CV Charging Sucks?

[Username1]

In my opinion the standard charging method for l-ion should be CC instead of CC/CV. If you think I'm wrong please read this and tell me why because I must be missing something.

Constant current (CC) gives a linear increase in charge over time. Once max voltage is reached, you end up with a nearly full battery. How full depends on the c-rate used during charging, but I believe it's usually in the neighborhood of 80-90%. So why not ALSO use constant voltage (CV) to add the last 10-20%?

Well firstly it takes a very long time, CC is linear but CV isn't. This tradeoff in time is reason enough to consider skipping CV, but there's more... People often mention the benefits for battery lifespan by charging to a reduced level like 80-90%. Well this basically lines up with the state of charge you get by skipping CV.

So people are spending huge amounts of time to get the final 10-20%, while at the same time greatly reducing their battery lifespan. Even more stupid is people charging to a reduced voltage (for battery health) AND doing the CV phase... Wouldn't it be much faster to just charge to 4.2v and skip the CV phase to get a similar result?

In conclusion, CV users are wasting time and battery health in exchange for a measly 10-20% capacity. I can't see a justification for CC/CV being the standard. It only seems justified in very niche cases, but for the vast majority CC alone should be the standard in my opinion.

Spoiler

Edit: The very first reply was merged in from a similar old thread, where I asked about CC/CV vs CC-only with a higher cutoff voltage to achieve the same resting voltage.

 

[john61ct]

Your general theory is correct, there are many dozens of charging profile / methods that will result in the same exact Ah capacity utilisation / SoC%

However how they correlate specifically and the impact on longevity vs fire risk will vary a lot by product line / chemistry.

The ending voltage at rest is not a precise benchmark, since higher V may just be surface charge, not much usable mAh added.

CC-only stopping via HVC at 4.15V is ideal, no advantage filling past that, easy to automate with any charge source, any lower does not extend longevity. Charge rate should be above 0.2C or drop to 4.10V

However a more influential care factor is not your charge termination point as much as not letting them SIT at high SoC.

Only top up above 3.7-3.8V just before you are going to discharge.

Overnight maybe not a huge impact, but days certainly are, even dangerous as the packs wear.

If you are running very high C-rates, or fast charging in cold weather, ir storing in hot ambients, then fuggedaboudit, they won't last that long, the above will be within statistical error rates.

 

[Squilt]

Battery voltage jumps a bit when you start charging. It jumps by more with more current. If someone tried to charge their battery all the way with CC, charging would stop when the inflated voltage reaches the target and then the voltage would drop back down after current stops being applied and the battery wouldn't really be full.

You could keep charging but then the voltage would jump back up again and could go past what the battery is rated for, damaging the cells.

 

[Username1]

Battery voltage jumps a bit when you start charging. It jumps by more with more current. If someone tried to charge their battery all the way with CC, charging would stop when the inflated voltage reaches the target and then the voltage would drop back down after current stops being applied and the battery wouldn't really be full.

You could keep charging but then the voltage would jump back up again and could go past what the battery is rated for, damaging the cells.

That's correct.

Many CC/CV chargers can be set to a lower max voltage such 4.1v or 4.15v per cell, in order to prolong battery health. Instead of lowering the max voltage, you can simply set it to the full 4.2v and it will naturally charge to around 80-90% while charging much faster by skipping CV.

 

[Squilt]

That's correct.

Many CC/CV chargers can be set to a lower max voltage such 4.1v or 4.15v per cell, in order to prolong battery health. Instead of lowering the max voltage, you can simply set it to the full 4.2v and it will naturally charge to around 80-90% while charging much faster by skipping CV.

Well then it's just an argument about charging to 100% or not. CV is just the only good way to do that. I agree with you by the way, I don't intentionally charge li-ion past 80%. But I understand why people would.

 

[Username1]

Well then it's just an argument about charging to 100% or not. CV is just the only good way to do that. I agree with you by the way, I don't intentionally charge li-ion past 80%. But I understand why people would.

Sure it's partly about advocating for lower states of charge, but it's also about NOT using CV as the standard charging method for the masses.

The ONLY purpose of CV is to cram 100% charge into battery. Manufacturers from cars to phones all recommend a lower state of charge. The exact percentage is somewhat arbitrary, there's nothing magical about 80% for example, but lower is better.

My point is that the standard charging protocol should be CC only, which provides much faster charging and a lowered state of charge in the (very reasonable) 80-90% region. CC/CV should be a NICHE charging method since it greatly increases charging time, sacrifices lifespan, and can cause rapid degradation for batteries stored at 100% by uniformed people.

Imagine CC was already the standard and you told people you have a charger which increases their charge 10-20%, but cuts their battery lifespan in half, and takes 2 extra hours to charge. It would sound very niche and unreasonable.

 

[eee291]

You might be able to achieve this effect with those chargers that have 3 potentiometers inside, ones is for low current cutoff (.25A or so).
I wonder how high the value can be set. If you could crank it up to ~90% of CC that would be pretty much do it.

 

[Username1]

You might be able to achieve this effect with those chargers that have 3 potentiometers inside, ones is for low current cutoff (.25A or so).
I wonder how high the value can be set. If you could crank it up to ~90% of CC that would be pretty much do it.

Sure but what's the point, to add a tiny bit of extra charge while being only a little slower than CC?

Why not just do away with CV entirely for the average person/device?

 

[eee291]

I was just suggesting a method of how one might be able to avhieve a 80% cutoff without having to unplug the charger when the battery reaches SOC.

Why not just do away with CV entirely for the average person/device?

Because of range (anxiety) obviously, just look at the way EVs are marketed. Almost no one mentions durability or lifespan nearly as much as range.
As for other devices like smartphones and laptops, by the time the battery degrades to a noticeable degree it'll already have been replaced by a newer model anyways.
Lithium batteries will also degrade when using them at elevated temperatures. Look at the lifespan of active cooled batteries vs natural convection.

 

[neptronix]

I sometimes want that last 10% for a long journey.

 

[Hillhater]

Do you know of any (reputable) chargers that are CC only ?
i have numerous lithium chargers , RC, Ebike , Car 12v LiFe, etc…and they are all CC/CV with no option to select CC only.
Even a common 24v DC “clone” power supply i sometines use to bulk charge, appears to defalt to CV before peak charge is reached ?

 

[From-A-To-B]

Seems like you’re on an ideological rampage against CV charging. Just cut your charge at 80 or 90 percent and you’ve effectively utilized the CC portion of a CC-CV charger. I don’t get what the rub is.

By the time a charger hits the CV portion of the charge curve, all the heavy lifting of CC is complete and it’s just taking extra time to top up the battery. If you don’t like that part of the charge curve, don’t use it.

Alternatively, buy a charger with a custom set endpoint from Aliexpress or adjust the charger you have to terminate the charge at a lower voltage.

I sometimes want that last 10% for a long journey.

Same.

 

[Username1]

I can charge to 80 or 90% no problem using an adjustable CC/CV charger I bought for this purpose. It's not a practical issue I'm personally having, it's about CC/CV as the default method being bad for most customers.

If you released a PEV with a CC only charger that displayed 100% after charging, the customer would be none the wiser. They'd look at advertised range and displayed percentages. Meanwhile they'd benefit from fast linear charging and increased battery lifespan.

Average riders (most of which are uninformed) often charge to 100% and leave it there until their next ride, which could be days or longer. Having it at roughly 80-90% would certainly help battery longevity. Not to mention leaving room for regen braking on a full battery, less chance of overcharging a cell etc.

CV throws out all these benefits for a tiny range increase. I'd say make the CV feature disabled by default, and allow enabling via app with a warning that it degrades battery, greatly increases charging time, warning about storing it at full charge, and the small estimated range increase. I bet most users wouldn't enable it if the PEV was advertised based on CC figures to begin with.

 

[Hillhater]

. Just cut your charge at 80 or 90 percent and you’ve effectively utilized the CC portion of a CC-CV charger. I don’t get what the rub is.

By the time a charger hits the CV portion of the charge curve, all the heavy lifting of CC is complete…..

i dont think that will work .
most chargers are “profiled”
if you set a cc/cv charger to charge at say 4.0v to charge a 4.2 max charge cell, it will still switch to cv at 4.0v and then continue to add charge as the chemistry “settles” to give a “off charge” resting voltage of 4.0v
if you manually remove the charge when the CC reaches 4.0, the off charge resting voltage will drop back to 3.9v or something.
If you use CC only , you would have to “calibrate” the shut off voltage to allow for that surface charge to dissipate down to the 80% voltage setting you are targeting.

 

[rafalg]

Hi, i think there are some logic flaws in the discussion - you can't throw out the CV part from CC/CV charging because current and voltage are linked together by the Ohm's law. The current is controlled by limiting the voltage in the charging circuit and if you want the charging to stop at some point you have to set some limit on the voltage. Either automatic, by setting max Volt = 4.2 on the charger, or by pulling the plug just at the right moment.

 

[Username1]

Hi, i think there are some logic flaws in the discussion - you can't throw out the CV part from CC/CV charging because current and voltage are linked together by the Ohm's law. The current is controlled by limiting the voltage in the charging circuit and if you want the charging to stop at some point you have to set some limit on the voltage. Either automatic, by setting max Volt = 4.2 on the charger, or by pulling the plug just at the right moment.

What I'm talking about is a charger that does constant current until the chosen max voltage is reached (4.2v for example), and then shuts off instead of continuing to hold the voltage while the current decreases to near zero.

In other words, skipping the CV phase by automatically shutting off the charger when max voltage is reached.

 

[Squilt]

Hi, i think there are some logic flaws in the discussion - you can't throw out the CV part from CC/CV charging because current and voltage are linked together by the Ohm's law. The current is controlled by limiting the voltage in the charging circuit and if you want the charging to stop at some point you have to set some limit on the voltage. Either automatic, by setting max Volt = 4.2 on the charger, or by pulling the plug just at the right moment.

It's called constant current because the current stays constant, opposed to holding the voltage constant. How either is achieved is irrelevant to the argument.

 

[rafalg]

OK, i see. I'm only pointing out that you can't charge with constant current without having some pre-defined limit on voltage. Without Volt limit the charging would go on indefinitely, and you would have the constant current all the way until the battery (or charger) explodes.
But luckily every power supply has some limit on voltage and will not go above that - upon getting close to the limit the current will drop gradually to zero (so if you dont pull the plug you're getting the CV phase anyway, no way around that)

 

[Hillhater]

every power supply has some limit on voltage and will not go above that - upon getting close to the limit the current will drop gradually to zero

True, ..but a someone pointed out earlier that “zero” current setting, could be configured to be CC setting -5% say .
So if your charge rate is 10 amps, have the CV cut off current set to 9.5 amps ?

 

[Hillhater]

If so, wouldn't it make more sense to just use CC charging?

Why do you feel it neccessary to keep starting new threads on the same subject ?
This is the 3rd thread you have started with the same question ….how many answers do you need ?

 

[rafalg]

Yep, you can cut off earlier, before reaching the battery voltage limit. All valid and OK, but you're leaving some capacity unused and don't give the balancer a chance to do its job
Or you can set the voltage limit above the battery rated voltage, for example 4.5V instead of 4.2V, and then you could charge 'constant current' all the way until the battery is 100% charged. This is difficult because there's a risk of overcharging and you don't know exactly what should be the cutoff moment. That's why you set the limit on voltage and don't have to worry about overcharging - the physics guarantee current will drop and charging slow down to a stop when you're at battery max voltage. So in my opinion me the 'CV' phase doesn't suck at all