nicd charger for li-ion

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.

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 ?

SentinelAeon said:

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

Click to expand...

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.

You might have a look here:


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.

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.

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.

SentinelAeon said:

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.

Click to expand...

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.

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.

SentinelAeon said:

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.

Click to expand...

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

I totaly agree that those connectors are great option, but sometimes its not possible due to size constraint, or worse, you already are in a situation and have to deal with it.

I will have to do some modifying of a pocket battery spot welder and that lipo soldered to the board is making me really uneasy. If i want to remove it, i have to desolder it, which will seriously heat the battery - if battery has juice, there is potential for fire. On the other hand, if i dont remove it, i have a short hazard while modifying, where again, if battery has juice, its dangerous. So either way you look at it, it would be great if battery was without juice. But as far as i understood, even if i drain it to 2V for a very short amount of time, damage is permanent.

Today i was doing on 18650. First i had to disasembly the pack that was welded together. Even though the batteries voltage was very low, i still managed to get the sparks. The fact that all around the positive terminal we have negative casing, with some puny plastic between didnt help. My pliers barely touched the negative part of the case and managed to get through 2 layers and made a spark.

SentinelAeon said:

I totaly agree that those connectors are great option, but sometimes its not possible due to size constraint, or worse, you already are in a situation and have to deal with it.

I will have to do some modifying of a pocket battery spot welder and that lipo soldered to the board is making me really uneasy. If i want to remove it, i have to desolder it, which will seriously heat the battery - if battery has juice, there is potential for fire. On the other hand, if i dont remove it, i have a short hazard while modifying, where again, if battery has juice, its dangerous. So either way you look at it, it would be great if battery was without juice. But as far as i understood, even if i drain it to 2V for a very short amount of time, damage is permanent.

Today i was doing on 18650. First i had to disasembly the pack that was welded together. Even though the batteries voltage was very low, i still managed to get the sparks. The fact that all around the positive terminal we have negative casing, with some puny plastic between didnt help. My pliers barely touched the negative part of the case and managed to get through 2 layers and made a spark.

Click to expand...

Hi,

You could try using some of that cloth electrical tape to cover the case end when soldering. That might help. The cloth type would be better than the vinyl type. I ran into this a couple times too with various types of cells. If the solder bridges over or if the soldering tip bridges the gap on the board or on the cell or on any wires to the cell, it could be a big problem.

From what I remember the lowest voltage that is deemed safe for the cell is 2.5 volts, although the higher you stay the better. If two or more cells are in series you have to be certain that they are all individually limited like that.

It sounds like you also want to use a battery that is soldered to a PC board by unsoldering it and that is difficult. If you do not need the PC board you can try cutting the board where the cell is so that the cell and the board section become separate from the main board. This way you can use the stubby traces left on the cell board to solder to. This is only if you do not need the main PC board anymore because once you cut the cell away there will be a big square hole in the board and all the traces in that area will be cut. The smaller board will still have heavy traces to the battery so you can solder to the traces rather than the battery. Of course this also has to be able to fit in your new application for the battery.

Btw, not sure if i told you why i was asking u about nicd charger for liion. A good friend has an old electric tool, it was quite expensive when new (about 500$), not sure how much its worth now, but clearly a friend is very fond of it. I owe him big, so i wanted to do a nicd to liion conversion of that tool for him, since i got tons of 18650 20A cells (tool originaly has sub c nicd cells). Conversion itself is really easy, the problem is the charger. The original charger is really nice, charges with constant 4A and has an active fan blowing through the battery pack. So i was thinking how i could use the original charger with the liion batteries. Sadly, due to difference between nicd and liion i couldnt come up with any good solution, except maybe opening the charger, removing the existing circuit and replace with liion circuit.

SentinelAeon said:

Btw, not sure if i told you why i was asking u about nicd charger for liion. A good friend has an old electric tool, it was quite expensive when new (about 500$), not sure how much its worth now, but clearly a friend is very fond of it. I owe him big, so i wanted to do a nicd to liion conversion of that tool for him, since i got tons of 18650 20A cells (tool originaly has sub c nicd cells). Conversion itself is really easy, the problem is the charger. The original charger is really nice, charges with constant 4A and has an active fan blowing through the battery pack. So i was thinking how i could use the original charger with the liion batteries. Sadly, due to difference between nicd and liion i couldnt come up with any good solution, except maybe opening the charger, removing the existing circuit and replace with liion circuit.

Click to expand...

Exactly!

SentinelAeon said:

A good friend has an old electric tool, it was quite expensive when new (about 500$), not sure how much its worth now, but clearly a friend is very fond of it. I owe him big, so i wanted to do a nicd to liion conversion of that tool for him

Click to expand...

The safest way is to use some standard 18V (20V) power tool battery, that has its own, safe, charger.
You can buy adapters for some types.

I designed my own to suit the cheap Aldi batteries; the can be screwed to the upper half of the old NiCd battery casing & that fixed in place, so you have an interchangeable lithium pack.

eg.

A nice mod you did. But as far as i understood the friend, he really wanted to keep the tools as is, at least from the outside. I ordered some aliexpress sub c batteries. Were supposed to be 2200mAh, in reality they were 850mAh. Got half the money back, but im pretty sure he will say capacity isnt enough. And reputable sub c batteries in my country are of insane price, i would prolly pay 100$ for 2 packs of this ancient technology. So yeah, disasembling the charger would prolly be the best way.

Btw, i really like this charger of his. Batteries are 12x sub c. It charged at 70W, so over 4A. Now, without cooling the batteries would get very hot and it would start charging. But the battery has holes where it attaches to a charger so a fan blows air through. And at 4A charging with 2 batteries, you basicaly never run out. You use 1 battery while speed charging the second battery. Its really neat. And im pretty sure the batteries arent rated for 4A charging.

Im wondering what would happen if you used the same aproach to charging liion batteries. Lets say battery is rated for 1C charging at 2A, but you charged at 4A, cooling it and monitoring the temperature. Its not healthy for the battery, but is it safe ? Liions are nasty.

SentinelAeon said:

Im wondering what would happen if you used the same aproach to charging liion batteries. Lets say battery is rated for 1C charging at 2A, but you charged at 4A, cooling it and monitoring the temperature. Its not healthy for the battery, but is it safe ? Liions are nasty.

Click to expand...

You need to take extensive precautions while charging Li-Ion batteries, there's plenty of data out there about it. But basically, if you're fast charging, you only partially charge, and you also carefully monitor the battery temperature while doing so, and while maintaining all the other charging parameters.

Personally, I always build slow and steady charging circuits, and charge the batteries fully - in our products long battery life is more important than fast recharging.

In one of the products, I designed the PCB with space for two current measuring resistors (in parallel), it has a single 0.1 ohm resistor fitted, with the option for fitting a second and doubling the charging current - but I've never had occasion to try that.

We buy all our batteries from 'proper' UK sources, one is here:


As well as building new products, we also repair various products, and manufacture replacement battery packs where the manufacturers have discontinued them (where we can).



Here's a partially filled box ready for dispatch later in the week, when the rest are finished.





Some more, partly built, We get through a LOT of batteries

Some ref material, attached. om charging and safety.