Battery Discharge rating and Motor Stall Current

Hello,

I’m building an obstacle avoidance robot and would like some clarification on safe operation limits.

Li-Ion battery pack has a max discgarge rating of 5A via the internal PCB.

The motor spec sheet shows a 2.2A stall current.

My question is ; how much buffer should be between stall current and max discharge current?

As much as I’d like to avoid stalling the motors there’s always a potential for it to happen.

Are there options for limiting the stall current? Such as a current sensing response? If the motors stall I would like to have the robot be able to react to it.

Will the battery PCB only allow 5A discharge or will reaching 5A shut something off?

Also I’m using this motor driver.
https://www.amazon.com/Driver-Module-Board-H-bridge-Irf3205/dp/B00SVCGE2M

Looking forward to learning some more great insight from the community !

If the motors draws 2.2A stall, the battery protection circuitry of 5A limit gives plenty of overhead. I would not be concerned about using a motor with that kind of safety margin.

As to what happens when you exceed 5A, That all depends on how the pcb is designed for the battery charge/discharge protection. Some may just allow 5A to be drawn for it and if you try to exceed that, no more current can be drawn. Others may list a 5A discharge limit meaning that you can draw more than that but it may turn into a fireball.

Without datasheets for these battery protection circuits I would not rely on it to limit current, rather I would make sure your current draw is within its safe limits.

You can use a current shunt resistor or hall sensing to monitor the current with an arduino and shut down the bridge or reduce duty cycle if you feel additional current limiting is required.

Thanks so much for the info alka.

Under load at max efficiency the motors will only be running @ 0.34A. It shouldn't ever have to reach that much torque. If It does, due to my generic use, it's safe to assume it will be heading towards a stall on the motor.

This is what I'm trying to accomplish. A protective cut off as not to overload any parts.

So can I use this ACS-712 current sensor and just have it stop the motors once it detects a preset Amperage coming off the battery ? I will measure the starting draw under it's max load and set the cut off signal above that.

https://www.amazon.com/gp/aw/d/B00COD8KJY/ref=mp_s_a_1_1?ie=UTF8&qid=1493235088&sr=8-1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=acs+712&dpPl=1&dpID=41qtu0S9zxL&ref=plSrch

Or might you have a better recommended sensor to use?

Then have the response from the board be similar to that of : if the sensors had detected an obstacle. Stop, back up etc. Assuming linear movements for example simplicity.

Also, Sounds like the battery protection depends on the quality of the battery in terms of what a 5A max discharge really means.

Sounds like you are on the right track, even a 0.25 Ohm resistor might be fine for such a small current draw.

A 0.25 Ohm resistor would be dropping 0.5 volts( easy to run into arduino adc) at 2 amps current and would be burning 1 watt at that current draw. At its regular running current the resistor would only drop 0.085v and using less than 0.03 Watts ( very small). Should be able to find a 1 or more watt resistor pretty cheap.

The current sense IC will work too, makes things easier for higher current for sure.
What voltage are you running ? Check out the datasheet for the acs712 chip and make sure that it will suit your needs , won’t work off a single cell 4.2v lithium for example.

Currently using a 7.4V 2S2P Lithium Ion pack. Look like the chip needs 5V.

Currently I am using an L298N motor controller, it has a 5V output supply for the arduino which I could split to power this as well.

Although the L298N driver only allows for 2A max per channel which is why I posted the earlier board as a higher amp alternative but it is costly if I'm making more than one.

Here's a theory;

Can I use the L298N motor driver with a soft start; and utilize the current sensor to cut off at 3A to protect all pieces ? Motors from actually stalling; motor driver from max amperage and battery from max discharge. The soft start to lower overall amp draw from the motors on the starting side.

I'm not very good with ohms and resistors so I'd like to avoid them for now. Although I may tinker with that idea once I've completed this circuit without them.

Although the L298N driver only allows for 2A max per channel which is why I posted the earlier board as a higher amp alternative but it is costly if I'm making more than one.

With the L298 you can count on about 1 ampere/channel. See https://www.rugged-circuits.com/the-motor-driver-myth/

How about something like this.

This really is a small motor, no need to buy an expensive high voltage bridge. I am not a fan of the irf3205 bridge. You can parralel the outputs of either the drv8833 or the l298.

The DRV8833 would work much better than the L298. It handles more current and does not suffer the up to 4V internal voltage drop.

Awesome. Ill definitely have to look into the DRV8833. Is there something similar that doesnt require soldering?

And great to know I can run either the L298N controller or DRV8833 in parallel. This would mean one controller per motor correct?

Im curious about the 4 channel L293 expansion board drivers. Considering it has two L293's, can i use this and just run each side parallel? M1+M2 > Left motor M3+M4 > Right motor

You really should learn to solder.

It is not difficult and is essential for reliable connections on something that vibrates or moves, like a robot. Plenty of on line tutorials.

I can solder I just am challenging myself to build something with readily available pieces is all.

Would combing the 4 channel l293 expansion board into two parralel channels work ?

Hey all,

So I've agreed to go with the DRV8833 as highly recommended. I had a few questions regarding the protections on board. What kind of protections are absolutely critical when selecting a motor driver?

This is going to be for a permanent solution within a robot.

DRV8833 claims built-in protection against Reverse-voltage, Under-voltage, Over-current Over-temperature

What else should I ensure I have?Or, what is necessary for safe long long term operations?

Battery can supply more than entire system should ever call for. The power pack shows : Over charge protection Over discharge protection Over current protection Short circuit protection

Motors are within operating limits of DRV8833 driver. 1.5A stall - 600ma draw at max efficiency

I'm not sure if I'm missing anything. This robot will be a gift to a friend so want to ensure everything will work properly without the need to adjust or replace down the road.