current. If you take a look at a spec sheet for the fuse you've used, I'm pretty sure you'll see that you have to greatly exceed the rating for it to open quickly (but still not near as quickly as a PWM cycle).

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