Details Of Charging Nicad Cells

[woolyhead 15]

I know this subject has been worked over many times before but I just took an interest in it and thought someone might be interested enough to answer my question. Here's my story so far:- I don't possess a constant current charger so I use a cheap commercial type. which is not very constant current. It isn't constant voltage either but is a voltage source in series with a resistor chosen to give charging currents of about C/10.  It doesn't sense the reduced slope, the dV/dT that occurs when the cells are fully charged. I read in a survey of NiCad chargers that this reduction is very small and can't easily be seen unless the charge current is at least C/3. .The article also said that low cost commercial chargers use a thermal cut out and this is not very good design because by the time the cell gets hot enough to trigger it that's damaging for the cell. They also monitor the charge current and when it falls sufficiently they switch the charge to trickle charge. (This backs up my statement that it isn't a constant current charger). I monitored the individual voltage of every cell as it charged using my commercial charger and was quite surprised to see them go up to 1.4 and 1.5 and in one case 1.635 volts. I had thought they would all stop at 1.35 volts when almost fully charged but still on charge. The sum total voltage is 18.38, equal to the off load voltage from the charger thus proving that it isn't a constant current charger.                                   

My question is can anyone give me a circuit diagram for a constant current charger supplying 12 off D cells, all  in series connection and requiring a constant charge current of C/3 so as to see the full charge dV/dT reduction when it occurs. I would also like to have a circuit for sensing this reduction in dV/dT if possible.

Edited August 17, 2020 by woolyhead

[apelike 10]

You can find plenty on the internet if you do a simple search for constant current charger, a LM317 Voltage regulator IC is the popular choice for building one. If you want adjustable voltage and adjustable current then a L200 will do it, think I use that one in my bence PSU. Ebay also have plenty of prebuilt ones but normally sourced from China so take a while to arrive and there are plenty of tut's on youtube also.

 

 

[woolyhead 15]

Thanks apelike

[Ghozer 112]

if it's just NiCad, don't worry too much, you can constantly trickle charge these, and they kinda have their own protection due to the way the cells are made, and the chemical composition..

That's why they are often used in solar applications like solar garden lights etc, as they can be charged, and wont over-charge etc...

Lithium batteries on the other hand, need a charge controller, to be charged at the correct speed and rate, and not over voltage etc...

You only need around a 20-24v supply, use a 200 Ohm  10+ watt resistor resistor on the positive line, and just connect them in series over the supply....

You may need something to measure the voltage to read if they are charged, or you could create a little feedback loop with an inductor/capacitor and transistor to light an LED when charged..

Edited August 18, 2020 by Ghozer

[woolyhead 15]

Thanks Ghozer.  Your idea would produce a charge current of about 100 mA which is C/10 for a 1Ahr cell.  This is a rather slow charge rate because NiCads can take a higher proportion of C and this makes the charging quicker and more efficient. Since I'm using D cells which have a C value of 4Ahrs  I was really looking for a constant current charger giving about 2A and also a circuit for detecting the negative deltaV point. I looked up those references,  apelike, but couldn't find either such circuit. Most of those shown were not proper constant current supplies.  Ghozer's circuit is almost constant current, independent of the cell voltage, but I need 2A so if I increase the dc voltage to say 100V and use  a 50 ohm resistor I should get 2A, But the wattage in the resistor would be 200W. That seems a bit extravagant in terms of power. If I reduce the voltage to 50V and use a 25 ohm resistor the wattage is then 100W. If I retain the 25V supply and use a 12.5 ohm resistor the wattage is still 50W.  All these options seem like a hammer to crack a walnut.  Since the cell voltage is only about  1.6V or so while on charge at most and only changes by about 0.6V from a starting voltage of say 1V,  it shouldn't need a very big resistor value to make the current independent of cell voltage. Suppose I use 4V supply and a resistor of 1.35 ohms, the wattage would be  about 5.4W. The current would be (4-1)/1.35 = 2.22A at the beginning , falling to (4-1.6)/1.35 =  1.78A.  This is 2A plus or minus 10% at the expense of only 5.4W. in the resistor. What do you think of this scheme? Is the current constant enough for an NDV circuit to work? I think the NDV reduction in cell voltage is of the order of 8mV.

Edited August 18, 2020 by woolyhead

[Ghozer 112]

Your idea about upping the voltage and using a bigger resistor is on track, the other way you could go about it is to create a buck/boost circuit, or make it a switching charger circuit (kinda like a switching power supply) - but i'm not sure how the batteries would take to having rapid switching/partial wave cycle charging etc, so you might need a beefy cap to stabilize or something...

You do seem to be on the right track though  - but I'll have a think - it's not something I have done myself, and not really designed any circuits for a while...

[woolyhead 15]

Looking at many of the circuits shown on the net for giving constant currents, they mean by this term that the CIRCUIT APPLIES A VOLTAGE to the battery and whatever current tries to flow, the circuit limits it, even if the battery voltage drops. If the battery voltage rises the "constant" current no longer stays constant but reduces. That's all very well but the term constant current really means that THE CIRCUIT APPLIES A CONSTANT CURRENT to the battery and this is maintained constant whatever the battery voltage does/is. So whether the battery voltage rises or falls the current remains constant. with a proper constant current charger. There is a clear difference between these two  types of charger. To monitor the negative dV/dT that occurs when the battery is fully charged the current source must be one of the latter kind, a genuine constant current type. Pulse charging is something else and I don't know much about it yet but I think it offers advantages over the dc type in that the battery responds better. I look forward to seeing any ideas you come up with. Since writing this I found the following comment on Battery University about pulse charging:-

"Interspersing discharge pulses between charge pulses is known to improve charge acceptance of nickel-based batteries. Commonly referred to as a “burp” or “reverse load” charge, this method assists in the recombination of gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers. The method is also said to reduce the “memory” effect as the battery is being exercised with pulses.  While pulse charging may be valuable for NiCad and NiMH batteries, this method does not apply to lead- and lithium-based systems as these batteries work best with a pure DC voltage."

 

Edited August 19, 2020 by woolyhead