Electronics > Beginners
Why would an Arduino kit specifically ask to use unprotected 18650 cells?
Ranayna:
Hi everyone,
some time ago I got a "Smart Car" kit very much like this: https://www.amazon.com/SunFounder-Arduino-Obstacle-Avoiding-Tracing/dp/B01DU3TKPY (this one is not exactly the same car, but same manufacturer, I cannot link to their webpage at the moment)
The car is powered by two 18650 cells in series, that are not included in the kit. Regardless of the very "chinglish" manual, there is one thing in the manual that I found strange: It explicitly requires the use of unprotected cells. I remember that this was also listed on the amazon product page back when I bought it, but that page is not available anymore.
The batteries are directly connected (via a switch) to a regulator module that outputs 5 volt. As far as I can see, there is no battery protection inherent in that module, and the batteries have to be externally charged anyway. If you want, I can test the unloaded dropout voltage of the regulator module.
I am currently using two protected cells and everything I tested so far works as expected. I have read a lot about lithium batteries, I'm sure a lot of it is scare-mongering, but I still would not want to use unprotected cells in a device that is not protecting them against deep discharge.
What could be the reason to require unprotected cells?
I can think of two things:
- protected cells are slightly longer, and might not fit the battery holder. Indeed the cells I am using now are a very tight fit, but the kit even contains two ribbons that can be threaded under the batteries to easily remove them from the holder.
- the protection might kick in if high current is drawn, which in turn might cause the user to think that the kit is busted. I do not have the kit fully working yet, so I was not able to test the current draw.
Any thoughts?
JS:
Liiely to be your last guess, high current draw when motors are working might trigger protection and plating with that for the designer was too much of a hassle.
JS
spec:
--- Quote from: Ranayna on January 02, 2019, 12:19:35 pm ---Hi everyone,
some time ago I got a "Smart Car" kit very much like this: https://www.amazon.com/SunFounder-Arduino-Obstacle-Avoiding-Tracing/dp/B01DU3TKPY (this one is not exactly the same car, but same manufacturer, I cannot link to their webpage at the moment)
The car is powered by two 18650 cells in series, that are not included in the kit. Regardless of the very "chinglish" manual, there is one thing in the manual that I found strange: It explicitly requires the use of unprotected cells. I remember that this was also listed on the amazon product page back when I bought it, but that page is not available anymore.
The batteries are directly connected (via a switch) to a regulator module that outputs 5 volt. As far as I can see, there is no battery protection inherent in that module, and the batteries have to be externally charged anyway. If you want, I can test the unloaded dropout voltage of the regulator module.
I am currently using two protected cells and everything I tested so far works as expected. I have read a lot about lithium batteries, I'm sure a lot of it is scare-mongering, but I still would not want to use unprotected cells in a device that is not protecting them against deep discharge.
What could be the reason to require unprotected cells?
I can think of two things:
- protected cells are slightly longer, and might not fit the battery holder. Indeed the cells I am using now are a very tight fit, but the kit even contains two ribbons that can be threaded under the batteries to easily remove them from the holder.
- the protection might kick in if high current is drawn, which in turn might cause the user to think that the kit is busted. I do not have the kit fully working yet, so I was not able to test the current draw.
Any thoughts?
--- End quote ---
Hi Ranayna,
It is also possible that the words do not have the correct meaning and should read, Protected batteries are not required but can be used. Other examples of corrupted meaning are:
On a cassette recorder, Pressing the red record button while playing can be fatal.
On a steam iron, Distilled water must not be used.
The other thing is that first-line manufacturer's (Panasonic, Sanyo, Varta, Sony, LG, Toshiba, Samsung) 18650 LiIion cells are protected. They have a thermal fuse inside and a strong case with controlled blow-off vents.
There is little danger in discharging LiIon batteries even down to 0V. I have done it many times. The only consequence of discharging below a certain minimum voltage is that the electrodes can get damaged and the cells internal resistance can increase and the capacity decrease. This damage is proportional to the time that the cell is left in the undercharged state.
There are literally thousands of undercharged power tool and laptop LiIon battery packs discarded around the world without any explosions. I have three undercharged laptop battery packs in the study, each containing nine 18650 LiIon cells, making a total of twenty seven cells with no consequences whatsoever.
Ranayna:
I know that deep discharging a lithium cell by itself is not a fire risk, but as far as I have understood it, charging a deep discharged cell can become a problem, due to the damage you described.
I will provide a link to the manual, and more details about the regulator module this evening.
spec:
--- Quote from: Ranayna on January 02, 2019, 03:05:49 pm ---I know that deep discharging a lithium cell by itself is not a fire risk, but as far as I have understood it, charging a deep discharged cell can become a problem, due to the damage you described.
--- End quote ---
I have not found any problem with recharging under charged LiIon batteries, using a constant current of C/10, and even the most fervent scare mongers have been able to come up with an actual occurrence of a hazardous occurrence, let alone explosions, death, house burning down, and the end of the world.
The statement (not getting at you) does not make sense either, because a LiIon battery charger will not attempt to charge a LiIon battery that has a terminal voltage below a certain level, so how can there be a problem?
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