Why limit battery current in a controller?

Many controllers have a battery current limit that is maybe half of the phase current.
I wonder why?
From what I understand the only current that matters is the phase current, that is what stress the mosfets?
Some controllers have a battery current limit close to the phase current limit, like nucular.
I know of at least one controller that dosent care at all about battery current (sfoc5) and only limit phase current.
When I asked the developer he said battery current dosent matter (for the controller)

My simplistic way of looking at it is that battery current limits protect the battery, and phase current limits protect the motor.

That dosent give a reason for the controller to have a limit that you cant go over. That is a reason to have a limit that the user can decide. My question is why many controllers have a max battery amp limit that often is a lot lower than the max allowable phase amp.
Is it protecting the controller in a way that I dont understand?
I am not interested in battery protection in this question.
And I dont think the phase amp limit is to protect the motor, but the controller.

A controller is a buck converter, at low rpm lower voltages means higher amps to make high torque.

Thus by definition phase current is greater than what is pulled from the battery.

The battery amp limit is an overall power limit. You can use a more powerful controller, but there will always be some reason to limit controller power to what other components can provide/tolerate.

Higher phase amps don't increase total power; they trade off against volts under some conditions.

I like to use a battery and motor that both are capable of greater power than the controller I use with them. The controller is the only one of the three that actively limits power by design, rather than running into its physical limitations and suffering overheating, accelerated wear, or damage.

When someone make a controller it has some hardware limits, lets say it has 24mosfets rated for 100V.

The manufacturer decides that it can safely handle 88V and 450phaseA, so they set the overvoltage protection to 89V and max phase amp the user can set to 450.

Then they decide to set the max battery amp the user can set to 120A. What I wonder is why do the manufacturer choose to set this limit? Does it protect the controller in some way?
I dont mean square wave controllers, that dosent have any phase amp sensors. I understand the importance of limiting battery amp in those.
I mean sine wave and foc controllers that have phase amp sensors.

Or do they really just decide that people who buy this controller dosent have batterys that can handle mare than 120A?

Here is an example:

https://ebikes.ca/tools/simulator.html?bopen=true&motor=MX4503&batt=cust_88_0.001_8&cont=cust_120_450_0.03_V&hp=0&cont_b=cust_500_450_0.03_V&motor_b=MX4503&batt_b=cust_88_0.001_8&hp_b=0

I just took the example controller in the post with the battery current limit and without it.
Its performance is very limited with the battery current limit, without it the controller deliver almost 30kw.
I used mxus 3kw motor in the example, so it cant really handle it :wink:

j bjork said:

What I wonder is why do the manufacturer choose to set this limit?

Click to expand...

In general there is the equation battery amps = duty cycle * phase amps. So especially at low speeds, where the BEMF is low, the phase amps are much higher than the battery current as it needs only low low duty cycles to generate high phase amps.

So even you have a lot of phase amps and therefore a lot of torque e.g. when rinding uphill, the battery isn't stressed very much.

At higher speed, where you need more duty cycle to overcome the BEMF, you will have to limit the battery current to prevent the BMS of the battery from shutting down the whole system due to overcurrent.
So you can give 80 amps to the motor without problems (for a short time ) for high torque at low speed. But you can't give 80 amps to the motor at higher speed, as the BMS of the battery would switch off.

Regards
stancecoke

I have some idea.

There is a software algorithm in play. Inherent to the PWM driving of BLDC motors.

I want to learn more, though, also.

stancecoke said:

j bjork said:

What I wonder is why do the manufacturer choose to set this limit?

Click to expand...

In general there is the equation battery amps = duty cycle * phase amps. So especially at low speeds, where the BEMF is low, the phase amps are much higher than the battery current as it needs only low low duty cycles to generate high phase amps.

So even you have a lot of phase amps and therefore a lot of torque e.g. when rinding uphill, the battery isn't stressed very much.

At higher speed, where you need more duty cycle to overcome the BEMF, you will have to limit the battery current to prevent the BMS of the battery from shutting down the whole system due to overcurrent.
So you can give 80 amps to the motor without problems (for a short time ) for high torque at low speed. But you can't give 80 amps to the motor at higher speed, as the BMS of the battery would switch off.

Regards
stancecoke

Click to expand...

Yes, I understand that. And that is a reason to have a limit for battery amp that the user can choose to fit his aplication. Another is to just limit the power. Or to protect the motor.

What I question is the reason to set a battery amp limit that the user cant go over, that limits the performance the controller otherwise would be capable of.
The manufacturer of the controller dont know what the users motor or battery is capable of, so why limit the controllers performance for a reason that dosent seem to really matter for the controller?

j bjork said:

What I question is the reason to set a battery amp limit that the user cant go over,

Click to expand...

I guess 95% of all motor controllers are built for OEMs that order a specific setup for a predefinded combinatian of motor and battery. So there is no need to make anything settable for the end user, this is even not wanted, to keep the vehicle in legal conditions.

The few motor controllers for DIY projects is no big business...

Therefore, we develop open source firmware for common controllers to get full access on all parameters

regards
stancecoke

Well, that might be an answer.
Or when controllers are sold in kits with hub motors they want higher phase amps to get good starting torque, but dont want the buyer to be able to go crazy with the battery amps and brake things.

I am mostly interested in if those that have designed a controller, or have enough understanding of the design can see a reason to limit battery amp to protect the controller.

I guess heat may be a reason, with limited battery amp the phase amps drops as soon as you pick up some speed. If you dont limit battery amp, or have a high limit it will keep full phase amps up to peak power.
So a lot more time to heat up the mosfets and controller, but there are temp sensors for that right?

But most controllers that I come to think about that has low battery amp limits compared to phase amp, are the Chinese ones.
So maybe lack of sufficient heat control is the answer?