Another La Crosse Meltdown

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.

Hello Nikon,

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

Tom

:eww:

thankfully it didnt vent anything, use gloves

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.

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

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.

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

Tom

Click to expand...

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??

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

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

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.

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.!.

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.

koala said:

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

Click to expand...

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

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?

modamag said:

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

Click to expand...

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

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

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.

wptski said:

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

Click to expand...

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

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

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.

Click to expand...

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.

Click to expand...

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:

SilverFox said:

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

Click to expand...

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