# Another La Crosse Meltdown



## nikon (Aug 16, 2006)

I've never used my La Crosse BC-900 charger very much and hadn't encountered any problems with it until last night. I put four Energizer 2500mA batteries in to charge at the lowest setting (200mA). This was for a charge only, not a discharge/charge cycle. I came back awhile later to check on it and found it still running and in the condition shown in the picture below. 

Was it ever determined that there's a warranty on these things? I can't remember the name of the website where I bought it, but it was the one that introduced these things awhile back.


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## SilverFox (Aug 16, 2006)

Hello Nikon,

La Crosse has a 1 year warranty on this charger. Contact them and let them know what happened.

Tom


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## Illum (Aug 16, 2006)

:eww:

thankfully it didnt vent anything, use gloves


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## jtr1962 (Aug 16, 2006)

I wonder if there's a problem with the cells rather than the charger. Maybe the charger doesn't get a proper end of charge signal at 200 mA, overcharges the cells, and the cells destroy themselves by some internal mechanism. The simple fact is that at 200 mA the cells probably aren't going above 1.5 volts. That's at most 0.3 watts of power being dissipated in each cell. At worst they should reach 100°F or so. Either the cells are defective, or something causes the charger to switch to a higher charge rate by itself. I'll also add that it's not a good idea to charge high-capacity AA NiMH at 200 mA unless you also connect the charger to a timer to turn it off once you reach about 120% of the cells capacity. That would be 15 hours for 2500 mAh cells when charging at 200 mA. In order to give a good end of charge signal I read that you need to charge at 0.3C or better. That's 750 mA or more in the case of the Energizers. In any case, I always run mine with a fan now, even on the lowest settings, just in case.


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## SilverFox (Aug 16, 2006)

Hello Jtr,

That is a very interesting observation. I would be tempted to send the whole thing to Energizer and say that their cells malfunctioned and destroyed the charger.

GP states that their cells can handle a 0.1C continuous charge for a year without problems.

Tom


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## wptski (Aug 16, 2006)

SilverFox said:


> Hello Jtr,
> 
> That is a very interesting observation. I would be tempted to send the whole thing to Energizer and say that their cells malfunctioned and destroyed the charger.
> 
> ...


Tom:

Isn't this the same situation that happened several times before! Charging >2.0Ah AA cells at 200ma and they miss termination.

This is what Matthias Schulze cautioned me about using these latest high capacity AA cells. He said to charge them at least 1C to insure that they don't miss termination.

What about the high temperature cutoff in the BC900??


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## SilverFox (Aug 16, 2006)

Hello Bill,

Yes, it is the same situation as we have seen before.

Still, if the chargers high temperature sensor is functioning properly, there is no reason the charger should melt down at rates below 0.1C.

Tom


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## willchueh (Aug 16, 2006)

Hello folks,

Can't say I am an expert on the BC-900, but judging from the photos posted I believe the problem was not due to a "missed termination." 

In the BC-900 design, it uses a very small package MOSFET (SOT23) as a linear step-down regulator which has to disspate power due to the difference in voltage between the power supply (4V) and the battery (1.5V). The power disspated by the MOSFET is the current times the voltage difference. At peak charging current, the wattage disspated is considerable. 

This is not a typical design as most chargers today use a "switching buck design" which uses a inductor, transistor, diode and capacitor to convert the voltage. This design usually gives 70-80% conversion efficiency even at a large voltage difference, but more expensive to make. 

Back to the point, I believe it is possible that the MOSFET failed due to either a surge in the Gate-to-Source voltage causing the gate oxide to break. Even more likely it is due to the aging of the MOSFET since it is taking a huge toll close or beyond its power disspation limit. As a result, the probability of MOSFET failure increases. 

When a MOSFET fails, it can either become always OPEN or always SHORTED. In the latter case, there would no longer be a current limiting component between the charger and the power supply causing the all the current to rush to the batteries. Furthermore, since the MOSFET is always SHORTED there's no way to stop the charging even if the microcontrolluer unit sends the signal to stop the current. Therefore, it doesn't matter if there is a thermocouple or voltage-based termination, since the MCU is no longer able to control the charging current.



Just my two cents. 

William


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## josean (Aug 16, 2006)

Your explanation has a lot of sense. Should La Crosse consider your suggestions and revise the design, or at least they could add a disipator to the MOSFET.


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## wptski (Aug 16, 2006)

William:

This forum has seen more BC900 meltdowns at 200ma than higher currents and most often on the first charge on new cells! My BC900 meltdown was at 700ma on a group of 16, 2.3Ah Duracells that were cycled 5 or 6 times after the 2 or 3rd batch of four cells. My didn't melt as badly as the one pictured above and this is the worst that I can remember too.!.


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## koala (Aug 16, 2006)

I don't know if this has been tested before. How much heat can the batteries put out at 200ma charge current? 

Charging current - 0.200A
Charginng voltage - 2V(maximum assumption)
Number of cells - 4

So in total 0.2 x 2 x 4 = 1.6watts.

If no energy is going in to the reservoir then it's going to be converted in to heat. I am not sure how 'hot' is 1.6watts so maybe someone can simulate this overcharging with some crappy cells and note the maximum temperature, please...

I've never encounter any problems with my v33 but I have shelved it, too scary to use.


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## willchueh (Aug 16, 2006)

koala said:


> I don't know if this has been tested before. How much heat can the batteries put out at 200ma charge current?



If the charger is indeed charging at 200mA, almost all batteries of good quality can take continuous overcharging without significant overheating or venting. Our battery lab constantly test battery overcharging at this rate to see impact to battery life. I believe we have tested some for as long as 10 month.

koala, according to your calculation the power dissipation of the AA battery charged at 200mA is at worst 0.4W assuming all input power is lost as heat. A AA battery has a very large surface area to dissipate the heat so temperature naturally won't be too high. 

William


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## modamag (Aug 16, 2006)

William, first off "WELCOME To CPF! Why don't you stay for a while"

Although your explanation on the FET failure makes sense. But I'm a bit confused.
I have couple of these charger. From time to time one of the channel would would charge my Energizer 2500mA @ 200mA charge to the capacity of +3500 mAh.

But then I take that cell out and put another cell in there then it would be fine (charge to ~2500mAh.

I then discharge the bad cell (on CBAII) and recharge it again. It charges fine to ~2500mAh.

So the question is can the FET be intermittently bad?


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## willchueh (Aug 16, 2006)

modamag said:


> From time to time one of the channel would would charge my Energizer 2500mA @ 200mA charge to the capacity of +3500 mAh.



modamag, 

In your case, it is not due to a MOSFET failure. Once they fail, they are done. 

The BC-900 uses four termination methods: Negative Delta V, Maximum Voltage, temperature and time. Negative Delta V usually won't work at a current of 0.1C as the voltage drop is too small be consistently picked up by the MCU. I believe that the BC-900 is missing the termination point in your case (and terminating probably by maximum voltage - LaCrosse probably chose a relatively high value to avoid a false termination). However, as described in my post earlier, this is not significant enough to cause battery to overheat and melt plastic (plastic melts at around 90 deg C). 

The best practice for using 0.1C charge is the following:

1) Discharge to battery to 1.0V/cell.
2) Charge it at 0.1C for 16 hours. 

This way, you know the battery is empty before recharging. Therefore, you can use time to control the charge. This is recommended by many battery manufacturers and our practice at Maha.

William


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## modamag (Aug 16, 2006)

Ah! now that much clearer. Thank you for the insight. I'll charge them @0.5C from now on.


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## wptski (Aug 16, 2006)

William:

I don't understand how a charger can use all four types of termination that you've listed. I've graphed the MAHA C204W watching temperature and voltage with a scope. The Dt/dt or 1.8F/min rise occured just before ZeroDeltaV and therefore before -DeltaV.

A BC900 charging cells over 2Ah at 1A may pause for high temperature(125F-130F) several times.


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## willchueh (Aug 16, 2006)

wptski said:


> I don't understand how a charger can use all four types of termination that you've listed.



Typically charger designers use multiple termination methods to safeguard against overcharging. Of course, the termination is usually due to one of the conditions been met (this is, however, dependent on the algorithm - the MH-C808M for example uses more than one variable at the same time). 

Negative Delta V is usually pretty reliable. However, certain aged battery might not provide sufficient delta V and therefore may require temperature as a backup. 

Generally speaking, the following priorities are followed:

#1: Voltage (Negative Delta V or other algrithm)
#2: Temperature
#3: Time

William


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## SilverFox (Aug 16, 2006)

Hello William,

Thank you for your comments. I have been trying to understand the mode of failure for some time now, and your analysis seems to make the most sense.

I would suppose that for those of us that are continuing to use this charger, we should insure adequate air circulation around the charging unit, and charge on a heat resistant surface. Is there any easy way to apply additional heat sinking to the MOSFET?

Tom


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## jtr1962 (Aug 16, 2006)

willchueh said:


> In the BC-900 design, it uses a very small package MOSFET (SOT23) as a linear step-down regulator which has to disspate power due to the difference in voltage between the power supply (4V) and the battery (1.5V). The power disspated by the MOSFET is the current times the voltage difference. At peak charging current, the wattage disspated is considerable.


Hence the reason why I'm now running my two BC-900s on a 2.8V power supply. See this thread. The stock supply is around 3.0 to 3.1 volts. My modded supplies give about 2.8 volts no load and about 2.6 volts at the charger when all four charging stations are at 1000 mA. The MOSFETs get warm but not very hot like before. I agree 100% that the MOSFETs are overstressed at the stock power supply voltage. In fact, it really concerned me the first time I noticed how hot they were getting.



> The BC-900 uses four termination methods: Negative Delta V, Maximum Voltage, temperature and time.


I'm not sure that the BC-900 uses maximum voltage to terminate at all. I've charged some AAA rechargeable alkalines while watching everything very carefully. At the 1000 mA charge rate the voltage actually goes all the way to ~2.10 volts before they terminate. I don't know whether the charger is using maximum voltage to stop the charge at that point or if the cells just happen to have a negative delta V like NiMH/Nicad once they reach full charge. I've observed the same behavoir with plain old alkalines as well (yes, I know they're not supposed to be recharged but I did it once as an experiment under continual observation).

BTW, welcome to CPF! :thumbsup:


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## willchueh (Aug 16, 2006)

SilverFox said:


> Is there any easy way to apply additional heat sinking to the MOSFET?



Tom,

Frankly speaking, I believe LaCrosse used the improper MOSFET packaging (SOT23) for this application. Most SOT23 are only rated for Pd = 1.0 ~ 1.25W. Given their linear design, the best case efficiency = (Vin - Vout)/Vin. 

The best bet would be to replace the MOSFET with a bigger packaging ones, say SO-8 or TO-252. But there might be problem fitting these components in. 

William


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## willchueh (Aug 16, 2006)

jtr1962 said:


> Hence the reason why I'm now running my two BC-900s on a 2.8V power supply. See this thread.



jtr1962,

I believe you got the right idea. However, there is a slight problem. BC-900 uses a single output power supply. When you reduce the output voltage of the PSU, you are simultanesouly reducing the voltage to the MOSFET and the voltage to microcontroller. 

The BC-900 uses a form of inexpensive mounting called Chip-On-Board (COB) to cut cost. If you've seen the guts of the charger, it is under the black epoxy. It is basically a silicon wafer without a case. These types of ICs are pretty sensitive to input voltage. Without knowing the exact specs, your supply voltage to the IC could be below the required voltage and may cause stability (latching) and reference voltage problems. 

All the chargers we design almost always have a secondary voltage regulator (1% regulated output) right before the controller IC. However, I believe the BC-900 lacks this.

The best way would be to just reduce the voltage to the MOSFET but not to the main IC. I would imagine that, however, it is not an easy modification.

William


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## jtr1962 (Aug 16, 2006)

willchueh said:


> Without knowing the exact specs, your supply voltage to the IC could be below the required voltage and may cause stability (latching) and reference voltage problems.


I've had the chargers running continuously refreshing batteries for the last week to see if there are any problems. So far, so good. The voltage readings still seem correct even at the reduced power supply voltage so the lower voltage isn't causing problems with the reference voltage.

Yes, you could reduce the voltage to the MOSFET but not the main IC by putting a Schottkey diode in series with the MOSFET drain. Of course, that will still result in the same amount of heat being dissipated on the circuit board even if it is spread among two components instead of one. I consider the plastic case too small and poorly ventilated to dissipate that amount of heat.

BTW, some here have said that the voltages displayed by the charger are incorrect. To a first approximation this was true unless the charger was modified with added bypass capacitors. See this thread for details. However, once modified the voltages seem accurate to within 0.01 or 0.02 volts. The problem is that you can't simply measure the voltage at the battery terminals while it's _in the charger_ and compare that to the value displayed by the BC-900. Since the BC-900 pulses the charging current, it measures battery voltage during the time when the MOSFET is off so as to get a reading independent of voltage drops in the contacts and PC board traces. However, if you try to measure the voltage at the battery terminals with a multimeter you'll end up with an average of the voltage while charging and the resting voltage. The readings will be higher than what the BC-900 displays if it's charging, and lower if it's discharging (yes, it pulse discharges as well). I just thought this info would be useful. Also note that it's important to measure the battery voltage right after you pull it out of the charger since it will immediately start to drop once it's no longer receiving charging current. I find that if I measure within 10 seconds of pulling the cell the measured voltage pretty much agrees with what the BC-900 displayed.


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## willchueh (Aug 16, 2006)

jtr1962 said:


> The problem is that you can't simply measure the voltage at the battery terminals while it's _in the charger_ and compare that to the value displayed by the BC-900.



Engineers usually refer this voltage as the "off-line voltage." A quick and dirty way to see this voltage is simply connect a scope to the battery terminal. You'll be able to observe the up and downs in the voltage. 

The only tricky thing is a battery is typically modeled as a resistor and a capacitor in parallel (plus other stuff) so there is a capacitive discharge effect when the current pauses.

William


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## wptski (Aug 17, 2006)

I had four "HR" stamped made in Japan 2500mAh Energizer AA cells so I discharged two at a time on a Triton and ICE charger down to .9V/cell, so each cell isn't really equal. I have two cells charging at 200ma on a v33 BC900 monitoring the temperature on both with K-type bead probes on a Fluke 54-II.

Since I don't want to leave it unattended, I'll only be able to let it charge for a max of 12 hrs. if it don't terminate or melt by then!  I'll be able to post a graph tomorrow evening but right now after almost four hours, they are at 86F which is 8F higher from the start.


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## evan9162 (Aug 17, 2006)

I can't believe that LaCrosse is using an SOT-23 package to disspate upwards of 1W. I wouldn't dare touch that level of power dissipation in a package smaller than SOT-223, and would rather use a DPAK or D2PAK instead. Sounds like a poor design decision.


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## wptski (Aug 18, 2006)

I haven't looked at the log so temperature maybe a bit higher! One cell finished charging this morning at 11:12, 2150mAh at 101F. The other cell was still charging at 12:01, 2310mAh at 97F. So the calculation above for 100F at 200ma was accurate.


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## wptski (Aug 18, 2006)

I didn't post a graph from the 200ma temperature test because there wasn't anything really interesting to look at.

Below is a temperature graph of two 2.5Ah Energizer AA cells again in Slots 1/2 but at 1A charging rate. The green dash line is Slot #2 and the blue line is Slot #1. Notice how Slot #1 turned off for high temperature lower than Slot #2 was allowed to go and finish the charging cycle. Slot #1 cooled down and restarted. It finished charging but was allowed to go to higher temperature which had cut it off earlier!! No fan is used.

Looks like I cut off the time scale words on the pic, it's 1,2,3 hrs.


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## koala (Aug 21, 2006)

Stack em up! Double/parallel the tiny mosfets . Maybe a tiny heatsink will cure it...

I don't mean to bash BC-900. The electronics in it is a joke. It's the cheapest looking circuit board I've ever seen. It seems like the whole charging system depends alot on the software.

I'll try to find out what's the max operating temperature of the mosfets in BC900 running at this week.

Question: I have an Infrared Thermometer I use it for everything. I find it pretty accurate and very effective. Is there anything I should be aware of as I will be pointing it directly on the tiny surface on the mosfets of the BC900.


Vince.



willchueh said:


> Tom,
> 
> Frankly speaking, I believe LaCrosse used the improper MOSFET packaging (SOT23) for this application. Most SOT23 are only rated for Pd = 1.0 ~ 1.25W. Given their linear design, the best case efficiency = (Vin - Vout)/Vin.
> 
> ...


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## wptski (Aug 21, 2006)

koala said:


> Stack em up! Double/parallel the tiny mosfets . Maybe a tiny heatsink will cure it...
> 
> I don't mean to bash BC-900. The electronics in it is a joke. It's the cheapest looking circuit board I've ever seen. It seems like the whole charging system depends alot on the software.
> 
> ...


You might want to experiment on something of similar size. The low end IR temperature probes have 1" spot at 8" most often and they suggest staying at least 8" away from the target.


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## tacoal (Aug 22, 2006)

The MOSFET used in BC-900 is NTGS3443, 2A, 20V.


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## jtr1962 (Aug 22, 2006)

That MOSFET would be fine if they were using a switching type circuit. For the linear circuit in the BC-900 it's marginal at best.

BTW, both my BC-900s have been happily operating on ~2.8V with the modified supplies for 11 days continously now. I haven't noticed any problems at all. The only thing I've noted is that all the batteries I've refreshed are reading lower capacity than the last time. Some are as much as 25% less. I'm guessing the warmer room temperature in my work room is the culprit (80+ °F versus ~65°F last time). I noticed the same thing even with the stock supply so it's not a voltage reference problem.


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## wptski (Aug 22, 2006)

jtr1962 said:


> That MOSFET would be fine if they were using a switching type circuit. For the linear circuit in the BC-900 it's marginal at best.
> 
> BTW, both my BC-900s have been happily operating on ~2.8V with the modified supplies for 11 days continously now. I haven't noticed any problems at all. The only thing I've noted is that all the batteries I've refreshed are reading lower capacity than the last time. Some are as much as 25% less. I'm guessing the warmer room temperature in my work room is the culprit (80+ °F versus ~65°F last time). I noticed the same thing even with the stock supply so it's not a voltage reference problem.


Your saying that higher temperature gives lower capacity would go against the grain of RC'rs wanting to run hot packs. Some chargers like the ICE has provisions to allow timing so packs are hot just when they need to be used.

So, what about using a fan likes some are doing? Does that cause less capacity too?

There's some odd stuff about high temperature cufoff too because form my graph posted above you can see that it cutoff at a lower temperature once but allowed a higher temperature when it neared termination.


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## evan9162 (Aug 22, 2006)

tacoal said:


> The MOSFET used in BC-900 is NTGS3443, 2A, 20V.



unless there are huge copper pads coming off the leads of this device, it's maximum power disssipation is just 0.5W.


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## jtr1962 (Aug 22, 2006)

wptski said:


> Your saying that higher temperature gives lower capacity would go against the grain of RC'rs wanting to run hot packs. Some chargers like the ICE has provisions to allow timing so packs are hot just when they need to be used.


I don't know why my cells are coming out at lower capacity. I thought higher temperature might be a reason.



> There's some odd stuff about high temperature cufoff too because form my graph posted above you can see that it cutoff at a lower temperature once but allowed a higher temperature when it neared termination.


That's when it's charging. When discharging even if the cells are warm at the end of charge they'll cool off fairly quickly with or without a fan. Yes, I use a fan which is on all the time in order to keep my cells from overheating. I do this with any charger capable of charging over 500 mA, not just the BC-900. I just don't trust the charger to shut down in the event the cells overheat. Also, I modded my BC-900 to have a lower cutoff temperature because the cells could get too hot with the stock setup. If I don't use a fan at the higher charge rates the charger would go into thermal shutdown long before the cells were charged, and the charging would take much longer to complete.


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## willchueh (Aug 22, 2006)

It would be interesting to monitor the MOSFET package temperature. Then compensate for the thermal gradient across the junction and case. Compare that to spec to see if it's close to the "kill zone."

William


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## willchueh (Aug 22, 2006)

tacoal said:


> The MOSFET used in BC-900 is NTGS3443, 2A, 20V.



Thanks tacoal for the exact packaging information. Based on the spec, the maximum disspation for minmal thermal padding is only 0.5W. Even if the thermal padding is improved, at best it can take 1W (and quoting the spec, this requires "package mounted onto a 2 in square FR-4 board with 0.06" thick cupper, which I doubt there is room in the BC-900). 

William


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## wptski (Aug 22, 2006)

jtr1962 said:


> I don't know why my cells are coming out at lower capacity. I thought higher temperature might be a reason.
> 
> 
> That's when it's charging. When discharging even if the cells are warm at the end of charge they'll cool off fairly quickly with or without a fan. Yes, I use a fan which is on all the time in order to keep my cells from overheating. I do this with any charger capable of charging over 500 mA, not just the BC-900. I just don't trust the charger to shut down in the event the cells overheat. Also, I modded my BC-900 to have a lower cutoff temperature because the cells could get too hot with the stock setup. If I don't use a fan at the higher charge rates the charger would go into thermal shutdown long before the cells were charged, and the charging would take much longer to complete.


By my graphing, it takes about 15-20 minutes to resume charging after a temperature pause.


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## Bones (Feb 11, 2010)

Bah, wrong thread, but interesting reading nonetheless...


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## alphazeta (Feb 13, 2010)

......


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