# What does CC, CV and dV/dt stand for?



## Mr. Nobody (Jan 11, 2015)

As the title states, curious to know what these mean: CC, CV and dV/dt ?

MY Nitecore charger mention these Legends, but do not explain there technical meaning. I cur

rently have two 18650 nitecore 3400mAh batteries in the charger. One says it is CC the Other is CV.


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## mvyrmnd (Jan 11, 2015)

Mr. Nobody said:


> As the title states, curious to know what these mean: CC, CV and dV/dt ?
> 
> MY Nitecore charger mention these Legends, but do not explain there technical meaning. I cur
> 
> rently have two 18650 nitecore 3400mAh batteries in the charger. One says it is CC the Other is CV.



CC/CV = Constant Current / Constant Voltage

The battery is charged to ~80% at a set current - eg 1 Amp. The charger then switches to charge at a set voltage - eg 4.2V and the battery absorbs whatever current it can at those volts.


dV/dt = Delta Voltage / Delta Time (read: Change in voltage / Change in time)

I'm no expert in this, so I'll let someone else explain


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## HKJ (Jan 12, 2015)

When charging LiIon batteries you first feed them a constant current (CC), when the voltage reach maximum (4.2 volt), you hold the voltage constant (CV) and wait until the current drops to termination current, before stopping the charger.







The -dv/dt is a method to detect when a NiMH battery is full. -dv is voltage drop and dt is time, i.e. a voltage drop over time.






On the above curve you can see the red voltage line has a small drop at the end, it is the -dv/dt and signals the battery is full.


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## StorminMatt (Jan 12, 2015)

CC/CV refers to a constant current, constant voltage charging algorithm. Most of the charging is accomplished by sending a constant current through the cell. But once a specified voltage is reached, the charging changes to constant voltage to top off the battery. When the battery is topped off at a constant voltage, the charging current decreases as the batterybis charged until it drops to a sufficiently low value that the charge is terminated. All Li-Ion chemistries are charged in this manner, including LiFePO4 cells (which actually discharge similar to NiMH). Lead acid batteries also charge by constant current, constant voltage.

dV/dt refers to the rate of change of voltage. This is a relevant quantity when it comes to the termination of the charging of NiMH batteries. Like Li-Ion, NiMH batteries start off charging at constant current. But unlike Li-Ion, there is no switch to constant voltage charging. Rather, charging is always at constant current until termination. As a NiMH battery is charged at constant current, the voltage will increase, reach a peak, and then decrease to a lower level. When the voltage starts decreasing, the battery is being overcharged. So the charging circuitry looks at the rate of change of the voltage (or simply the change in voltage). When charging voltage starts to decrease, charging is finished.


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## Grijon (Jan 12, 2015)

Oh my goodness, y'all are awesome!

lovecpf


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## chillinn (Jan 12, 2015)

What is the relationship between -dV/dt and cell temperature? Is there one? If not, I was completely misinterpreting -dV/dt before this thread.

Another question not worth a new thread... I have the analog MM you threw away as a child. The NiteCore D4 has a voltimeter built-in, of course. According to the manual, when you put the cell on the charger, it doesn't start charging for 6 seconds but applies something to "wake" the cell. But I noticed it immediately gives a digital voltage reading in the hundreths, far more accurate than my lil analog MM. Is it ok to use this as a rechargeable cell, battery tester (pop cell on for a second to check the voltage, pop it off)? (no worries, I wouldn't put any primary on there even for a second)

Ug, few more... if time is not an issue, is it always best to charge at the lowest current available, for (healthy, new, well-behaved, NOT over-discharged) NiMH or Li-ion of any size? How common is it the cell would miss termination at a low current (because ... temperature spike at the end of charge doesn't get high enough for detection) and overcharge? How bad is overcharge?


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## HKJ (Jan 12, 2015)

chillinn said:


> What is the relationship between -dV/dt and cell temperature? Is there one? If not, I was completely misinterpreting -dV/dt before this thread.



Many people says it is the temperature raise that is reason for the voltage drop.



chillinn said:


> Another question not worth a new thread... I have the analog MM you threw away as a child. The NiteCore D4 has a voltimeter built-in, of course. According to the manual, when you put the cell on the charger, it doesn't start charging for 6 seconds but applies something to "wake" the cell. But I noticed it immediately gives a digital voltage reading in the hundreths, far more accurate than my lil analog MM. Is it ok to use this as a rechargeable cell, battery tester (pop cell on for a second to check the voltage, pop it off)? (no worries, I wouldn't put any primary on there even for a second)



As long as the chemistry is supported in the D4 it is fine.



chillinn said:


> One more... if time is not an issue, is it always best to charge at the lowest current available, for NiMH or Li-ion of any size?



No, with NiMH the termination may fail.
With LiIon you may apply a little bit less wear during charge, but you may finish at a higher voltage and that will increase the wear on the cell.


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## chillinn (Jan 12, 2015)

HKJ said:


> No, with NiMH the termination may fail.
> With LiIon you may apply a little bit less wear during charge, but you may finish at a higher voltage and that will increase the wear on the cell.



So the size of the cell determines what voltage range in which one should charge a cell? Only use the low voltage (150mA) for AAA/10440? or should I just drop the cells on there and let the charger decide (1-2 cells, 750mA I think, 3-4 cells, 375mA)?

HKJ -thanks for your reviews... chose D4 as my first "hobby" charger, and set aside desire for an analyzer for now, after viewing and viewing again and again your site reviews before pulling the trigger. I am cat with å canary in ma belly.


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## HKJ (Jan 12, 2015)

chillinn said:


> So the size of the cell determines what voltage range in which one should charge a cell? Only use the low voltage (150mA) for AAA/10440? or should I just drop the cells on there and let the charger decide (1-2 cells, 750mA I think, 3-4 cells, 375mA)?




With a maximum current of 750mA you only need to reduce current when smaller LiIon cell (Check table in my review), NiMH can take the full current, even AAA.


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## chillinn (Jan 12, 2015)

HKJ said:


> With a maximum current of 750mA you only need to reduce current when smaller LiIon cell *(Check table in my review),* NiMH can take the full current, even AAA.



_shi shi ni!!!*_

*Once upon a time, I dated a Manderin princess, a polyglot, and this is all I can say :/


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## gallon (Jan 12, 2015)

chillinn said:


> *So the size of the cell determines what voltage range in which one should charge a cell?* Only use the low voltage (150mA) for AAA/10440? or should I just drop the cells on there and let the charger decide (1-2 cells, 750mA I think, 3-4 cells, 375mA)?
> 
> HKJ -thanks for your reviews... chose D4 as my first "hobby" charger, and set aside desire for an analyzer for now, after viewing and viewing again and again your site reviews before pulling the trigger. I am cat with å canary in ma belly.



NO!

The charging voltage is related to the type of chemistry.

Charging current can be related to the size of cell.


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## Gauss163 (Jan 12, 2015)

mvyrmnd said:


> dV/dt = Delta Voltage / Delta Time (read: Change in voltage / Change in time)
> 
> I'm no expert in this, so I'll let someone else explain



The notation dV/dt comes from calculus. It denotes a derivative. While charging we can plot the voltage V(t) as a function of time. Then the derivative V'(t) = dV/dt denotes the instantaneous rate of change of voltage. More graphically, V'(t) denotes the slope of the tangent to the curve at time t. It's the limit of the slope of the secant line on the curve between the point t and t+h as the time delta h goes to 0. As an analogy, if we instead plotted D(t) = distance vs time of a car traveling along a straight route then the derivative D'(t) is simply the speed at time t.

A voltage vs. time charge graph of a NiMh/NiCd charge has a characteristic change in shape when the cell becomes full. This can be detected as a change in slope of the tangent dV/dt. For example, below is a description from one of Maxim's app notes. There are also variations on this, and other termination methods, e.g. see here.


APPLICATION NOTE 4496
* NiMH/NiCd Switchmode Battery Charger Has dV/dt Charge Termination*

[...] Several techniques are available for deciding when a NiCd or NiMh battery is fully charged. The most common of these relies on terminating the charge when the battery terminals reach a particular voltage level, based on a characteristic increase in the positive slope of voltage versus time.

This is not the best method, because the absolute value of termination voltage depends strongly on the ambient temperature and the charge rate (the "C" rate). The final result can therefore be an under- or over-charged battery. Overcharging a NiCd or NiMh battery is not as serious as for lithium batteries, which are much more sensitive to damage. NiCd and NiMh batteries are more rugged devices. Undercharging is a problem, simply because the store of charge will be less than expected.

Most of the better termination methods rely on the fact that charging transforms electrical energy into stored (potential) chemical energy. Charging is an endothermic process. That means the battery temperature not only doesn't rise; it actually falls slightly during a charge.

When a battery reaches full charge, the reactions that transform electrical energy to chemical energy cease. Any further electrical energy forced into the battery by the charger transforms to heat, which increases the battery temperature. At that point charging should stop, because the battery has stored 100% of its capacity.

You can sense this temperature increase and use it as a signal for termination of charging, but that measurement implies a thermal sensor in intimate contact with the battery—not always a feasible arrangement. You can also sense the temperature increase by changes in the battery's terminal voltage, which is a sensitive indicator of internal temperature changes. Thus, charging a battery produces a positive slope in the plot of voltage versus time. The positive slope turns negative when a NiCd battery reaches full charge, and goes to zero (flat) when a NiMh battery reaches full charge.

*Figure 1* shows the terminal voltage vs. time for a NiCd battery under charge. The time scale for change of slope, which can range from minutes to tens of minutes according to the battery size, depends on the thermal time constant of the battery and its enclosure. It also depends on the charge rate (i.e., the charging current), because the temperature increase and its rate of increase are functions of the battery's thermal capacity and of power delivered (which, in turn, is a function of charging current).






To detect slope changes, the charge controller must run a detection algorithm that makes sequenced voltage measurements at long time intervals and then stores the results for comparison. This capability, which cannot be implemented in analog form, must be performed by a combination of ADC, memory, timer, and sequencer.


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## gallon (Jan 12, 2015)

Gauss, that was excellent.


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