Troubleshooting Lithium Ion Balance/Protection Board (HX-3S-FL25A)

[pushpaw]

Hello!

I'm putting together a 3S lithium ion battery pack using 18650's and a balancing/protection board from ebay (model HX-3S-FL25A-A). After building the pack and connecting the balance board as per the instructions (starting with most negative and progressing in order toward most positive; B-, B1, B2, B+), the board appears to still be in protection mode, showing a voltage of only 11.8V on the P+/P- terminals despite the pack voltage being around 12.3V. The very minimal instructions for the board suggest "After the wire is connected, needs to charge first, then have output", but charging through the P terminals has no effect, showing only a few microamps of current when either charging or applying a load. I've tried discharging and re-charging the pack by bypassing the protection board, to see if perhaps a lower pack voltage might bring the board out of protection mode, but there was no apparent change. Also, the cells are all in perfect balace (+/- ~.01V), having been previously charged in parallel.

Knowing precious little about the design and operation of a circuit like this, what are some troubleshooting steps I might take in order to find out more about this problem? How can I determine if this is simply a bad board without ordering a replacement part? What points in the circuit might I want to look at in order to determine what state the board is in (e.g. overvoltage protection, undervoltage protection, balancing of which cells, etc.)? Is there any way that I might reset the protection circuit or otherwise 'force' it to re-evaluate its protected state?

For reference, here is the board in question on Banggood, and Aliexpress

Any suggestions, tips, or observations would be much appreciated!

[ArthurDent]

What voltage are you looking to feed to your load? I suspect that if the battery voltage is 12.3VDC there will be some drop between the batteries and the output/input terminal and 0.5VDC giving 11.8VDC output doesn't look unreasonable. This would be with batteries near full charge so you might expect around 10VDC output when the batteries have discharged to the point that they require recharge to start.

Perhaps the 4S board and 4 cells to give you a higher voltage is what you want. More information is needed. 

[m3vuv]

I recently brought a 4s bms board from ebay(chinese)  it was for a prc 320 radio battery conversion from nicad to l ion the output voltage was 6 volts lower than the output!,,my advice would be avoid el cheapo chinese stuff for this,,just my twp pences worth!!,to ad insult to injury the company said i had to order and pay for another one before they would replace or refund wtf!!

[pushpaw]

Ah, the issue is not so much that the voltage itself is insufficient for my needs but rather the current delivery: when presented with even a 50 ohm load the current is only ~40uA, which is a bit lower than I'd have hoped for from a 9P pack. I suppose I'm curious about how to diagnose the board before I order a replacement, partially because I worry that replacing it might not solve the problem (given that a similar situation has arisen for me before with this type of protection circuit). As for the voltage I'd like to drive a load at-- 11.8v would be perfectly sufficient.

[ArthurDent]

This may not solve the problem but here's a guess as to how the charger board works (see drawing). The top section has a bunch of 3 pin voltage comparators and they control the small 6 pin MOSFETs that are turned on when the batteries reach full charge to discharge individual cells through some load resistors.

The three 18600 cells are in series and B+ and P+ are connected together. P- allows charging through the big MOSFET switch array and through the low resistance high current sensing resistors to B- and these big MOSFET switches can turn the load on or off to protect the batteries from short circuit.

What you describe is that the MOSFET switches are only off and you can't get an output.  Check the gate voltage on the switches when the load is on P+ and P-. If the batteries are charged and the circuit is working properly, you should get an output. Also see if the batteries will charge through P+ and P-. If the batteries won't charge and the wiring is correct, there would appear to be a problem with the board.

Keep in mind that this forum is like calling your doctor and asking: "what is this pain in my foot?" It is hard to figure out what is what long distance plus I haven't owned one of these boards so judge my advice accordingly.   

[pushpaw]

While the foot analogy is spot-on, I now know the difference between a in innocuously-pained foot and a broken one! The gate voltage is nil, and upon being pulled up the board comes out of protected mode but only temporarily. Thanks for your insight!

[dagelf]

Firstly, the most likely reason for getting no output, is for one or more of your cells being either under- or over voltage. I know you say your cells were in perfect balance, but I stumbled on this post... checked everything in detail, and found that mine wasn't.

The spec for this board says:
 Overload Voltage: 4.25 - 4.5V
 Charging Voltage: 12.6-13V

Seeing as that you're bypassing the circuit to charge them, I would put money on the fact that your cells were over voltage, so the protection kicked in. (Or never switched off?  ) Or maybe your multimeter's 4.25V is not the same as the boards'? I'm sure you can tweak the voltage ranges with a resistor. But refer to this blog icon for my thoughts on that. 

Oooor... maybe resistors or mosfets are outliers in their tolerance ranges... If it's a resistor, just pop another smd resistor on top to fix that, I guess?  Or run the board at supercooled, or superheated temperatures to push those tolerances back in line! 

Secondly, I disagree with the foot analogy, because the foot is much more complex than most simple electronic circuits. But mostly because you shared the model number, so anyone could look up the schematics and specs.

Unrelated. I've thought I'd broken my foot at least 4 times in recent memory. I hardly ever wear shoes. But twice I was in a situation where I could do nothing but keep running... and after a minute or 2 of severe pain, the pain subsided somewhat... and when I got to my destination an hour in the one instance, and 2 hours later in the other, it was like nothing had happened. If I was home I would've put my foot on ice and it would probably have been sore for weeks, like it had at other times. If my foot was really broken then the severe pain would've continued for longer... or would've been too severe for me to keep running. I guess. 

Okay, where's my necromancer badge?