cv slow fill or cc fast fill charging?

[auraslip]

What gives more capacity?

Setting the voltage to my meanwell at 3.6v per cell (lifepo4) and slowly letting the current drop as it reaches 3.6v?

Setting the voltage higher and letting the psu deliver constant current right up until the HVC alarms go off?

The later is much quicker. Besides requiring prompt attention at HVC, does it have any downsides like less capacity?

 

[SamTexas]

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

auraslip, according to figure 1 and table 2 of the above article, reaching 4.2V only gives 85%. Maintaining 4.2V for a period of time will saturate the capacity, but will presumably shorten the cell life as well.

Obviously the article is about LiCo and LiPo, but I assume that the same holds true for LifePo4 as well after adjusting for the different ceiling voltage (3.6 vs 4.2)

 

[neptronix]

No answers about #2 so i'll bite.

Sounds like a bad idea.

+ batteries are stressed the most at their higher and lower voltage ranges. For example, near the end of charge and beginning of the charge, the internal resistance tends to rise. Thus, it would seem unwise to be putting in the full amount of amperage near at top, this is why every charger i have seen has CV mode.

+ overfilling a cell is never a good idea, and it is highly probable that you will do it to some degree with this method, since when you look at the charger, all you will see the elevated voltage and full amps being delivered to it. You would have to watch it like a hawk near the end. Even then, human error is a factor that should be eliminated.

+ CV mode does not take very long in the first place, even on a larger pack. If you are strapped for time, why not just unplug when the CV mode starts? you may cut your battery capacity short by 5%-10%... i dunno about you, but that safety margin, plus a life extension from using lower SOC, is worth it.

 

[liveforphysics]

If you go to very low resistance cells, your CV period is basicly doing nothing.

I CC only. On my packs I get a >98% fill that way. You need big heavy charge leads or remote pack sense taps from the charger to do it right, or the v-drop in your leads will decrease the final SOC.

 

[auraslip]

err, I guess I got my terms messed up. Meanwells don't really do CV right? At full voltage (3.6v a cell in my case) there is no current flowing between the battery and meanwell.

What I was referring to was the current tapering near the end of charge. From 3.3 to 3.6 is less than 10% capacity, yet because the current drops it takes more than it's fair share time to fill that top 10%.
If I turn it to 4v a cell, it quickly goes from 3.3v to 3.6v. I actually think this is safer because the alarm goes off quicker. If I just leave it on the meanwells at a lower charging voltage, the current will drop to zero without the alarm going off and I'll forget to get up and turn the thing off. Not that sitting overnight at 3.6v a cell is any different than leaving it on the charger when the pack had a bms. (fascinating fact, pings cells balance at 3.85v a cell, and when you leave it on the charger it stays at that voltage until you take it off. Dumb imho to sacrifice life cycles just to eek a tiny bit of capacity out of a fully balanced pack, but honestly who is gonna use the thing for 2,000 cycles anyways?)

 

[neptronix]

That 'current tapering off' bit is the CV part.

CC = constant current
CV = constant voltage ( while current is slowly tapering off as the battery holds that voltage on it's own )

Potentially overcharging your cells is not safer. What if you miss the alarm?
And there is nothing inherently unsafe about keeping your battery floating for a long period of time.

BTW, have you measured the ping charger to ensure that the cells themselves are actually 3.85v each? maybe it has a higher voltage into the BMS to make up for some kind of voltage drop in the pathway from the BMS to the cells..
just a theory..

That does seem weird to keep them elevated at such high voltages.

 

[auraslip]

Charging with a meanwell doesn't have cv - the current slowly tapers as the voltage nears full. Ohms law or something - lol - to tired to sound smart right now.

Ping has it setup like that because the cells balance quicker for whatever reason.

 

[neptronix]

Charging with a meanwell doesn't have cv - the current slowly tapers as the voltage nears full. Ohms law or something - lol - to tired to sound smart right now.

No, that is exactly what CV is. Constant voltage, variable ( tapering down in our case ) amps.

 

[auraslip]

that's what cv is, but meanwells don't do it. I'm charging right now. The cells are @ 3.38v and the current has dropped from 6.5a down to 4.5a, and by the time the cells are at 3.6v the current will be at 0a.

 

[neptronix]

uh huh.... are you measuring at the power supply's terminals, or at the battery? because you are going to see a different voltage at the battery than the supply. The battery voltage will be lower.

During this state, the charger will be putting out constant voltage, and gradually lowering the current.
The battery voltage will gradually be rising up to the target voltage at this time.

I've observed this while charging my lipo. Both with the meanwell and the iCharger.

 

[auraslip]

The battery and supply voltage will always be the same.

Ohms law: v/r = I

In this case the differences between the two voltages 16*3.6 - 16*3.3 = v

Or ( 57.6 - 52.8 )/.048 (using a headway pack here) = 100a

The power supply can't output 100a, so the voltage of the supply drops to a voltage that it can sustain or is limited too. We can find the supply voltage like:

( x-52.8 )/.048 = 5.6a

x = 53v

So the meanwell is just barely above the pack voltage.

 

[auraslip]

What I don't understand about cc/cv chargers is how they're still putting out current even though cell voltage rises as SOC rises. Wouldn't the extra current send the cell over the CV point?

 

[neptronix]

It's the difference between the battery's voltage and the power supply's voltage that results in an draw of amps.
It is not like a regular load where you are powering something.

The battery may be at your target voltage ( while it is connected ), but when taken off of power, will drop it's total pack voltage. The power supply in this instance is just giving it the energy it needs to sustain that voltage, which slowly goes downwards.

If you watch the readout of a balancing charger with a realtime graph, this would make more sense.

IE, when you start the charge, the pack will jump up by a certain amount of volts. if you stopped the charge immediately after, it would drop down.

The end of the battery charge is when the battery can hold that voltage on it's own.
So yeah, during that period, the power supply holds it's voltage steady and gives the battery the current it wants.. which gradually lessens.

It's much like filling a glass of water to the top. You can open the faucet super wide for 80% of the fill, but you have to gradually slow the flow of water to get the glass 100% full to the last drop.

 

[neptronix]

But more simply, you set a target voltage and the battery will eventually end up there and stay there.

 

[The Mighty Volt]

I like CC.

Nothing nicer than putting A123's onto a charger and letting them race up to 3.65v and then slowly draw less and less current as they charge up.

 

[Spacey]

What I have been doing with my Lipo pack (16 Cell 66V), is I balance charge using an i1010B+ when I can at home which takes ages.

At work I have my old Lifepo4 72V 5Amp Charger which I have adjusted to the the lowest I could get it which is 69V, I use this to bulk charge the Lipo pack and use Cell Log 8's to let me know when a cell hits 4.15V then I turn the charger off.

I was worried that the charger will be put full 5amp charge into the pack when it is near 4.15v a cell, is this harmful as it is not tapering down the amps as it gets close to full pack voltage?

 

[Spacey]

Reading from that link it says:

Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.

So it would be best not to leave a pack on charge for a long time letting the BMS keep cells in a floating charge of say 3.6V for Lifepo4....maybe better to have the pack on a timer for 1 hour charge and 1 hour balance for the BMS so 2 hours total?

 

[fechter]

Several people have observed the Meanwell supplies don't behave like true CV sources when charging batteries for some reason. The current should max out at the CC settting until the cells reach the CV point, then the voltage should stay there until the current drops to zero. This doesn't seem to be the case with some Meanwells.

That said, most commercial chargers go CC until the voltage reaches a somewhat high value, then shut off. This gives a faster charging time. If you hold the voltage at a 'safe' CV level and let the current taper off, you'll get a few more electrons in there, but it will take much longer.

What's better for the batteries? Not sure. It is said that holding the cells at the maximum voltage for a long time will reduce their cycle life. I think it has to be a really long time to make any difference, like a constant float charger that's plugged in for months. There is quite a bit of research on this for LiCo cells. The same principles seem to apply to all lithium chemistries.

 

[neptronix]

HM!

Well i'll tell ya, my meanwell from sureelectronics, which has the proper logos on it, and engraved top, does the current tapering thing.

But there are a lot of fakes/old versions/etc so i see why this can't be agreed on !

 

[auraslip]

Dude, it's called ohms law for a reason. If the differences in battery voltage and supply voltage are as great as you say they are, then by ohms law current would be tremendous.

 

[liveforphysics]

Reading from that link it says:

Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.

So it would be best not to leave a pack on charge for a long time letting the BMS keep cells in a floating charge of say 3.6V for Lifepo4....maybe better to have the pack on a timer for 1 hour charge and 1 hour balance for the BMS so 2 hours total?

No. It can never overcharge on a CV supply. Its a physical impossibility. (unless the CV supply were to have a mechanical malfunction and shift to outputting a higher voltage for some reason which any charger would equally suffer from).


You plug a pack into a meanwell (or any regulated power supply fixed at your desired HVC voltage), and it supplies all the current it can (generally watching an internal shunt voltage and regulating it to be whatever it's capable of without hurting itself, this is the CC mode) until the pack reaches the HVC voltage which it can't exceed (because it's a regulated supply). Once it reaches this point, current MUST taper off, no matter what sort of power supply etc, because it can only make current move into the cells because they have a difference in voltage. The reason current flows in the CV stage is because the resting voltage of the pack is still below the regulated voltage of the supply, it's just at the supplies voltage because of the resistance of the pack being pulled up by the charge current (exactly how it's a lower pack voltage while discharging because of sag, this is just reverse-sag). The resistance in the wires also plays a factor in how soon it reaches the CV stage, because it's also got it's own voltage drop across the wires. As the resting SOC of the pack approaches the regulated supply voltage, the potential keeps decreasing, and the resistance of the system is fixed, so current tapers off. As the pack gets closer and closer to full, the current approaches the limit zero, and the charging decreases to mA, then uA, then nA, and eventually matches the rate of the cells own self discharge if you left it for weeks, but it can never over-charge.

 

[neptronix]

Auraslip,
these are current limiting power supplies, which is the entire reason why we are using them as battery chargers.
That is why nothing explodes when you plug it in to the battery. :lol:

Do some tests yourself to understand it. I think you have read some stuff online and understand some theory, but are missing a few pieces of the puzzle.

 

[auraslip]

Thanks for the explanation LFP. I think I understand now. While the packs resting voltage is below the CV voltage, current will still flow as it's the only thing holding it at CV. Perhaps a better way to think of it is Topping voltage. The voltage where current slowly tapers to nil as the battery pack catches up.

Speaking of CV. What happens if you leave a dis-balanced pack at CV for a long time? Like 8 cells are 3.65 and 8 cells at 3.4v?

these are current limiting power supplies, which is the entire reason why we are using them as battery chargers.

The current limiting is done by reducing the voltage. I think that's the piece of the puzzle you're missing.

You're right that battery voltage will be lower than supply voltage; otherwise current wouldn't flow. But the difference is very small.

 

[liveforphysics]

The current limiting is done by the same mechanism that it's done in a motor controller. It watches a shunt to know current, and then alters the PWM% as needed to maintain it at its safe limit. (and you can hack it just like a controller by tweaking the shunts, or adding an external board to let you trim the CC mode current value with a little pot (fetcher/gary sells them, or you can make it yourself for $4.)

 

[neptronix]

Thanks for the explanation LFP. I think I understand now. While the packs resting voltage is below the CV voltage, current will still flow as it's the only thing holding it at CV. Perhaps a better way to think of it is Topping voltage. The voltage where current slowly tapers to nil as the battery pack catches up.

Speaking of CV. What happens if you leave a dis-balanced pack at CV for a long time? Like 8 cells are 3.65 and 8 cells at 3.4v?

The current limiting is done by reducing the voltage. I think that's the piece of the puzzle you're missing.

You're right that battery voltage will be lower than supply voltage; otherwise current wouldn't flow. But the difference is very small.

I'm not missing anything.. it doesn't matter how the power supply limits current, just that it limits current
The current limiting is not done by reducing voltage, the difference in voltage is due to other factors.
If the current limiting was done by reducing voltage, and by no other means, it would still try to flow thousands of amps ( at a lower voltage )

If that was true, you would see a huge difference in amps from when you plug a drained pack in VS plugged in one that was nearly full.
You would also see a difference in amps when you adjust the voltage too.

Like LFP said, the current limiting works just like your motor controller, for example, there are shunts in the meanwell that determine how much current to flow.. adjust the shunt, and you adjust the amp limiting.

As for your disbalanced pack goes, if no BMS is involved, all that the power source connected sees is the voltage of the pack. if you have 15 cells at 0v and 1 cell at 54v on a 16S pack, it sees the total voltage at the discharge lead at 54v.. it knows no different.