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.