# Ripple effect on batteries



## HYPRDRV (Aug 20, 2009)

I’m going to a LiFePo4 pack and read somewhere that the “ripple effect” when charging a pack of batteries (for that matter any battery) with a genset with A/C output rectified to D/C may affect the life of the batteries. Since I really don’t know what this effect does or is can anyone shed some light on charging batteries with this type of setup and how this generally affects batteries? For that matter is it more of a problem with LiFePo4’s?

Thanks,
Steve in Solomons MD


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## riba2233 (Apr 29, 2015)

There is some info on ES forum that it might even have positive effect.


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## HYPRDRV (Aug 20, 2009)

I'm Sorry Riba, ES forum? You got a link?

Steve in Solomons MD


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## riba2233 (Apr 29, 2015)

Oh sorry, it's endless sphere forum:

https://endless-sphere.com/forums/

I don't have exact link, but there are few experts there, so you can open a topic and ask. 

But I'm sure that you'll also get answer on this forum too


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## Moltenmetal (Mar 20, 2014)

High voltage is bad for Li-ion batteries irrespective of chemistry because it causes electrochemical damage to the electrolyte. Understanding the chemistry I can say that high voltage is very likely to be bad news whether it happens for seconds in a row or for a few milliseconds 120 times per second.

The question is whether or not the particular charger's circuitry is actually capable of raising the cell voltage above an unacceptable level during each little ripple at the end of charge. I haven't examined the AC performance of the cells so can't comment on how good they are at dampening out 120 Hz- but you would imagine that they might work pretty well as (giant) electrolytic capacitors at a frequency this low. I'm sure someone has looked at this though.

You'd have to talk to particular BMS makers to find out whether or not the celltop boards would trip early based on the ripple voltage, or ignore it and hence not adequately protect cells against ripple.


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## dcb (Dec 5, 2009)

suggesting "it is good for them" without any supporting evidence is ridiculous.


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## HYPRDRV (Aug 20, 2009)

Let's see if I can be more specific on the conditions.
The genset outputs 120V A/C and is rectified to 160V DC (adjustable) at about 80A. There won't be any charger in line, the genset would be "disconnected" once the pack voltage reached a set point somewhere around 155V. I do have an Orion BMS being installed. 

Steve in Solomons MD


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## arklan (Dec 10, 2012)

ok im a bit confused here, going by ohms law;
the higher the voltage difference between battery and charge, the higher the amount of amps that will be transferred for a given battery...

battery chargers that have the constant current phase would maintain the voltage at an exact amount above the batteries voltage so that it could maintain a steady and constant amount of amps until it reaches the constant voltage phase right?

so when the voltage goes up on the peak of its cycle, it will try to transfer more amps, the battery would soak this up as more amps but i dont see how the battery would be raised to an over voltage state?

with my wind turbine the voltage has ripple and fluctuates all over the place when measured at the battery end the voltage is pretty steady and the controller just dumps when the batteries r full.

if you mean the voltage difference between battery and charger being high enough to go over the batteries c rating then sure, but youd need something pretty crazy for that for a traction pack...
the 48v system i have at the farm uses 100v solar panels and never has a problem.

so im just a bit confused as to why it would be such a big deal


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## dcb (Dec 5, 2009)

mostly because they are likely the most expensive part, and it would require lots of testing to say anything with any degree of certainty.

They are subject to lots of pulsing during normal use, and they have fairly low IR, though current needs to be limited, so theoretically, it might be fine, but the manufacturers all specify CONSTANT current followed by CONSTANT voltage for charging...

So there should be some healthy doubt about the practice, but someone needs to try it sooner or later. I kind-of am by using the reactive charger on my elec-trak @ 40v, and there are a lot of other variables there. But it should always be a proceed at your own risk till proven, not just speculated about.

As for being "good for them", that is probably confusion with Lead Acid and desulfating talking.


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## bigmotherwhale (Apr 15, 2011)

Reflex charging has been studied with li ion chemistry and it had a positive effect on cycle life, mainly because the batteries stayed cooler during charging.

A ripple may give the battery a rest period so it could also have a positive effect, i very much doubt that any negative effects could occur without a much sharper transient.

I would not be worried about a low frequency ripple during charging at all, your batteries are not going to be affected, (opinion) as said before the cells will act as a capacitor anyhow, if you are concerned for some reason then just use a filter capacitor.

see:
http://ieeexplore.ieee.org/xpl/logi...6917161/6931119/06931148.pdf?arnumber=6931148

http://www.esrf.eu/home/UsersAndScience/Publications/Highlights/highlights-2014/SOM/SOM06.html


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## dcb (Dec 5, 2009)

bigmotherwhale said:


> Reflex charging has been studied with li ion chemistry and it had a positive effect on cycle life,


Where did it say that?!?




bigmotherwhale said:


> http://ieeexplore.ieee.org/xpl/logi...6917161/6931119/06931148.pdf?arnumber=6931148


I see data for one cycle, for one cell, claiming a %0.73 increase in capacity, for a specific algorithm, not rectified AC.


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## bigmotherwhale (Apr 15, 2011)

find it yourself i cant be bothered, when i looked into it in detail most of the conclusions were positive. The more detailed studies you have to pay for... 

its merely a comparison.

reflex charging is usually accomplished by using inductors or capacitors using very high reverse pulsed currents. 
why would a slight modulation in dc current at low frequency be detrimental if non symmetrical AC charging has had any positive effect at all, the effect on the ion transfer must be minimal in one case and stop and reverse on the other.


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## dcb (Dec 5, 2009)

you're full of crap though. I REALLY can't be bothered.

edit, since I am interested, a recent paper titled:
Lithium-ion batteries: Evaluation study of different charging methodologies based on aging process

tried several negative pulse methods, they were ALL worse for capacity lifespan than constant current.

Besides, the RMS (heating) value is going to be higher if you are throwing large pulses of current at your battery.


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## bigmotherwhale (Apr 15, 2011)

i found this and i thought it might be applicable: 

http://www.researchgate.net/publica...Charging_Frequency_Study_for_Li-Ion_Batteries


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## dcb (Dec 5, 2009)

That's better 

though that sinewave with offset isn't practical in the single stage pfc they mentioned, not for a high voltage battery, maybe for a single cell it is. Also not they NEVER go negative on the pulses. It isn't anything like rectified current into an existing battery either (which basically bangs on and off as it hits the battery voltage level).

here is high voltage single stage pfc, inductor on the line side of the switch, pulsing the crap out of the battery I(v2). I wouldn't recommend it.

http://www.diyelectriccar.com/forums/attachment.php?attachmentid=44657&d=1440658626

you can imagine this is what the single stage version of the emotorwerks charger is doing to your battery as well.


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## bigmotherwhale (Apr 15, 2011)

high voltage pulses into a li ion battery seem to be the opposite of whats suggested as being good for them, 

should be great for ni cad or lead acid, however.


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## dcb (Dec 5, 2009)

I mean, this is what single stage pfc (as mentioned in the article) looks like when you have a high voltage battery, i.e. one that is above peak input voltage. The pulses represent current, the voltage is pretty well fixed by the battery at 401v in that diagram.

If you had a single cell battery @ 3.2v, it is the opposite issue, the rectified input is almost always greater than the battery, so shaping it to a sinewave (not a rectified sinewave) is fairly easy, though it will affect the pf, but that is kind of a ridiculous concern for one cell.

If you want best pfc (i.e. for a high powered battery charge) and sinusoidal charging current again not rectified sinewave), you need a pfc front end boost stage and a storage section, and a bucking shaping stage. Though the storage section can take advantage of the dips in input voltage, which is nice.

I couldn't really tell if the 1000 cycles were actual testing or wordplay on estimated values though, just saying.

also, you will need to be able to handle substantially larger peak currents in the sinewave version.


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## bigmotherwhale (Apr 15, 2011)

surely you would just need a capacitor with some switching to create a ripple?

are we talking about 3 phase or single phase ripple?

what effect on current does the ripple in voltage cause when charging?

effects on the chemistry of batteries using different charging methods does seem to have a huge effect. Im sure there are alot of people researching it at different levels.

The really interesting part is how the frequency seems so important. There is alot going on in there. 
Ive done a few experiments in the past with some good results. especially with nicad and nimh the internal resistance drops very quickly and they take more charge but they are completely different to Li cells. 
Maybee someone should try out some different lithium cells, small scale in the lab to see how capacity is affected.


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## dcb (Dec 5, 2009)

bigmotherwhale said:


> surely you would just need a capacitor with some switching to create a ripple?


It needs another inductor, a current source. The battery is a voltage clamp (without a second stage downstream of the capacitors).



bigmotherwhale said:


> are we talking about 3 phase or single phase ripple?


Me? I'm only worried about @home, single phase stuff. Someone with commercial intent might be into 3 phase. I'm just a hobbiest w/no such designs.



bigmotherwhale said:


> what effect on current does the ripple in voltage cause when charging?


well, it creates a ripple in the current to some degree, depending how much effort you put into eliminating that ripple (usually caps in-between the stages). And this *might* be good news in that department, assuming 100hz and 120hz don't make a big difference. If you can align the battery sinewave peaks with the input peaks. I took a peek at the resulting pfc without correction, and I'm sure it is workable. You might not need a storage section (caps) and a second 'stage' but just an inductor and a diode downstream of the primary section.

Still it is only a 3 battery test, but interesting.


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## Tony Bogs (Apr 12, 2014)

Paper describing a model that I'm going to use for my SiC LLC charger design and the effects of pulse loads on batteries:
users.ece.gatech.edu/rincon-mora/publicat/jrnls/tec05_batt_mdl.pdf

Less tech details, good description of pulse charging:
http://orbit.dtu.dk/files/9852018/Paper_EV_charge.pdf

I agree that a charger should have: an EMC filter, PFC (boost), isolation, and a buck stage for the required V/I charge profile.
Many residential installations have 3phase outlets for high power applications like cooking, heating and cooling. 
2 10kW outlets in my case (unused at the moment).


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## Moltenmetal (Mar 20, 2014)

Tony: I wish we had 3 phase available in residential here in Canada. There might be some available in multi-unit apartment blocks, but you can't get in a home here unless you make it yourself with an inverter from the split-phase 240/120 VAC which is normal here.

As to the effect of pulse charging: not at all the same thing as a fast ripple from unfiltered rectified AC, but in a vehicle with regenerative braking, every drive results in alternating pulses of discharge followed by brief and relatively shallow recharge, but at currents much larger than you tend to use during a normal recharge. I suspect there's research into the lifecycle implications of that operating mode on Li-ion chemistries. Have you seen any that you can refer me to?


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## bigmotherwhale (Apr 15, 2011)

i came across a paper a few moths ago describing exactly that, the conclusion was that regenerative braking increases battery life as the battery is damaged by deep discharge more than being charged at high currents periodically.
so a battery doing the same trip every day one with and one without regenerative braking, the one with would have suffered less capacity drop as the result of a low soc.


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## arklan (Dec 10, 2012)

could u use the controller to charge somehow? it has the filtering and the motor is an inductor..


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## bigmotherwhale (Apr 15, 2011)

arklan said:


> could u use the controller to charge somehow? it has the filtering and the motor is an inductor..


Thats been done and its very efficient weight wise, why it isnt done more often i dont know.


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## dcb (Dec 5, 2009)

probably because a charger isn't that big an expense or consumer of space in a car (and the battery manufacturers haven't signed off on anything but constant current/voltage). 

A motorcycle might make more sense for motor-charger as space is at a premium. I do wonder what jddcircuit makes of this discussion. I *think* his patent makes for some significant pulsing/ripple, not %100 sure on that.


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## Tony Bogs (Apr 12, 2014)

Sorry for the late response. 
Moltenmetal, this might be interesting for you.
An epic presentation of Dalhousie's prof Dahn, explaining why Li-ion batteries die. Especially interesting is the section at app. 25' where he compares different chemistries:
http://evobsession.com/li-ion-batteries-die/
Spoiler: SEI buildup is what kills Li-ion. An approach from an electrochemical perspective regarding charging, discharging and regen braking:
http://depts.washington.edu/maple/pubs/53_JES_SEI_layer_growth_and_capacityfade.pdf

I believe the paper supports bigmotherwhale's remark about DOD as the main cause of failure.

And CV charging is also bad for the lifespan. No surprise there for me!


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## Moltenmetal (Mar 20, 2014)

I'd seen the Dalhousie presentation before- will check out the other link.

Solid electrolyte interphase getting thicker and blocking off pores in the anode graphite material is what I recall from that talk. The root cause was time at high voltage, particularly at high temperature. Certain electrolyte additives reduce the rate at which this happens, forestalling it while they last. Regen would not cause much of a problem there, but it seems unlikely that SEI buildup and cell reversal are the only two death modes for these cells.


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## Moltenmetal (Mar 20, 2014)

The paper in the latter link was very interesting- thanks Tony! 

It was based almost entirely on mathematical modelling, using driving cycles which for the most part used far more energy recovery from regenerative braking than is observed in real driving from what I've read elsewhere and observed in my own vehicles (my home-built EV and my several Priuses over the past 7 years). 

The only test data that I noted (rather than model predictions) was a test of LiFePO4 cells for 1000 cycles of dynamic stress test (DST) cycling with regeneration versus rest periods (zero current) of equal duration to the regen cycles. In that testing, the capacity loss was modestly but significantly less for the batteries on the regen cycle than those on the rest only cycle.

Their modelling on the other cycles indicated, as far as I can tell from a brief read, that the more regen you do, the more time you spend at a high SOC and hence the more SEI growth and resulting capacity loss tends to occur.

They also suggest that CV charging (i.e. after CC for top balancing purposes) is a bad idea, as it causes most of the SEI growth. I'd concluded that already based on the Dalhousie paper.


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## Tony Bogs (Apr 12, 2014)

Yeah, there are other negative effects besides SEI buildup. But SEI is the main kill factor. 

The models show varying results for regen braking, depending on the drive cycle (they used about eight drive cycle standards from different parts of the world). 
No reasons there to stop using regen braking, especially when real world tests indicate a positive effect.

Although a low DOD combined with a maximum SOC of 50% could increase the lifespan, nobody in their right mind is going to sacrifice half the driving range.

Back to charging, my conclusions:
- no CV charging
- charge as fast as possible while monitoring the battery temperature


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## dcb (Dec 5, 2009)

the main thing consumers can do is not leave it hot and charged. I.e. use a timer to charge it so it is ready to go when you are. Also a faster charge can be useful for limiting the time sitting at charge.

I think this is fairy independent of ripple, and the bulk of regen isn't near the higher state of charge where things fall off a cliff.

It would be nice to use Jeff Dahns approach to testing different charge profiles on different chemistries, but he is going to be busy working for Tesla now https://chargedevs.com/newswire/tesla-hires-prominent-battery-researcher-jeff-dahn/

edit, I REALLY like this comment on that page though  "Get the battery into a flywheel"


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## Tony Bogs (Apr 12, 2014)

Ripple could be useful for the reactive part of the battery impedance to reduce loss. Needs further investigation.


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