BMS not passing current

[TlfT]

I have a 15s XiaoXiang BMS. After I built my 15s5p pack with LG M50LT cells, current was going through fine. Now a week later, I only very very briefly see ~half voltage at C-/P- before it rapidly drops down to 0.4 volts.

I opened the pack and tested all the voltages at the balance leads. Each cell group is between 3.5 and 3.6 volts, where I left them after capacity testing. There is full (53v) voltage at B-.

Is there anything that could be going on, or did I get an odd BMS?

Thank You

 

[amberwolf]

Sounds like the BMS is just doing it's job of protecting the cells and user.

That big a difference in voltage, at just about half charged for some of the cells, means there is a difference in properties (capacity, resistnace, etc) between cells, so the BMS has probably detected this and shut down to prevent damage to the cells that can lead to a fire.

Some BMS will protect against not only HVC or LVC excursions, but also from a difference in voltage (delta) between cells greater than some amount--commonly 0.1v, because that is a huge difference in cell properties to cause that.

Some BMS with this function can have it edited to be less, or more, but bypassing a protection to use poorly matched cells is not a great idea.

It's a better idea to replace all the cells with identical, matched cells, that all have the same properties (not similar, but identical). Then you have a better-performing pack that will last longer.

It's not always practical to do this, because it is very hard to locate matched cells, but sometimes you can come close by reusing cells from recycled EV packs (since those are much more likely to use matched cells in the first place for various reasons, whereas cell vendors and ebike/etc pack builders are mcuh much less likely to spend money on the expensive time consuming step of matching them).


If your cells *are* matched, then to have it operate correctly you just need to charge them all up to identical voltages (most easily done at either full or empty) so that they are all at identical SoC. If you have a balancing BMS (not all have balancers) then it will do this for you once you've manually gotten them close enough to allow the BMS to stay out of protection modes, just by leaving the charger on there until it stops cycling on and off (could take hours, days or even weeks for a badly-imbalanced pack).

 

[docw009]

You're measuring 26-30 volts at B- relative to C- or P-, Then it decays to .40V. Let's look at that later. It probably means the BMS has shut down, More importantly, what's the voltage between the positive pin on the output connector and P- ? I bet you get nome at all.

Why don't you check the balance connector. Walk the probe up from B0 to B15, and make sure each point has a voltage, and that it increases steadily by your 3.6V average. If it worked originally, probably a balance wire became disconnected. It's not uncommon.

 

[TlfT]

Thanks guys.

Main positive from the XT60 connector to b- is 53v, main positive to c-/p- shows 25v for a fraction of a second then falls down to 0.4v.

I tested all the leads sequentially at the BMS PCB, they are 3.44v-3.61v. Tugged on all them to be sure and they are good.

When I connect a charger it doesn't switch on to charging.

Is that 0.17v difference enough to trip the bms? Must be. Do I tear the pack apart to get all the cells exactly balanced? Or buy a new BMS that isn't so touchy?

This is the BMS I have:

Li-ion/Lifepo4 3s-20s 30a BMS WITH BALANCE

Note: These are not compatible with our bluetooth modules or computer interfaces. --------------------Here are 30a BMS units with 50ma balance function. We carry 3s-20s and both Lithium Ion (Li-Ion) and Lithium Iron Phosphate (Lifepo4). The lithium ion version also works with Lithium Polymer...

batteryhookup.com

 

[Chalo]

You don't have to take the pack completely apart to balance it. You can balance the cells manually with a power supply at their bus bars or you can balance them with an active balance board, which attaches like a BMS, but does a different job.

 

[E-HP]

You don't have to take the pack completely apart to balance it. You can balance the cells manually with a power supply at their bus bars or you can balance them with an active balance board, which attaches like a BMS, but does a different job.

This would work after cutting off the connector.

https://a.co/d/fjnQDka

 

[amberwolf]

I tested all the leads sequentially at the BMS PCB, they are 3.44v-3.61v.

As noted, that's very bad imbalance. I recommend that you look up a charge curve chart for your cells, and see how much capacity difference there is between those voltages, and it might help you understand why it is a problem.

EDIT: Here's one version; I marked the chart with the voltages above intersecting the capacity line for a 5A discharge. It shows a whole amp-hour difference in capacity / state of charge between those two voltages. That's huge, essentially 20% of the whole capacity.

Test of LG 21700 M50 5000mAh (Grey)

When I connect a charger it doesn't switch on to charging.

Is that 0.17v difference enough to trip the bms? Must be.

A good BMS *should* prevent usage in that scenario. A pack that had gotten into that condition during use would be either damaged or faulty or very aged.

Do the cells you have all have identical capacities and resistances? If not, this type of scenario is likely to happen during actual use over time, and will get worse as the cells age.

Do I tear the pack apart to get all the cells exactly balanced?

Just charge thru the balance leads if you don't have access to the actual cell group interconnects.

The only reason to tear it apart would be to replace cells (groups) that are mismatched in properties (capacity, resistance, etc) to prevent this kind of problem from happening.

Or buy a new BMS that isn't so touchy?

You *want* a BMS that is designed to protect you against faults and problems like this, so you don't end up with a cell failure that ends up causing a fire.

This is the BMS I have:

Li-ion/Lifepo4 3s-20s 30a BMS WITH BALANCE

Note: These are not compatible with our bluetooth modules or computer interfaces. --------------------Here are 30a BMS units with 50ma balance function. We carry 3s-20s and both Lithium Ion (Li-Ion) and Lithium Iron Phosphate (Lifepo4). The lithium ion version also works with Lithium Polymer...

batteryhookup.com

Unfortunatley they don't provide any actual documentation on them, or link to the manufacturer's page for that specific model (or an actual model that could be looked up to find the docs), so we can't say what specific features and protections it has beyond the few limits they do provide on that page.

XiaoXang makes a lot of types and models, and they generally look similar enough that without specific info about the exact one in use, you'll have a tough time knowing complete details about it.

If you do want to replace it, you could, but I would recommend one that has bluetooth and a reliable easy-to-use app that is available in your language, so that you can at minimum do remote monitoring without opening up the pack to find out what's likely to be causing a specific problem. Some of them also allow changing some of the limits in the BMS...but I don't recommend changing them to make them more relaxed, only more restrictive (safer, more conservative).

 

[TlfT]

Thank You for taking the time to help me and pull a graph.

I tested the capacities and they were all within ~100mAh of each other, from ~4950 to ~5050. I tried to bring the cells back to 3.5 before building. I guess I was too concerned with matching the capacities of all 15 series that I ignored voltages, and wound up with two so far apart. I'll get everything all balanced up and sleep better knowing my BMS is smart where I am foolish.

 

[docw009]

I tested all the leads sequentially at the BMS PCB, they are 3.44v-3.61v. Tugged on all them to be sure and they are good.

When I connect a charger it doesn't switch on to charging.

Is that 0.17v difference enough to trip the bms? Must be. Do I tear the pack apart to get all the cells exactly balanced? Or buy a new BMS that isn't so touchy?

I've seen badly unbalanced batteries have cells off well over a half volt, and the BMS still worked, until the bad cells went under 3V, Maybe the top dollar units will selectively shut off at 1/4 volt delta, but the twenty five dollar rigs do not care about the difference between cell groups., All they look at is the voltage on each cell group being above LVC and below Vmax.

Well, you have verified that the battery is healthy in my opinion. If there is no reason for the BMS to not charge or discharge, then the BMS must be at fault, Maybe Battery Hookup will send you a replacement.

In the winter months, it's easy to blow up a BMS with static discharge, I had two identical units that were working perfectly, but I took off the heat shield to look at something. Both quit working after i put my fingers all over the circuitry,