Why do we need CC/CV for charging Lipo ?

[Bazaki]

In another thread which I can't find I asked the same question, but a new topic won't harm anyone.

People say it is better to charge a Lipo to 4,10v per cell for a longer lifetime.

A lipo charger and even my mastech power supply has the cc/cv function. If we set our charger to 4,10v at 1c per cell the charger will charge to about 80% with full amps, then CV will start and slowly the cell will reach 4,10v and sit there for a while untill the amps are zero.

This last 20% of charging will take a lot of time.

Why don't we just charge to about 4,13v per cell without CV and stop charging when it reaches 4,13v ? The voltage will drop to about 4,10v when we stop charging which we would like to have. I can't see the point of using CC/CV, only when you want your pack 100% full to 4,20v then it is impossible and we need CC/CV for sure. Also for a lead acid battery we need it I think.

Am I missing some information maybe ?

 

[cell_man]

What you're suggesting seems quite reasonable. If you've got a means to monitor the charge/discharge it should be fairly straightforward to prove or disprove. I would say that you can likely get the same amount of charge as you would with 4.1V using CC/CV by just using CC to a slightly higher voltage, but I wouldn't know how much higher voltage that would require. Just by comparing the charged Whrs at the end of the CC stage and the end of a standard CV stage should give you some idea of the percentage added in this stage, plus I've seen lots of charge graphs around that should also give this info.

 

[Bazaki]

I think charging tot 4,13v and cut-off will result to a drop of 0,03v, so that would be to 4,10v.
I charge my 30ah pack with 30amps, so then it will drop more than when I would charge it with 15amp I think. Maybe then it would drop tot 4,11v

But that is what we could find out in practice.

So cc/cv is completely useless for most situations ?

 

[dogman dan]

No genius here, but isnt that slow end of the charge needed because if you do it fast there is lots of possiblitiy of overcharging?. And isn't that when the fun begins? As in Foooom? Or if no fireworks with todays lipo, at least you ruined em with an overcharge?

 

[Bazaki]

I talk about lipo that can have 4,20v per cell. So 4,13v is far away from any danger.

 

[AussieJester]

I would be charging to 4.15v your still well under the limits of the 4.20v a cell so your pack life will
benefit plus you get that lil more range...

KiM

 

[Bazaki]

Any thoughts from other people maybe ?

It is now a few months later and I still wonder why we need intelligent Lipo or CC/CV chargers.

It would be much faster also, charge with full power to 4,13v per cell and cut of the power, a few seconds later all cells are nicely 4,10v

Can convince me that I'm wrong or that I forget something ?

 

[hydro-one]

seems very reasonable. I do this when i want to get max amps out of my meanwell. turn the voltage up, so that cc stays on longer. then manually cut the power at cellog alarm of 4.20. then it drops down to about 4.12. only problem with this is its manual. no redundancy in saftey if i forget it, pack iz toast!!

i have ruined some lipos this way. so now i just set and forget at 4.15 per cell and put up with the slower end of charge.

mike

 

[liveforphysics]

Only the SOC of the chemistry matters, so if the Ri*I charging voltage swing pulls the cell up to 4.4v (just an example), it makes no difference to the cell, as long as it's state of charge is still down at something like 4.1-4.2 etc.

They make chargers like this, it's just called CC charging, and the charger simply needs to interrupt at some interval, wait a moment for the capacitance of the cell to reach equilibrium with the SOC voltage, sample the SOC voltage, and decide to continue CC charging if it's not yet reached the cut-off threshold.

The reason CC/CV is used is about 50% for safety/simplicity, and 50% because the cell stops having as many active surfaces for lithium ions to nest into the carbon (kinda like a game of musical chairs where the number of chairs keeps getting reduced), so you increase the number of metallic lithium formation when ions are energized by charge current to cross the separator but sit on the lap of another li ion rather than the carbon.

When I'm rushing to charge a pack, and watching it, I crank up the power supply voltage to 4.5v/cell (which puts it back into CC), and just click it off every minute or so to check. NOT recommended for newbs. NOT recommended for failure prone cheesy china chargers. Fine for an expert user to reduce charge time at the expense of a little cell life.

 

[dogman dan]

Nice post. Even makes sense to me. I had the feeling it was not a you can't thing so much as you don't want to try to manufacture and liability insure such a charger.

 

[Bazaki]

I also set my powersupply to about 4,50v per cell for Lipo but only when I am watching the charging process.
Or I have my cell log connected and will beep when it reaches 4,18v per cell. I won't leave the charger.

But if we build an LVC from the Celllog or other device than we could charge a lipo with full power till 4,18v per cell and the power will cut-off, the voltage will drop a bit to let's say 4,15v per cell.

 

[rscamp]

The charge rate also affects what SOC you will end up with. At high rates, the battery will end up less 'full' at a given CC cutoff voltage. Whereas with CC-CV the battery will end up at the same SOC at termination regardless of the charge rate during the CC portion.

 

[liveforphysics]

The charge rate also affects what SOC you will end up with. At high rates, the battery will end up less 'full' at a given CC cutoff voltage. Whereas with CC-CV the battery will end up at the same SOC at termination regardless of the charge rate during the CC portion.

If your charger is setup to terminate at some fraction of charge current, then yes. If you're stopping the charge current to sample the cell voltages, then if the cell voltage is at 4.XXv after 1sec or so (for low Ri cells), then it's got the same SOC as if you spent 10 hours slowly creeping it up the same voltage.

Again though, this is NOT a good idea for newbs, and I've even taken packs to 4.3-4.4x by mistake by getting interrupted by phone calls etc while charging at 5-10c rates (because even just missing the window by 30seconds can make a BIG difference when the total charge time is 6-10mins.)

Note to newbs: Just forget you ever heard of CC charging.

Note to experts: Pay close attention and interrupt to sample voltage often if CC charging.

Note to guys with programming guru skills: Make a little arduino (or whatever) board that interfaces with a meanwell charger to interrupt charging at perhaps 15 second intervals, sample pack voltage after waiting 1 second, and then re-activate the charger if the voltage is below your termination SOC voltage. Then you can just dial in your meanwell to something like 5v/cell, and CC charge all the way to your desired SOC voltage (ie, 4.15v resting voltage), and it cuts the charger off.

 

[rscamp]

The charge rate also affects what SOC you will end up with. At high rates, the battery will end up less 'full' at a given CC cutoff voltage. Whereas with CC-CV the battery will end up at the same SOC at termination regardless of the charge rate during the CC portion.

If your charger is setup to terminate at some fraction of charge current, then yes. If you're stopping the charge current to sample the cell voltages, then if the cell voltage is at 4.XXv after 1sec or so (for low Ri cells), then it's got the same SOC as if you spent 10 hours slowly creeping it up the same voltage.

I was referring to CC charge termination at a preset voltage - i.e. no current tapering and no zero-current sampling.