nicd charger for li-ion

[SentinelAeon]

Charging liion 18650 batteries with nicd/nimh charger is a big no. But i would like to understand why that is so, in what ways the charging is different for different chemistry.

Let's take a hipotetical scenario where we have a standard 4S pack with a BMS. The BMS has several protection functions, it prevents cells to go under 2.8V and prevents them from going over 4.2V. But the BMS itself has very high current limit protection, far higher than this cells can handle, it has no dedicated charging circuit. And we have a nicd/nimh charger that charges with constant current of 1A.

This scenario would not be adviced but i would like to understand why and what would be the negatives of charging in such a way. I hope you can help me understand this better. Thank you

 

[rjenkinsgb]

The BMS safety cut-out voltage should be slightly higher than the ideal full-charge voltage, it will over-stress the cells.

The correct charger would hold at the correct full charge voltage and allow the current to drop off (to ~10% the capacity, 1/10 C) then shut off until the cells discharge a few percent.

That constant voltage stage is vital for correct, safe, charging.

 

[SentinelAeon]

if we assume that BMS cuts off at 4.2V, then we are left with the other problem, namely that nicd charger usualy ramps up the voltage slowly, while liion will always stay at max voltage and keep constant current. Am i right to assume that if you were to charge liion with nicd charger, that would degrade cells fast ? Would it even charge them ? I can assume that its not safe, but is heat/current/voltage the only concern or is it something else, chemical maybe ?

 

[MrAl]

Charging liion 18650 batteries with nicd/nimh charger is a big no. But i would like to understand why that is so, in what ways the charging is different for different chemistry.

Let's take a hipotetical scenario where we have a standard 4S pack with a BMS. The BMS has several protection functions, it prevents cells to go under 2.8V and prevents them from going over 4.2V. But the BMS itself has very high current limit protection, far higher than this cells can handle, it has no dedicated charging circuit. And we have a nicd/nimh charger that charges with constant current of 1A.

This scenario would not be adviced but i would like to understand why and what would be the negatives of charging in such a way. I hope you can help me understand this better. Thank you

Hi,

There are different types of NiCd chargers and none of them would work with Li-ion. I'll explain the NiCd ones first.

The first type is the current limited NiCd charger. It uses a transformer and current limit resistor. It is crude, but it works. This could not be used with Li-ion unless it was the right voltage AND you monitored the voltage level manually with a voltmeter.

The second type is the dv/dt method NiCd charger. It works by detecting a small drop in voltage once the cell has been fully charged. That would never work with Li-ion because those cells do not exhibit a drop in voltage during the charge phase. It might work to some extent if you monitored the voltage manually with a voltmeter.

There are other types that monitor the temperature of the NiCd cell and stop when the temperature gets to a certain level. Again you could only use that with a Li-ion if you monitored the voltage level manually with a voltmeter.

The other thing those three would need is they would have to have a limited current output. Some would limit to 1 amp others could go much higher. If they went higher than the Li-ion cell the cell could explode and start a really bad fire.

Now charging an Li-ion cell is very different. The charger has to have a limited current to start with, and then once the cell gets close to full charge the charger has to have a limited voltage output. These are two requirements that are critical. If either of them is not observed properly, the cell could explode and catch fire and start a really bad fire.
The current limit cannot go over the limit spec for the cell being charged. The voltage cannot go over the max voltage for the cell which is usually 4.200 volts but some can go higher. You have to know the specs of your cell and the specs of the charger. The current limit and voltage limit has to be correct for the cell. If either is too high it could cause some serious damage to the surroundings. You can find out what happens by looking for some You Tube videos with cells that explode and catch fire. You don't want that it is very dangerous that's why it is always best to use a charger made for the type of cell being charged, and that the limits match the cell. You can however charge at less current and have a lower voltage cutout point, but you cannot go over the specs of the cell.

 

[Nigel Goodwin]

You might have a look here:

Build a Lithium-Ion Battery Charger on Arduino | μF

The tutorial of a DIY Lithium-Ion battery charger implemented on Arduino with several advanced features like state-of-charge estimation, EEPROM logging, command-line interface and more...

www.microfarad.de

Which describes the charging process quite well, and the reasons for why you do it that way - and it's worth checking the source code, to see how he implemented it.

 

[Tony Stewart]

NiCd 1.45 V to 1.6 V charge with C/10 and thermal rise to cutoff and 1.2 V to 1.1 V range for use.
Li Ion is 4.0, 4.1 or 4.2 to charge CC to CV to charge and CC/10 typ to cutoff for 3.7 to 3V for use.

Not even similar.

 

[SentinelAeon]

Thank you for the great explanation guys. I would like to ask some more questions if thats alright. I often find myself with dealing with liion or lipo connected to a circuit and i have to do some modification and cant physicaly unplug the liion or lipo battery. Even though i discharge the battery to about 2.8V, it can still make some nasty sparks if i accidently short it for a moment.

So i know that deeply discharging liion or lipo will permanently damage the cell. But is there any time limit. Would that apply even if i were to deeply discharge it just for 30 minutes in order to do the necesary work without worrying about shorting it ? Im not talking about 0V, but rather 2V where it essentialy has almost no capacity left in it.

 

[Galgso]

So i know that deeply discharging liion or lipo will permanently damage the cell. But is there any time limit. Would that apply even if i were to deeply discharge it just for 30 minutes in order to do the necesary work without worrying about shorting it ? Im not talking about 0V, but rather 2V where it essentialy has almost no capacity left in it.

First of all, there is really no point in discharging to 2v as you will hardly get any extra capacity out of the cell. More on that in a minute. About the time limit, it's more about the voltage than the time. Discharging to a really low voltage like 2v will negatively affect the capacity of the cell, meaning next time you charge it up you will get less amp hours out of it. This will occur regardless of the time it took you to get to 2v; discharging to two volts from 4.2v in 30mins and discharging to 2v from 4.2v over several hours will both decrease the capacity of the cell. It should however be noted that higher discharge currents are typically worse for the cell than lower discharge currents. Getting back to my point about very low discharge cutoff voltages being pointless, take a look at this graph here (link below):

Link: https://www.researchgate.net/figure/Li-ion-battery-discharge-voltage-curve_fig5_363575973

Notice how you will barely get any extra capacity if you go down to 2v versus 3v. It is much better to discharge down to a higher voltage like 3.6v and get a longer lifespan for your cells than try to discharge down to 2v and squeeze every last joule out of the cell at the expense of decreased longevity. Whenever I use Li-Ions I discharge them to no lower than 3.6v.

 

[SentinelAeon]

Thank you for the info. I wasn't thinking about getting more capacity out of it. It was more of a safety measure. I will give you a simple exampe. I have a very small lipo soldered to a mini spot welding board. The traces connecting the battery to the board are extremely thick and extremely short. So in order to desolder this battery, i will have to heat up the traces very fast and lots of heat will be going to the battery. I would feel a lot safer doing that if the battery was at 2V as opposed to 2.8V. Because even if battery has just 10% capacity remaining, its gonna be nasty if it catches fire. Another option would be to not desolder the battery and just work on the spot welding board while the battery is attached, to replace mosfets and such. But in this case again, its possible that i accidently short the terminals with a metal tool or something. And i would feel a lot better the battery is at 2V as opposed to 2.8V. Shorting lipo even at 2.8V will be a lightshow

That is why i was asking if im doing permanent damage to the battery if i discharge it deeply for 20 minutes while i do the work. I ask myself the safe question when spot welding 18650 cells. Even at 2.8V, if you accidently drop battery tab over the cells and make a short, there is gonna be nasty sparks.

 

[Galgso]

Thank you for the info. I wasn't thinking about getting more capacity out of it. It was more of a safety measure. I will give you a simple exampe. I have a very small lipo soldered to a mini spot welding board. The traces connecting the battery to the board are extremely thick and extremely short. So in order to desolder this battery, i will have to heat up the traces very fast and lots of heat will be going to the battery. I would feel a lot safer doing that if the battery was at 2V as opposed to 2.8V. Because even if battery has just 10% capacity remaining, its gonna be nasty if it catches fire. Another option would be to not desolder the battery and just work on the spot welding board while the battery is attached, to replace mosfets and such. But in this case again, its possible that i accidently short the terminals with a metal tool or something. And i would feel a lot better the battery is at 2V as opposed to 2.8V. Shorting lipo even at 2.8V will be a lightshow

That is why i was asking if im doing permanent damage to the battery if i discharge it deeply for 20 minutes while i do the work. I ask myself the safe question when spot welding 18650 cells. Even at 2.8V, if you accidently drop battery tab over the cells and make a short, there is gonna be nasty sparks.

Having to constantly desolder batteries and then solder them back is not really a practical solution. You can get XT90 connectors or similar and just use these to disconnect the batteries when working on the board. It would theoretically indeed be safer if you discharged them to 2v but then you will negatively affect the lifespan of your battery. Better to get a proper connector. Alternatively, get some heavy duty screw terminals and use them instead