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]

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?

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]

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 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?

[GadgetBoy]

It could also be an issue with size. If it's expecting 18650 batteries, the battery holder might not be able to accommodate the increased length of a protected cell.

Sent from my ONEPLUS A3000 using Tapatalk

[amyk]

Are you sure there's no protection? Some pictures of the circuitry, or even better, a schematic, would definitively say so. Also, a picture of the wording in the manual will help.

0V seems to be in a "not good not bad" area with current lion technology. Reverse polarity (i.e. discharging so far the +/- switch places) is bad. There's a bigclivedotcom video about this, which also links to a scientific study:

[Ranayna]

Here is the manual:
https://www.sunfounder.com/learn/download/U21hcnRfQ2FyX0tpdF9mb3JfQXJkdWlub191c2VyX21hbnVhbC5wZGY=/dispi
This version is significantly revised compared to the printed one included in the kit, but both contain the warning in the same wording. The PDF has it on page 6.

The manual also contains schematics for all the used modules, except for the Arduino Uno clone. These show clearly that the modules do not have any deep discharge protection. One (page 18) is a switching regulator (LM2596), the other module (page 16) contains a linear regulator (78M05)

[amyk]

That is perplexing. I suspect it has to do with the current draw, as others have guessed, but then again, if your protection circuit is cutting out because of excess current, it's doing so for a reason...

[Kilrah]

That is perplexing. I suspect it has to do with the current draw, as others have guessed, but then again, if your protection circuit is cutting out because of excess current, it's doing so for a reason...

One of the possible reasons being that the device's operating current is above a particular cell protection circuit's trip current.

Most likely the manufacturer got multiple "it don't work" messages that were tracked down to some cells having protection circuits that tripped too easily with the motor inrush current and just decided recommending unprotected cells was the easiest.

[beanflying]

Cruddy Chinese 18650's not able to deliver the current needed causing the protection circuitry to kick in when firing up a motor under load as has been mentioned.

A quick look at the manual and maybe some heavier wire from the battery holder to the DC-DC converter would be nice too.

Within reason don't be to worried about the cells going down to 3V the LM2596 will fall out of regulation so you should be able to pickup a slowing of the vehicle. You could as an extra do a little trickery on Pin 5 of it to provide low voltage cutout of the regulator and that would switch it off. It is shown tied to ground (on) but if the voltage on that pin is 1.3 or up it turns off. http://www.ti.com/lit/ds/symlink/lm2596.pdf

[KL27x]

I have encountered the problem, before, in a battery powered ICSP programmer. The battery I used has an internal current limiter, and it trips when one of the circuit's outputs gets shorted. Good? Well, the circuit has its own short-circuit detection and cutout which would kick in a few mS later. But once the battery protection trips, then you have to put a float voltage on the leads of the cell to turn it back on, again. Due to the purpose of the device, some short circuits occur, now and then.

[Kilrah]

Cruddy Chinese 18650's not able to deliver the current needed causing the protection circuitry to kick in when firing up a motor under load as has been mentioned.

Not the same thing at all. That's supposing the cell's IR is so bad the voltage sags. But the likely problem has nothing to do with voltage, just the overcurrent/short circuit protection being set too low for what the motor draws when starting.

maybe some heavier wire from the battery holder to the DC-DC converter would be nice too.

In both these scenarios heavier wire would actually make it worse. The wire is after the protection, so thinner one would reduce both the voltage sag in the cell and the max current draw from it.

[beanflying]

That wasn't meant as a resolution to protection or lack of it, the wire from the holder just looked cruddy Give me decent known LiPos over random 18650's any day too.