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JDB/xiaoxiang BMS Overtemp Shutoff

kshopper2084

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Nov 21, 2021
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Hi all. I've finally gotten a chance to test/use my new LiFePO4 280Ah battery with the JDB 200A BMS in my motorcoach.

Over a 2 week trip we used the battery extensively and charged via the Renogy 60A DC-DC charger while driving, and also using the legacy house Freedom 20 inverter/charger.

The Freedom 20 doesn't have a Lithium charging mode (it's 25 years old!) but it does allow you set a few different charge profiles for warm/cold FLA, GEL, etc. batteries.

So everything worked out really well on the trip, however, I did notice that when charging using the Freedom 20 I was seeing some scary high temperature readings from the BMS mounted temp sensor. During one charge session the BMS shutdown due to overtemp.
I was using the following charge profile on the Freedom 20 during these events: Bulk=14.3v, Float=13.4v.
The Freedom 20 will happily charge at up to 100A.

Here are some shots at the cutoff time, and then 1 hr later when the BMS temp dropped to match the rest of the cells.

Screen Shot 2022-06-03 at 9.49.38 AM.png Screen Shot 2022-06-03 at 9.49.58 AM.png


Any thoughts on why the BMS temps would be so much higher than the cells in this scenario?

I then changed the Freedom 20 charge profile to use lower voltages: Bulk=13.9v, Float=13.3v.
Charging using this profile did not result in the extremely high BMS temperatures, and the charge cycle ended slightly before completely charging the battery:

Screen Shot 2022-06-03 at 9.50.48 AM.png

Also, when charging using the DC-DC 60A charger no high temps have been seen.

I am fine with using this charge profile but am curious as to why the BMS would overtemp like that?

Finally, I have one temp. sensor on the cells that will sometimes go to "off chart low" aka -255c while charging is in progress. This doesn't seem to cause any actual problems, and the other sensors remain accurate, but I have my notifications setup such that I see when the temps drop below zero and so I am flooded with notifications when this happens. Seems sporadic. Has anyone else seen this type of sensor problem?
 
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Assume you are ensuring either charger is not allowed to exceed maximum LFP charging voltage, Make sure Freedom 20 does not automatically enable lead-acid equilization cycle. You don't want any lead-acid temp charge voltage compensation. Usually just disconnecting battery temp sensor will default them to 25 deg C with no compensation adjustment.

Likely temp is caused by amount of current through BMS. Most Chinese BMS can only tolerate a continuous current of about half their advertised currrent rating. They can tolerate their rated current for about 5 minutes before they overheat.

Other potential issue is placing BMS in a confined space where it does not get enough heat dissipating ventilation or a space that gets high ambient temperature. Not great for LFP battery either.
 
Assume you are ensuring either charger is not allowed to exceed maximum LFP charging voltage, Make sure Freedom 20 does not automatically enable lead-acid equilization cycle. You don't want any lead-acid temp charge voltage compensation. Usually just disconnecting battery temp sensor will default them to 25 deg C with no compensation adjustment.

Likely temp is caused by amount of current through BMS. Most Chinese BMS can only tolerate a continuous current of about half their advertised currrent rating. They can tolerate their rated current for about 5 minutes before they overheat.

Other potential issue is placing BMS in a confined space where it does not get enough heat dissipating ventilation or a space that gets high ambient temperature. Not great for LFP battery either.
The Freedom 20 is configured to NOT equalize, and doesn't have dynamic temp. compensation, so those shouldn't be factors.

As to current, I suppose several hours of 100A charging could add up, but not clear why charging the same amount of time at slightly lower voltage would be so different, still outputting 100A. I also have used the batteries discharging well over 100A for 30mins at a time and not seen a similar spike in BMS temps.

The battery is housed in the same basement compartment in the RV as the Freedom 20, so air circulation could be an issue, since the charger will heat up while charging as well. however, the cells were at 40c while the BMS went to 85c. You'd think the cells would heat up more if ambient air temps were the issue.

Just looking for ideas, so far charging with lower voltage on the Freedom 20 seems to be resolving the issue, but I'll need to keep an eye on it.
 
I have an IR heat gun to double check those readings.

1) In the pic with the high temp, charge and discharge was shut off. What were the loads at this time?

2) How is this BMS housed And what were the outside temps?

3) Can you bring charging lower? Perhaps a gel setting would be lower.

The easy thing to do would be to say a new SCC to replace the Freedom 20 may be needed for your new lithium batteries, but ought to go through some troubleshooting first.
 
Following RCinFLA about heat in the BMS, does it have any positive ventilation? Any air moving across it. A little moving air goes a long way to keep electronics temp down.
 
I have an IR heat gun to double check those readings.

I have one too, but the battery isn't super easy to get to and I was busy with other things at the time.

1) In the pic with the high temp, charge and discharge was shut off. What were the loads at this time?
Not sure, there were loads for sure but we were connected to 50A shore power so the battery itself wasn't under any load.

2) How is this BMS housed And what were the outside temps?
It's inside a regular Group 27? battery box, with the open gaps in the top cover. Outside temps were probably mid 20s C.
3) Can you bring charging lower? Perhaps a gel setting would be lower.

Yes, this is what I've done. It had been set to Bulk=14.3v, Float=13.4v.

...and after the hightemp cutoff event I changed it to : Bulk=13.9v, Float=13.3v.

Based on a few charge cycles since the setting change not seeing anywhere close to the same high temp as before (39.9C vs 85.6C) , so seems to be helping, though it doesn't quite get the battery 100% charged. That isn't a huge concern though, as more often the DC-DC charger off the alternator should charge it fully.

I just don't understand quite why there would be such a difference.

The easy thing to do would be to say a new SCC to replace the Freedom 20 may be needed for your new lithium batteries, but ought to go through some troubleshooting first.
Yes, this is my plan. Trying to keep as many original components as feasible.
 
Following RCinFLA about heat in the BMS, does it have any positive ventilation? Any air moving across it. A little moving air goes a long way to keep electronics temp down.
No positive ventilation. Stored in a Group 27 battery box, with the openings in the lid. BMS is tape mounted to the side of one end of the cells, with neoprene rubber between it and the cell, with the BMS heatsink on the outside edge.

Battery is inside the utility closet next the Freedom 20 unit, so certainly the Freedom 20 would throw some heat as well.
 
No positive ventilation. Stored in a Group 27 battery box, with the openings in the lid. BMS is tape mounted to the side of one end of the cells, with neoprene rubber between it and the cell, with the BMS heatsink on the outside edge.

Battery is inside the utility closet next the Freedom 20 unit, so certainly the Freedom 20 would throw some heat as well.
I have played with some adjustable CC/CV boards with heatsinks. By the time they get to about 50% of their rated output, they need a fan. The heatsink is very hot to the touch. Sounds like your BMS (all closed in) is suffering from lack of ventilation, at that power level.
 
I think you should put a scope on the output of the Freedom. See what the switching looks like in a 25-year-old charger.
 
If you have two charging points, and the DC to DC alternator charger works fine, but the Freedom didn’t, then you have tried what I would like a lower charge voltage prior to swapping the freedom out.

If you can stay off the freedom one day charge with your alternator only, that is the only other thing I may do. Low temps with the alternator but high when the freedom is on points to the freedom. Consistently high in both circumstances points to the BMS.

EDIT: With the price of lithiums, I’d honestly replace the 25 year old freedom anyway.
 
Alright guys, thanks for all the suggestions.

While I do think airflow could be an issue, so far the temps have only really spiked when I had the Freedom set to charge at the higher voltage (Bulk=14.3v, Float=13.4v). I'm going to keep using it at the lower setting (Bulk=13.9v, Float=13.3v) for now and monitor it closely to see how it does. Of course, the higher voltage setting could be a red herring, and maybe it got that hot because the Freedom was working harder in general with additional loads at the same time, causing the air in the utility closet to get hotter, etc.

As for trashing the old Freedom 20, well I take pride in keeping working things working if I can. There is zero chance that a modern era piece of electronics is going to run fine for 25 years, I'll tell you that much. :) I was planning on removing the Freedom this fall at the end of the season to clean it, reflow any cracked solder joints, and replace the capacitors. Depending on what I find then I may replace at that time.

If I see another temp. spike on the BMS I will try to verify with my IR gun as well.

Any thoughts on my one temp. sensor that goes off-chart low sporadically while charging? Anyone seen that? Maybe I should contact the vendor to see about a replacement sensor. At the end of the day I have 3 temp sensors, could get by with 2 so I could disable the errant one if becomes a problem.
 
No on the BMS to battery difference, but the BMS I have does not monitor the BMS temp, only two spots on the battery, so my measurements have been through an occasional IR gun reading.

It’d be interesting to see what the BMS manufacturer says. All batteries you buy off the shelf have internal BMSs, and running at 20 - 25 degrees c ambient temp is not that hot like the 85 you see. IMO adding a fan to it may not fix but might mask whatever the problem is.
 
Even though the zFreedom reads correctly, I do wonder if the DC pulses like a PWM SCC. Only a scope would tell that, not a DMM.

This could pulse to more than the SCC wants to see.

Lead acid is more tolerant of this pulsing than lithium.
 
Even though the zFreedom reads correctly, I do wonder if the DC pulses like a PWM SCC. Only a scope would tell that, not a DMM.

This could pulse to more than the SCC wants to see.

Lead acid is more tolerant of this pulsing than lithium.
Interesting. Certainly possible. The inverter is also a modified sine-wave so it's technology of the era.

I think using the lower voltage charge settings should be taking the Float cycle out of the equation, since it's configured for 3.3v, whereas the settings I was using when I got the high temp situation was 3.4v float.

I should really verify the voltage being output by the charger as well since it could be varying a substantial amount with old capacitors, etc. in this thing. Entirely possible the 14.4v bulk charge was actually substantially higher as well.
 
What BMS temps do you experience when you discharge at ~100a for the same amount of time? If it's significantly lower, I'd say the cleanliness of the DC charge output from your Freedom is suspect. As chrisski mentioned, only a scope will reveal these types of issues. With our JBD 200a BMS, BMS temps remain similar given the same charge and discharge c rate. For us, voltage (<=14.6v) has never been a factor in terms of BMS temps, only current.
 
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What BMS temps do you experience when you discharge at ~100a for the same amount of time? If it's significantly lower, I'd say the cleanliness of the DC charge output from your Freedom is suspect. As chrisski mentioned, only a scope will reveal these types of issues. With our JBD 200a BMS, BMS temps remain similar given the same charge and discharge c rate. Voltage (<=14.6v) has never been a factor in terms of BMS temps, only current.
Discharge temps. have always been nominal (30c).

I wonder if the cleanliness of the incoming 50A AC might be a factor?
 
Discharge temps. have always been nominal (30c).

I wonder if the cleanliness of the incoming 50A AC might be a factor?
So, 30c discharge vs. 85c charge BMS temps at the same 100a. Maybe the charge mosfets aren't making good thermal contact with the heatsink. Or, the Freedom is producing dirty DC. If dirty DC (14.3v at 100a) is indeed heating up the charge mosfets, there's a chance it may be having a negative effect on the cells, too.

Dirty ac on the Freedom input is certainly a possibility. Can't imagine why a properly operating charger wouldn't be able to filter it out. Have you encountered the same overheating 85c charge mosfets using different ac power sources for the Freedom?
 
So, 30c discharge vs. 85c charge BMS temps at the same 100a. Maybe the charge mosfets aren't making good thermal contact with the heatsink. Or, the Freedom is producing dirty DC. If dirty DC (14.3v at 100a) is indeed heating up the charge mosfets, there's a chance it may be having a negative effect on the cells, too.

Dirty ac on the Freedom input is certainly a possibility. Can't imagine why a properly operating charger wouldn't be able to filter it out. Have you encountered the same overheating 85c charge mosfets using different ac power sources for the Freedom?
Well except for the data is inconsistent. I've only had the high temp charging a couple of times, they could have both been when using the same campground power source, I can't remember anymore. I do know that since I've lowered the charge voltage the problem hasn't recurred, but only a few charge cycles with that setting too.

I think I need more data.
 
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