What is the difference between CC, CV, and -dV/dt?

6 posts / 0 new
Last post
passive101
Offline
Last seen: 5 days 18 hours ago
Joined: 05/01/2019 - 04:11
Posts: 55
What is the difference between CC, CV, and -dV/dt?

I just got a Nitecore UMS4 charger. I have so far noticed that my Eneloops charge with -dV/dt, and my protected 18650 is charging with CC

hiuintahs
Offline
Last seen: 2 hours 58 min ago
Joined: 06/15/2014 - 12:24
Posts: 73
Location: Utah

Lithium Ion batteries charge with CC / CV. That is constant current followed by constant voltage. The way that is accomplished is for the charger to charge with a fixed current until the voltage on the battery climbs to 4.20v and then hold the voltage at 4.20v until the current decays to about 10% of the original charging current.

I guess you could charge an NiMh battery the same way only replace the voltage with 1.50v. But that is not the proper way to charge NiMh batteries. There is a characteristic known of NiMh batteries where the voltage on the NiMh battery actually drops a little when fully charged. That is known as the change in voltage over time (dv/dt). The value is in mili-volts and the magnitude kind of dependent on the charge current. So you hold a constant current until you see that phenomena on an NiMh battery. Some chargers will then hold the charge current for a few minutes past that point just to make sure. Once you reach dv/dt on an NiMh battery and if you continually charge……….the charge current gets changed into heat and depending on the charge current such as 1 amp……..it will eventually get real hot. Thus its a good idea to have temperature sensing as a protection means in the event dv/dt wasn’t detected.

NiMh charging is a little more difficult than the CC/CV charging for a lithium ion. HKJ published a report on a variety of NiMh chargers and you can see how these manufacturers went about it. They are not all the same but lots of similarities.

https://lygte-info.dk/info/batteryChargingNiMH%20UK.html

Pete7874
Pete7874's picture
Offline
Last seen: 5 hours 13 min ago
Joined: 11/23/2011 - 16:47
Posts: 2548
Location: USA

hiuintahs wrote:
There is a characteristic known of NiMh batteries where the voltage on the NiMh battery actually drops a little when fully charged.

AFAIK, it doesn’t drop, but it stops rising or begins rising extremely slowly, which means very little change in voltage over time. That is how the charger knows that it’s time to terminate.

Voltage only starts dropping when charging current has been stopped.

hiuintahs
Offline
Last seen: 2 hours 58 min ago
Joined: 06/15/2014 - 12:24
Posts: 73
Location: Utah

Pete7874 wrote:
hiuintahs wrote:
There is a characteristic known of NiMh batteries where the voltage on the NiMh battery actually drops a little when fully charged.

AFAIK, it doesn’t drop, but it stops rising or begins rising extremely slowly, which means very little change in voltage over time. That is how the charger knows that it’s time to terminate.

Voltage only starts dropping when charging current has been stopped.

No it actually does drop a little when charging current is constant. You’ll notice that the term isn’t dv/dt but actually -dv/dt as noted in the title of this thread. I designed an NiMh charger and had to understand -dv/dt so as to be able to write the software to detect it. It’s not much of a drop but it’s there and then the voltage may rise just slightly again. But at that point it won’t rise much further at all and turns the charge current into heat. When developing this charger and trying to get my detect point accurate, I missed a termination and saw what happens to the battery. Voltage did not go any higher or possibly just a couple of hundredths of a volt and the battery got hot. Mostly during the charging process the voltage is climbing. But when at full charge the voltage stops climbing and actually drops a tiny bit.

If you look through the graphs of HKJ’s document on NiMh charging you can see it. It’s obvious that not all chargers have this technique down well.

Pete7874
Pete7874's picture
Offline
Last seen: 5 hours 13 min ago
Joined: 11/23/2011 - 16:47
Posts: 2548
Location: USA

Thanks for clarifying.

passive101
Offline
Last seen: 5 days 18 hours ago
Joined: 05/01/2019 - 04:11
Posts: 55

Neat. That’s interesting how they have to do that to charge them up.