Charging a huge battery presents feedback loop problems?

[treez]

Hello,
We are designing a 7.4kW offline LLC battery charger for EV. The (lithium) battery is 300-410VDC.
As you know, We are closing the feedback loop on the output current (into the battery).
Do you believe that this presents a tricky dynamic situation for us?

I am used to closing the loop in previous SMPS’s on output voltage, with a resistance type load. In other words, there is a load pole of 1/(2.pi.R.C)…
Where
R is the load
C is the SMPS output capacitance.

As you know, in output voltage regulated SMPS’s the addition of an enormous load capacitance often brings about instability in an SMPS…due to the big decrease in phase margin that it causes.

Our huge lithium battery is obviously an enormous output capacitance, and we are somewhat concerned about this situation of decreased phase margin.
We are wondering, that instead of using an analog feedback loop, comprising an error amplifier, with feedback compensation R’s and C’s, perhaps we would instead be more advised to implement a very slow “try it and see”, incremental type of feedback loop? That is, we could set up for a very low initial charge current, and then just bit-by-bit, reduce the frequency of the LLC converter, until the output current is the right value, and then just continually monitor the current from there on. This would be handled by a microcontroller. Obviously we would have an overcurrent shutdown comparator ‘overseeing’ the whole process.

Which type of feedback loop would you advise for this?

- - - Updated - - -

Just as an after note , the attached ltspice simulation shows that a slight change in LLC frequency from 58.8khz to 60.2khz results in a large 10 Amp change in charge current

 

[treez]

I now think its an absolute must that we do a simple microcontroller based feedback loop, (overseen by a fast comparator)...the micro will simply decrease the LLC frequency slightly every 30ms, then measure the current, then if its still below set point, do it again, etc...doesn't matter if it takes 20 seconds to set the current, because it takes 4 hrs to charge.
The battery load has ridiculously low dynamic impedance, let an analog feedback loop loose on this and it would be "boom!". Besides, its simpler and easier to do it the slow "set-and-check","set-and-check","set-and-check"..... way

 

[D.A.(Tony)Stewart]

You could also include a 50mV shunt and regulate f with a ramp control with a feedback loop.

Don't you have a control system?

 

[treez]

we will use the ICE2 LLC controller from Infineon....but we will command its frequency via an external micro, not allow the ICE2's analog feedback loop to do it. The micro will monitor the output current and do repetitive "set and check" . That will be the control system...with a comparator overlooking.

 

[treez]

Please check over this LLC converter current regulation circuitry?
Hello,
As discussed, we are designing a 7.4kW battery charger using an LLC converter (SMPS) charging stage.
On connection of a battery, we will set the charging current by first of all setting the LLC converter’s operating frequency to a very high frequency, and then slowly, slowly, bit-by-bit, we will creep the switching frequency (of the LLC converter) down, until the charging current builds up to the value that we want (20A max).
We will gradually decrement the switching frequency by gradually reducing the current drawn from the “FREQ” pin of the ICE2SH01G LLC control chip.
We will do this gradual frequency changing using a microcontroller. The microcontroller will effect the drawing of current out of the FREQ pin of the ICE2SH01G controller by adjusting the voltage at the output of buffer opamps which feed the FREQ pin of the ICE2SH01G controller, via resistors. This is as shown in the attached schematic.

Can you see any problems with this method?

ICE2HS01G LLC controller datasheet:
https://www.infineon.com/dgdl/Infin...n.pdf?fileId=db3a30432a40a650012a458289712b4c

 

[treez]

Hello, this is a new situation but relates to the above...

Do you know what is meant by “pulse charging a lithium electric vehicle battery for the purpose of detecting individual damaged cells”? We understand “cell balancing” and how to detect imbalance, but don’t know why doing it needs the battery charge current to be pulsed on and off repeatedly. This all means that the battery charger design that we have done for our customer to date will have to be scrapped..

We have been designing an offline 3kw battery charger for an Electric vehicle battery which has a voltage of 300-410VDC.
We have designed it with an interleaved Boost PFC followed by an LLC converter. We designed it with an extremely simple, slow, non-dynamic feedback loop because we assumed that since it takes 4 hours to charge the battery, it wouldn’t matter if the microcontroller takes some 30 seconds to get the current up to the maximum charging current level. (obviously this is all “Overseen” by an overcurrent comparator).
The micro was meant to gradually increment the charging current to the maximum level by gradually adjusting the LLC converter’s switching frequency and dead time..(as well as us altering the output voltage of the Boost PFC at times).

..However, we have now found out that this is totally unsuitable. In fact, the charging of an electric vehicle battery requires an extremely fast feedback loop bandwidth of the charger. This is because toward the end of the battery charge, the charge current must be pulsed on and off repeatedly. This is for the purpose of detecting damaged cells. The current pulses must get up into regulation within about 100ms. This is far faster than our slow, incremental feedback loop can handle.

We cannot find any detail on the nature of the current pulses required for this damaged cell detection phase, and our customer cannot provide any such detail, since they have irregular contact with their battery supplier.

Do you know the detail of this pulsed current regime? –eg, to what level must the current rise? (C/2 etc), and how accurate must the current be?,…and how fast must the current get into regulation during a pulse?, and how many current pulses are to be delivered.?

 

[D.A.(Tony)Stewart]

cell mismatch is are more than simply DC voltage mismatch. It is capacity (Ah) and load regulation (ESR/Load)

It is historically measured by S.G. and CCA which is directly related to SoC and ESR.

Since sulphation is the build up of insulation on the lead plates from H2SO4 , it degrades ESR and thus pulse voltage with a pulse current. Pulse discharge with DC is well known to reduce sulphation and extend battery life.

It means your design spec was incomplete.

There are several successful products. The one we made for a client was a like BIC lighter with 10x 100ns pulses from low ESR caps. activated only by the charger or alternator . It went directly across motive power or truck batteries for the life of the battery. The company , SOlartech was sold years ago to another company who has this product now qualified for Volvo trucks.

You might be able to modify your design with a buck pulse charge that can supply sufficiently fast current pulses < 1% rate over top of DC .charging.

Most of reports are not public due to IP protection and resonant frequency is basically harmonically driven to cause ultrasonic cleaning.

Radiated EMI design is a challenge that necessitates skill in far field EMC design.

 

[treez]

thanks, I thought sulfation only applied to lead acids? I just now added the word 'lithium' in the post #6 to avoid this ambiguity) I like the idea of the Buck charger though.

 

[BradtheRad]

“pulse charging a lithium electric vehicle battery for the purpose of detecting individual damaged cells”?

The pulse charging could serve as an early warning that a cell is starting to go bad, when you measure voltage on it during a strong current pulse.

I tried measuring voltage on my rechargeable cells while charging. I have several of them, years old and in various states of health. (My charger is 'dumb'.) Good batteries go a few tenths of a volt above nominal. Bad cells are either (a) much higher voltage due to high impedance indicating reduced capacity, or (b) near zero due to internal short and won't take a charge.

When I take the high impedance bad cells out of the charger, and measure voltage, they show normal. Yet they are going bad.

I had a fast charger for nicad/nimh AA. It monitored voltage as it rose, then turned off when it started to drop. The drop in voltage was a sign the batteries were full, and that incoming juice was no longer being converted into charging the battery chemistry. Clues like this may be useful in care and upkeep for the EV batteries.

 

[treez]

thanks, I still don't understand why you have to pulse current into a battery so as to see if any of its cells have gone faulty. I mean, surely to see if a cell has gone faulty, you just measure the voltage across it and compare it with the others whilst its charging. Why the need for pulsing the current?

- - - Updated - - -

ok, the testing of battery cells for abnormality by pulsing current in them is called "electrochemical dynamic response"

Nowhere has any great detail on it. It seems to me like possibly a slight waste of time. I mean, a battery is going to die over time, so is there any point in detecting it. Also, surely just plain old measurement of cell voltage whilst charging and discharging would , I would have thought, been quite an adequate and satisfactory way of detecting damaged cells?
OK supposing a battery was going to get put in a vehicle that had to go on some crucial mission out in space or whatever, then sure, it would be worth doing electrochemical dynamic testing on it first, but for a car?...is it worth it?...does it give much more information than just measuring the cell voltages whilst charging in a non-pulsed way? And supposing one does detect cell abnormality through "electrochemical dynamic testing"...so what?....is there a way of fixing the cell?...I don't think so....whats the point in detecting something with an expensive detection method if there's no way of fixing it?
This pulse testing seems a waste of time?..
I mean, you don't have to pulse current into a battery in order to get cell balancing done. Also, I've never heard of anyone finding out if they're overcharging a battery by looking at what happens in the battery when its pulsed with current. The "electrochemical dynamic response" is to do with finding out the cell health, not finding out whether the cell is fully charged or not?
"Electrochemical dynamic response" testing seems a waste of time for domestic cars. It's a super sensitive way of detecting the slightest defects in lithium cells, and will likely result in EV car batteries being trashed prematurely , which is bad for the environment?
Why should we make a battery charger with a super fast feedback loop bandwidth, just for the sake of a battery cell testing mechanism which is a total waste of time.?

 

[treez]

Actually, we believe we will conform to it. We will indeed use a faster analog feedback loop with compensation R's and C's. Driving a load like an enormous capacitor (which a battery is), is not so much of a problem when you are controlling the current into it.

 

[BradtheRad]

"Electrochemical dynamic response" testing seems a waste of time for domestic cars. It's a super sensitive way of detecting the slightest defects in lithium cells, and will likely result in EV car batteries being trashed prematurely , which is bad for the environment?

This could have more to do with the nature of the consumer. Buyers expect a warranty that these expensive batteries will last 10 years or 100,000 miles... in spite of abuse, hostile environments, over-charging, over-discharging, etc. And you can count on human beings to come up with any excuse that will oblige the car dealer to replace the entire battery bank for free (whether or not under warranty).

Perhaps there are battery problems that reveal themselves during high-Ampere pulsed charging that do not show up normally, or do not show up until later when more damage is done.

Suppose just one battery is ailing. The dealer wants any means available to locate it, so he can say, "We replaced a bad battery." That is preferable to saying "Lucky you, we have to replace your entire battery bank, because we couldn't find the bad battery."

I don't know about EV batteries, but aging nicads are prone to develop 'whiskers', metallic dendrites that grow through the electrolyte, and short out the terminals. In the same vein, these battery-powered cars are still comparatively new, and who knows what problems they will develop over the years, and what is needed in the way of upkeep, etc.?

 

[treez]

Thankyou, I appreciate your comments BradThe Rad, though as you will appreciate, we are "postulating" about the dynamic pulse test......who knows if it serves any practical purpose whatsoever, the battery manufacturers keep all detail of it to themselves, we cannot even find out what the pulse current level is, or the rise time required, etc etc

 

[D.A.(Tony)Stewart]

In future when EV's become mainstream, this will pose major grid load fluctuations in addition to periodic loads off-peak hours that may require cloud controlled to reduce cost of peaks in so called off-peak pricing.

Thus pulse charging is not so much a battery issue as it is a network of micro-cell control issue, which has been modeled by some.

Is it intended to be just a J1772 Level 2 solution, with all required safety and charging system features? or more?

Consider SCADA requirements for precision measurement of voltage, current, power, energy, and frequency with remote telemetry over WiFi with low latency via commands from cloud-based servers which autonomous share grid capacity based on local electric conditions to prevent DT saturation.


i.e. not just simple on-off or 3 stage cycle or 4 stage cycle with pulse but smart energy wise options for user control of energy cost and rate of charge and longevity of battery.

 

[treez]

Sorry but I still don't see how doing "dynamic electrochemical testing" of batteries can be of much use, and even less so with regard to vehicle-to-grid systems.
By all means, if someone's sending a lunar buggy to mars, with a lithium battery in it, then subject that battery to dynamic pulse testing first, but for cars on terra firma, its just surely a waste of time.?
If a battery is unhealthy to the point where someone needs to change it, then that is obvious from other diagnostics without having to do "dynamic electrochemical testing" of the battery

 

[Easy peasy]

Pulse testing at the end of a charge ensures the good cells are not overcharged, but that you can still see a volt rise if any hi-Z cells are present.

Yes some simple analog current limit circuits do have to be very slow and damped when charging Lo-Z batteries with low res wires from charger to batt, usually the gain of the current limit power circuit is very high, so a faster clipping ckt can be used to ensure the peak o/p current never exceeds "x" it doesn't matter if this a bit oscilliatory as long as it doesn't interfere with measuring instantaneous V & I

 

[D.A.(Tony)Stewart]

US Military had great interest in Canpulse's unit and found it recovered NiCad and NiMH.

The oxidation rate depends on temperature rise. idle duration when not in use and State of Charge. 50~70% SoC is low leakage and degradation used for storage and shipping.

I suspect the same applies to LiPo. If it is used daily, perhaps not much benefit as the battery load is very much pulsed.

But if sitting under hot sun for all day and only used for short urban trips, it might be useful.

 

[Easy peasy]

Li Ion Phosphate cells are pretty good at keeping > 95% charge over months and months with no charger attached, we have cells under test here (50Ahr blocs) and they are surprisingly good, we have not had to do any cell balancing yet on our main test battery, and we can recharge at 10 amps average (pulsed 14A peaks) in 5 hours, with no heating....

 

[treez]

Pulse testing at the end of a charge ensures the good cells are not overcharged, but that you can still see a volt rise if any hi-Z cells are present.

Thanks, from this, if all you are looking for is a volt rise on pulsing, then it seems to me that the level of the pulse current would not have to be at all accurate..In such a case, we could go back to our mega-slow feedback loop and just throw a series of pulses at the battery, decrementing the LLC frequency each time, until the peak of the pulse was at a reasonable current level.....all overseen by a clipping comparator as before of course.

Another point is, those LiPO's seem to have given you excellent performance, and I really wonder if such high quality batteries really need "dynamic electrochemical testing". It smacks of over-engineering.

 

[treez]

Yes some simple analog current limit circuits do have to be very slow and damped when charging Lo-Z batteries with low res wires from charger to batt

thanks,
Regarding the control of current from a SMPS into a very low impedance battery…I believe the following “type” of feedback loop will be able to do it…the ringed RC network prevents the current from "running away".
As we know, a voltage regulation loop controlling a battery voltage (ie, a large capacitive output) is more difficult, but a current regulated loop into a low impedance load (eg big capacitor) is not so difficult.