HELP: 72v 50ah LIFEPO4 CATL Battery cuts out at around 70v

500wattz · Jul 25, 2024

Hello all,

I recently bought a brand new 72v 50ah LifePo4 battery off aliexpress from X N technology that uses CATL cells. The thing seems to have the capacity more like a 25ah than a 50ah. My pooched 41.6ah li-ion battery has better range than this thing.

It never charges past 81-82v. always has one cell that is high (3.6v around there) which prevents charging the battery to it's full capacity (lifepo4 charges to 87, not just 84 like li-ion, right?).

My first test run on a 40km commute one way had me stranded at the last 7km due to the bms tripping and shutting everything off, and i mean everything...because a cell or two got too low in voltage (low voltage protect). But before it got close to that, it was running pretty good. Sag was decent, from 81 down to 76-78 on full throttle which for me is 70 line amps, 200 phase, 5kw nominal output, field weakening strong.

my 2nd test run I was more conservative with the throttle (another 40km). At the last KM or two, the battery died. it always does this thing where at around 70v, it will go down to 69, then 64, then eventually nothing and completely offline (my voltmeter on the handlebar). I knew it was going to die so i just pedaled the bike home then did a little test run around the house to see when the battery will actually die die.

Here are two videos, my settings on my xiaoxiang BMS and a screen recording of bms monitoring cells and so forth right before the battery dies.

What gives? Did i purchase a lemon, or is there a setting or two that I have wrong that is causing this?

Cutout video:

BMS settings video:

harrisonpatm · Jul 25, 2024

Couple of things could be the culprit.

First, it does seem like while the pack as a whole might not be bad, you are correct in pointing out that cell 22 reaches top voltage before the rest of them. And when I skipped to the end of your video where you are showing the low volt cutoff, cell 22 is at about 2.6v, while the rest of them are around 2.8v. Based on that alone, it appears that you have a single weak cell that's bringing down the rest of the pack's capacity; ie, a pack is only as good as its weakest cell, and #22 appears to be the weak cell (#10 appears to be the second weakest cell). So, one possible answer to your question, is that you possible do have a pack with 1-3 bad cells, perhaps a lemon. You could potentially show the seller this data and see if they might offer you a refund, discount, replacement, ect.

Second thing is nobody's fault, it's just the way LiFePO4 behaves, relative to Li-Ion.

500wattz said:
it always does this thing where at around 70v, it will go down to 69, then 64, then eventually nothing and completely offline

That's just what LFP cells do when they get to around 3.0v.

If you look at the chart, you'll see that the voltage will drop off hard once it gets down to 3.0v. Especially if you continue to pull current from it, which you were doing in the video.

If it's any consolation, you're not doing anything wrong, regarding either BMS settings or use. LFP is just less energy dense that Li-ion, and its voltage drops off fast at the bottom of its capacity. You could definitely argue that you have 2 bad cells that are bringing down the capacity for the whole pack, and you could choose to top- or bottom-balance the pack depending on your preferred use case, to get a bit more range out of it.

Portugaline · Jul 25, 2024

I would return the pack ASAP. if everything is well connected this are very bad cells, probably used cells recycled to you.

500wattz · Jul 25, 2024

harrisonpatm said:
Couple of things could be the culprit.

First, it does seem like while the pack as a whole might not be bad, you are correct in pointing out that cell 22 reaches top voltage before the rest of them. And when I skipped to the end of your video where you are showing the low volt cutoff, cell 22 is at about 2.6v, while the rest of them are around 2.8v. Based on that alone, it appears that you have a single weak cell that's bringing down the rest of the pack's capacity; ie, a pack is only as good as its weakest cell, and #22 appears to be the weak cell (#10 appears to be the second weakest cell). So, one possible answer to your question, is that you possible do have a pack with 1-3 bad cells, perhaps a lemon. You could potentially show the seller this data and see if they might offer you a refund, discount, replacement, ect.

Second thing is nobody's fault, it's just the way LiFePO4 behaves, relative to Li-Ion.

That's just what LFP cells do when they get to around 3.0v.
View attachment 357039

If you look at the chart, you'll see that the voltage will drop off hard once it gets down to 3.0v. Especially if you continue to pull current from it, which you were doing in the video.

If it's any consolation, you're not doing anything wrong, regarding either BMS settings or use. LFP is just less energy dense that Li-ion, and its voltage drops off fast at the bottom of its capacity. You could definitely argue that you have 2 bad cells that are bringing down the capacity for the whole pack, and you could choose to top- or bottom-balance the pack depending on your preferred use case, to get a bit more range out of it.

Damn I got the wrong conception I thought energy density just meant size of cell for the capacity provided. I do have the bms set to auto balance always and seems to always balance during or after charging. I have a video of it doing that after charging when I left it charging while I went to bed, I'll post that video Soon.

The fact that this 50ah battery has way less range than my pooched li-ion one which is 40ah (10 amp hours less capacity) does boggle my mind. I thought lifepo4 is superior to li-ion, having more cycles and life and all that? I guess I just got the wrong battery in general.. Big sigh.

harrisonpatm · Jul 25, 2024

500wattz said:
The fact that this 50ah battery has way less range than my pooched li-ion one which is 40ah (10 amp hours less capacity) does boggle my mind. I thought lifepo4 is superior to li-ion, having more cycles and life and all that? I guess I just got the wrong battery in general.. Big sigh.

Not necessarily. Because one thing that I was getting at, is that you may not be getting the full 50ah that was advertised. One reason for that is that you seem to have 1 or 2 bad cells. As you point out, cell 22 reaches 3.65 volts and the BMS cuts off charging. Meaning 23 other cells could hold more energy, but weren't allowed to. By top balancing and/or replacing the bad cell(s), you will see more capacity.

Second reason: seller possibly lied about or inflated the 50ah rating. Can you tell us more about the pack? Can you access the cells themselves for individual cell data to double check?

Third reason: just because 50ah is more than 40ah doesn't mean energy density is the same for the bike as a whole. What do the two batteries weight? If the 40ah battery was Li-ion pouches, it was probably rather light. And the 50ah battery was LFP, so heavier as a rule. If I had to guess, I'd say that the new battery weighs 2-2.5x as much as the first. How much does the bike weigh as a whole? You mention pedaling home, so it must not be too heavy. Meaning the battery likely takes up a significant percentage of the total ride weight. And if you increased the weight of the bike bike adding a much heavier battery, it means it will require more energy to get up to the same speeds you were seeing previously.

Side note: if the two batteries weigh roughly the same, then you almost certainly got sold a LiFePO4 battery with exaggerated capacity. LiFePO4 weighs as much as 2x more than Li-Ion, for the same capacity. Meaning, even if you got the "right" battery - that is, a LiFePO4 battery rated for 50ah, that actually has 50ah - it's still gonna be heavy, and potentially too heavy for the performance you were expecting.

Hopefully this all makes sense, I know it's a lot of information. Thanks for sharing your experience because I think it's a good learning case for other newbies that are wanting to learning about the practical differences between choosing which battery chemistry is right for them. LIFePO4 has plenty of other things going for it: longer calendar and cycle life, typically higher discharge rates, less heating during discharging, lower incidence of thermal events...

500wattz · Jul 25, 2024

harrisonpatm said:
Not necessarily. Because one thing that I was getting at, is that you may not be getting the full 50ah that was advertised. One reason for that is that you seem to have 1 or 2 bad cells. As you point out, cell 22 reaches 3.65 volts and the BMS cuts off charging. Meaning 23 other cells could hold more energy, but weren't allowed to. By top balancing and/or replacing the bad cell(s), you will see more capacity.

Second reason: seller possibly lied about or inflated the 50ah rating. Can you tell us more about the pack? Can you access the cells themselves for individual cell data to double check?

Third reason: just because 50ah is more than 40ah doesn't mean energy density is the same for the bike as a whole. What do the two batteries weight? If the 40ah battery was Li-ion pouches, it was probably rather light. And the 50ah battery was LFP, so heavier as a rule. If I had to guess, I'd say that the new battery weighs 2-2.5x as much as the first. How much does the bike weigh as a whole? You mention pedaling home, so it must not be too heavy. Meaning the battery likely takes up a significant percentage of the total ride weight. And if you increased the weight of the bike bike adding a much heavier battery, it means it will require more energy to get up to the same speeds you were seeing previously.

Side note: if the two batteries weigh roughly the same, then you almost certainly got sold a LiFePO4 battery with exaggerated capacity. LiFePO4 weighs as much as 2x more than Li-Ion, for the same capacity. Meaning, even if you got the "right" battery - that is, a LiFePO4 battery rated for 50ah, that actually has 50ah - it's still gonna be heavy, and potentially too heavy for the performance you were expecting.

Hopefully this all makes sense, I know it's a lot of information. Thanks for sharing your experience because I think it's a good learning case for other newbies that are wanting to learning about the practical differences between choosing which battery chemistry is right for them. LIFePO4 has plenty of other things going for it: longer calendar and cycle life, typically higher discharge rates, less heating during discharging, lower incidence of thermal events...

Thank you for your information.

Old battery and new battery on a scale pics below.
I did not pedal home from far. 1km. Pedaling to work where it died was 7km grueling because the bike is a 26x4 fat tire bomber clone hat weighs about 250lbs I weigh 250lbs but the old battery did handle this no problem. Plus more.

I am going to take both my batteries to a battery guy to have both inspected for repair. I'll update as this goes along.

Here is the direct link to my purchase:

https://a.aliexpress.com/_mq9yGbm

harrisonpatm · Jul 25, 2024

Sorry to say, it seems like you likely got the short end of the stick on your new buy. When I click on the link, the page is gone, likely meaning the seller sold a few packs and wrapped up the shop.

The weights don't match up well, either. 36 pounds (16.5kg) for 3.6kwh (72v x 50ah)? That's 218 wh/kg, which is basically the very best quality LiFePO4 cell you could possibly get on the market right now. Not even counting the weight of spacers, wrapping, cables, ect. Safe to say, you don't have those level of cells in your pack.

500wattz · Jul 25, 2024

The link works but I'm in Canada maybe that's why. Let me find a better link.

harrisonpatm said:
Sorry to say, it seems like you likely got the short end of the stick on your new buy. When I click on the link, the page is gone, likely meaning the seller sold a few packs and wrapped up the shop.

The weights don't match up well, either. 36 pounds (16.5kg) for 3.6kwh (72v x 50ah)? That's 218 wh/kg, which is basically the very best quality LiFePO4 cell you could possibly get on the market right now. Not even counting the weight of spacers, wrapping, cables, ect. Safe to say, you don't have those level of cells in your pack.

500wattz · Jul 25, 2024

I read on another forum of another person who was having the exact same issues as me and his solution was to rebuild the battery with better connectors and use a single cell balancer to properly balance his cells. It also turned out the cells advertised were not in his battery enclosure...so now I'm very curious as to what could potentially be in mine. When I asked my seller, he sent me a pic of the CATL cells used. I also attached a screenshot of the aliexpress page that sells my battery.

PROGRESS UPDATE

After doing a full charge, I hit "reset capacity" on the xiaoxiang BMS app, then went on a 7km test run (hard riding), got home, plugged in the charger and NOW it reached 87v on full charge! The cells are imbalanced as hell, but once at full charge, they start to self-balance as per BMS function then float at around 84 for full charge.

I also realized that it was series 10 that tripped my cutout this morning. It hit 2.5v but none of the other cells were even close to hitting 2.5v. This is making me feel that this is more of a cell balancing issue than anything at this point.

Now with the battery charging properly, I shall do another test run and will monitor all the cells to see which one (if any this time) goes way out of balance to the point of prematurely tripping the BMS and not fully utilizing it's range.

Oh and I was told that this battery is 3600Wh so with that being said (if true) this assumed CATL battery SHOULD at the very least get me to work (40km one way) without tripping, I would think.

500wattz · Jul 27, 2024

UPDATE

After a few full charges, the cells seem more balanced after a full charge. I re-configured some settings on the BMS and on my controller. I set single cell cut of at 2.5v and set my controller to cut off if voltage hit 67v assuming that will happen before a single cell hits 2.5v. I also changed field weakening to the weakest/slowest setting.

2nd attempt commuting to work, I actually made it with 65% battery remaining! not a single cell got close to going under 3.0v. I was conservative with my throttle and didn't hammer 5kw for any extended period of time, maybe for hills and that was it.

I charged up to full and after my shift I went home, same distance (40km) but my commute home is all uphill. Again being conservative with my throttle and monitoring every single cell, Once again I barely, and I mean barely got home. Cell 10 seems to dip way more than the others and the group voltage is around 70v when a single cell dips to 2.5v for cutoff

This may fix itself with a few more charges and discharges as it might just be a balancing issue but if it does not I'm going to request warranty or cell 10.

Will update in a few. In the mean time, this is what the seller had to say about my lifepo4 battery and how the cells bounce around a lot.

I also attached screenshots of my cell status when I got to work. So going to work looks promising. Going home, well...I guess it's all the uphill riding.

500wattz · Jul 28, 2024

UPDATE

After another cycle or so, cells are more balanced and charges fully properly now. Discharge is better. I did a hard 30km ride, 5kw the entire time and saw my group voltage go under 70v without a single cell tripping. I monitored the cells closely. There doesn't seem to be a specific weak cell, just a random cell showing drainage quicker than others near the bottom end. This might get better with another cycle or so as I read somewhere that LiFePo4 cells need about 4 cycles to balance properly.

It is looking promising...if I can just get home without it dying or getting close to, I'll be happy. Tonight will be the test, will update.

Oh and my 10a chargers only output about 7amps. Seller partial refunded for the lack of 10amp charging. The gauge of the wiring on these chargers is pretty thin compared to my li-ion 10a charger and that kinda gave it away...but got partial refund so, good from the seller ^.^

Gruesome · Jul 28, 2024

500wattz said:
UPDATE

After a few full charges, the cells seem more balanced after a full charge. I re-configured some settings on the BMS and on my controller. I set single cell cut of at 2.5v and set my controller to cut off if voltage hit 67v assuming that will happen before a single cell hits 2.5v. I also changed field weakening to the weakest/slowest setting.

2nd attempt commuting to work, I actually made it with 65% battery remaining! not a single cell got close to going under 3.0v. I was conservative with my throttle and didn't hammer 5kw for any extended period of time, maybe for hills and that was it.

I charged up to full and after my shift I went home, same distance (40km) but my commute home is all uphill. Again being conservative with my throttle and monitoring every single cell, Once again I barely, and I mean barely got home. Cell 10 seems to dip way more than the others and the group voltage is around 70v when a single cell dips to 2.5v for cutoff

This may fix itself with a few more charges and discharges as it might just be a balancing issue but if it does not I'm going to request warranty or cell 10.

Will update in a few. In the mean time, this is what the seller had to say about my lifepo4 battery and how the cells bounce around a lot.

I also attached screenshots of my cell status when I got to work. So going to work looks promising. Going home, well...I guess it's all the uphill riding.

BS on two points in seller communication:
a) 3.35-3.40V is about 60% state of for LFP (see for example Understanding the LiFePO4 Voltage Chart. ), far from fully charged as seller claims
b) LFP is supposed to be more stable than Lithium-ion, exactly the opposite of what the vendor says.

500wattz · Jul 28, 2024

Gruesome said:
BS on two points in seller communication:
a) 3.35-3.40V is about 60% state of for LFP (see for example Understanding the LiFePO4 Voltage Chart. ), far from fully charged as seller claims
b) LFP is supposed to be more stable than Lithium-ion, exactly the opposite of what the vendor says.

Okay, thank you.

After a few cycles it does stay above 3.5v per cell on full charge so that's assuring on my end.
I have noticed improvement on sag on lower end of battery life, will update after my commute tonight.

I also read elsewhere that full charge per cell is 3.65v BUT once you disconnect the charger the resting charge will drop to 82-84v, about 3.4v per cell. Correct me if this is wrong though.

Also, seller said this on why the chargers don't output 10amps:
"The voltage and current of 110V cannot be set too high, as it is prone to damage
Only by reducing the current can the quality be stable"

Now we all know that slow charging is best for balance and battery life and that 1 amp per cell in series is the general rule of thumb for fast charging, so I have 10 cells in series, 10amp charger is safe (for li-ion anyway).

I am requesting confirmation if my new battery can indeed, handle true 10amp charging as i do touring with my ebike club.

Or maybe anyone here can confirm this for me? will a 72v 50ah assumed CATL lifepo4 battery handle 10amp charging?

Thanks again for everyone's help.

Gruesome · Jul 28, 2024

I think your seller is just saying whatever comes to his mind to hoodwink you, trying to see whether something will stick and get you off his case. Either that or he/she truly doesn't know what they are talking about. Maybe both. What is the supposed problem with 110V AC (it's actually 120V since 1954) and running 10 amps? Both 15A and 20A are standard AC breaker sizes, so I think we would know by now if there was some inherent limit.

Regarding cells in series and charging current: the same current has to flow through all cells (or groups of cells) hooked up in series. The cells might develop different voltage drops, depending on their charge capacity, but they have to accept the same current that flows through the whole string.
The current divides (is the sum of individual currents) over cells connected in parallel. So if you have a 10s3p battery, you can safely charge it at three times the rate for a single cell, and the voltage drop will be the sum of the voltages over all ten groups of three.

Diggs · Jul 28, 2024

With the substantial cost of the batteries we use I'm surprised more people don't buy an inexpensive tester to actually know what they bought. In turn, if returning a battery you have actual capacity numbers.

500wattz · Jul 29, 2024

With the substantial cost of the batteries we use I'm surprised more people don't buy an inexpensive tester to actually know what they bought. In turn, if returning a battery you have actual capacity numbers.

Diggs said:

I will definitely invest in this, thank you.

UPDATE

I did a hard 30km ride 5kw the entire time, nothing tripped. One cell was getting a bit low, but not low enough to trip anything. I changed single cell low voltage cutoff to 2.65v, might change it to 2.75v just experimenting to make sure I can get home okay.

Later that day I did my 40km ride to work and it was fine, had over 50% battery remaining. Going back home, I kept it at 3kw or under 98% of the time and I got home okay, but as usual, a single cell dips lower than the rest by around 0.5v. It climbs back up if I ease off the throttle and it's never the same cell that does this, but it is always a cell in the bottom end.

I'm at the 4th cycle now. I left the charger plugged in and will leave it there for 10 hours to allow the cells to balance. I will update again to see how this does on my way home from work tomorrow morning as the 4th cycle is when the cells are supposed to be fully balanced, according to what I read.

99t4 · Jul 29, 2024

Repeatedly balancing a battery pack with a poorly performing or defective cell does not actually repair anything. It just temporarily reduces the undesired effects of the problem until the next cycle.

500wattz · Jul 29, 2024

Wellva

99t4 said:
Repeatedly balancing a battery pack with a poorly performing or defective cell does not actually repair anything. It just temporarily reduces the undesired effects of the problem until the next cycle.

Well as mentioned before its not a specific cell that keeps dropping quicker than the others which makes me think it's a balancing issue. Time will tell.

500wattz · Aug 12, 2024

Update

Well, you were all right. Garbage cells and fake capacity. I took the battery to a specialist and he did a capacity test. This turd has the capacity of 24ah, not 50ah. Explains why at 70v group voltage the battery is dead.

I learned my lesson. Can't cheap out on a battery. I already forked out the proper dough to get the proper battery this time from a local legend, using Murata(Sony) VTC5a cells.

Currently disputing with the bank, hopefully I get my money back.

I will be definitely creating a YouTube video exposing this company to help others not make the poor choice that I did.

99t4 · Aug 12, 2024

It's the vendor linked in post #6 of this thread?

Portugaline · Aug 12, 2024

Portugaline said:
I would return the pack ASAP. if everything is well connected this are very bad cells, probably used cells recycled to you.

500wattz said:
Update

Well, you were all right. Garbage cells and fake capacity. I took the battery to a specialist and he did a capacity test. This turd has the capacity of 24ah, not 50ah. Explains why at 70v group voltage the battery is dead.

I learned my lesson. Can't cheap out on a battery. I already forked out the proper dough to get the proper battery this time from a local legend, using Murata(Sony) VTC5a cells.

Currently disputing with the bank, hopefully I get my money back.

I will be definitely creating a YouTube video exposing this company to help others not make the poor choice that I did.

As I told in the beginning of the post. Never accept a battery that is not perfectly balanced and avoid AliExpress at all cost for batteries.

HELP: 72v 50ah LIFEPO4 CATL Battery cuts out at around 70v

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