Lets talk about CC CV ....

[EasyGoing1]

So as Ive stated in other threads, I've been wanting to build a LiIon charger with an Arduino ... just because. Really though, I want to understand the process and get a better feel for battery technology in general.

When I started googling on this topic about a year ago, I saw a lot of the CC CV stuff being thrown around. Constant Current and Constant Voltage power supplies are not something that I've ever messed with.

It also took me a while to realize that in order to maintain a constant current, the power supply must adjust voltage - or else Ohm would be made a liar ... and we know he's not a liar.

And maintaining a constant voltage is not as difficult as maintaining a CC would be - in my opinion.

Charging a LiIon battery ... lets assume that the cell is a 4.2V cell and lets pretend that the C value of the battery is 3. Therefore, I can charge it at a CC of 1.5 amps or less and feel safe about it.

What I've been kinda struggling with, is in the situation where the battery may be kinda low at 3.2V and in order to get the current up to 1.5 I might have to apply a voltage across the battery HIGHER than 4.2 ... but everything I've read says that applying a voltage higher than its rated for is bad juju ...

Can someone comment on this? Because it seems contradictory to me that you would have to violate the batteries own laws in order to stay with spec of the battery during charging.

[stj]

Re: Lets talk about CC CV ....

if your not wanting to use a dedicated chip, take a look at the old L200 regulator.

[EasyGoing1]

Re: Lets talk about CC CV ....

if your not wanting to use a dedicated chip, take a look at the old L200 regulator.

Im not building this because I need one, im building it to UNDERSTAND everything involved in actually charging a LiIon ... its for knowledge, not function.

[eccerr0r]

Re: Lets talk about CC CV ....

Actually that is the problem with Li-Ion. You NEVER charge with a voltage over 4.2V - even if it's not drawing enough amps to charge quickly. So don't expect that 1.5 amps to charge that battery, likely the outcome is to throw that battery out and buy a new one.

It's possible to build a CVCC battery charger with a LM317 and a transistor. The transistor will do the current limiting and the LM317, the voltage limit.

This is an example of probably the simplest LM317 charger: http://danyk.cz/li-ion_en.html . Completely analog, no microcontroller needed. Only thing this lacks is charge completion.

[EasyGoing1]

Re: Lets talk about CC CV ....

Actually that is the problem with Li-Ion. You NEVER charge with a voltage over 4.2V - even if it's not drawing enough amps to charge quickly. So don't expect that 1.5 amps to charge that battery, likely the outcome is to throw that battery out and buy a new one.

It's possible to build a CVCC battery charger with a LM317 and a transistor. The transistor will do the current limiting and the LM317, the voltage limit.

This is an example of probably the simplest LM317 charger: http://danyk.cz/li-ion_en.html . Completely analog, no microcontroller needed. Only thing this lacks is charge completion.

From that link, "Stabilization of current is not as critical as the stabilizing voltage, so it is sufficient to stabilise it by a shunt resistor and NPN transistor"

Thats useful information.

Im kinda thinking that the internal resistance characteristics of the battery is the dictating factor on how much current its going to consume...

So it sounds like this statement would be accurate concerning the charging of a LiIon battery:

The constant current stage of charging must never allow the current to exceed one half of C, while also ensuring that the voltage applied to the battery never exceeds its rated value.

So if we start charging the battery and lets say we start out at 3.4 volts ... but at 3.4 volts, its only drawing .2 amps but rated for charging at 1 amp, then we increase the voltage going to the battery until one of two things happens:

We hit the max rated current for charging - at which point we stop raising voltage and only adjust it to maintain that current until we reach 4.2V ...

ok this is beginning to make sense now ...

[stj]

Re: Lets talk about CC CV ....

i hope your working on a paving stone or a large ceramic tile incase you set one of these things off!!!

[eccerr0r]

Re: Lets talk about CC CV ....

Indeed. Improvising lithium ion chargers is the same as working with explosives here

[EasyGoing1]

Re: Lets talk about CC CV ....

i hope your working on a paving stone or a large ceramic tile incase you set one of these things off!!!

You should see my breadboard while working on an LM317 project ... the plastic in about three rails was liquid ... but only for a few minutes ... lol

[EasyGoing1]

Re: Lets talk about CC CV ....

Indeed. Improvising lithium ion chargers is the same as working with explosives here

This photo was texted to me about an hour ago by my good friend ... he took it last night and yes, that is his back yard ...

[eccerr0r]

Re: Lets talk about CC CV ....

I think the worst liquefying experience I had was on my digitally controlled CC/CV PSU (in this case as it's a bench PSU, reaching 0V is a consideration...) where my gate drive was not quite high enough to fully saturate the transistor coupled with the tiny heatsink. I never measured the temperature of the MOSFET and it melt its insulator.

Fortunately the heatsink was isolated and the insulator wasn't technically needed, so fortunately no shorts and no collateral damage.

[EasyGoing1]

Re: Lets talk about CC CV ....

Fortunately the heatsink was isolated and the insulator wasn't technically needed, so fortunately no shorts and no collateral damage.

I still dont know why mine melted like it did ... Sometimes the wiring on the breadboard gets a little out of hand and there may have been a short somewhere is all I can think of ...

[stj]

Re: Lets talk about CC CV ....

the center pin on the lm317 is physically part of the rear tab!!

[EasyGoing1]

Re: Lets talk about CC CV ....

the center pin on the lm317 is physically part of the rear tab!!

Yes, I'm well aware of this ... the back tab was exposed to open air.