Why does my battery cut out at 51.5 V

RTLSHIP

10 kW
Joined
Jul 24, 2008
Messages
521
Hi Folks, I have a little used (2017) 48 v 12 ah lifepo4 pack that runs good for 4 miles then apparently the BMS shuts it down. After reboot, bike rides again but becomes even more sensitive to shutdown. Motor is about 1000 watts, controller is 25 amp, I think.
It is disappointing, especially when I read about how somebody can go 20 to 25 miles on a similar or smaller pack. This pack could not go more than 8 originally and my weight is 170.
It seems to hold charges above 53 and charges to 58. It has never been below 51 as this BMS seems too conservative to permit such.
Is 20 to 25 minutes moderately hard riding normal? It can hit 30 mph with a little tailwind.
 
Could be one or more cell groups has gone bad.

Is you pack fully balanced?

Can you check all cell groups once it has been on a charger for 12 hours?

Can you check all cell groups after it stops working?

:D :bolt:
 
RTLSHIP said:
12 ah
controller is 25 amp
Is 20 to 25 minutes moderately hard riding normal?
If that means drawing 25amp continously, then yes, that would be about normal, because you would be drawing 2C. Drawing 1C would be 12A, which you could do for nearly an hour before you run out of the 12Ah.

runs good for 4 miles then apparently the BMS shuts it down. After reboot, bike rides again but becomes even more sensitive to shutdown.
I'm not sure I understand your circumstances. If you are riding moderately hard, but only get 4 miles, in 20-25 minutes, how steep a hill are you climbing for it to take that long to go such a short distance?

Or is the first circumstance a totally different thing to the shutdown problem?

If neither, you may need to give much more complete details of exactly what the complete circumstances are for the ride, and exactly when and how the problem happens, etc.



If you're not actually able to get 12Ah (you might only get 10-11Ah for real out of a 12Ah pack, less as it ages), then it may have problematic cell groups.

What are the voltages on each of the cell groups, measured between each of the balance wires on the BMS, starting from the most negative battery wire?

Check this after it cuts out, write them all down and post them here.

Then recheck this after recharging, write them all down, and post them here.

This will begin to help us help you find the specific cause of the problem.
 
thanks for replies. First, I have not checked cells or BMS as pack says not to disassemble. BMS is internal so it can't be accessed without cutting up the heat shrink.
As for my riding, there are practically no hills down here. Just a few bridges and overpasses.
By riding moderately hard I mean going between 18 and 24 mph and occasional pedaling.
The pack is not even fully recharged this morning and the voltage is at 53.5 which is not a bad number.
Finally, I'm trying to sell this thing locally with info about the packs issues . It may be useful for solar powering purposes.
If I have no luck, then I may consider opening it up. I have a 12 yr old (May 2009) 48/15ah pack that runs for about 8 miles at 45.9 V. It charges like a 48 etc.
It is missing about 6.6. So it is a 16 with only 14 working series. Yet it has more range and capacity.
I can't even track my jogging distance with this newer imotorbattery 12 ah pack without cutouts. The only reason I got it was that nobody bid on it and I won by default price.
If this is normal for such a pack, then it is not compatible with current set up. I have tried a smaller 18 amp controller and results are the same.
 
Have you left it on the charger for at least 12 hours?

:bolt:
 
yes. and I'm doing 9 hours charging right now at work. Will test ride tonight for range. I will also use a different throttle just to rule out anything throttle related as I have been using a universal twist grip that may not be appropriate.
 
Voltage means zero wrt State of Health

test for capacity

under 80% nearing EoL

getting below 70% I call scrap ready forrecycling

sure there are exceptions, but if a big pac gets its sag voltage way down with only 1-2C discharge

for a few minutes

it just won't do what you need it to do
 
Either sell it as a bad battery, or open it, test the cell groups to find the bad one(s), or just do something else with it.

:D :bolt:
 
e-beach, I did nearly as you said and charged it for 9.5 hrs. It was 58.1 v when removed from charger at 4:20 pm. At 5:40 pack was 56.1 v. At 6:20 still 56.1v. I headed out for ride. Lots of headwinds and tailwinds. I decided to go only 1/4 throttle and covered 7.1 miles in under 36 minutes. 19.3 mph was max speed. First pack shut down was at 6.7 miles. Voltage after 7 miles is 51 but went up to 51.2 just before recharging.
I did not remove the twist throttle as it made little sense and was more work than I anticipated. Point is that when pack shuts down, throttle is only red. When running, it is all 3 colors red, green, yellow.
So I was able to improve range through moderate throttle. 4 miles in 20 minutes of hard riding vs tonight's results. Most of the time I was going 13 to 17 mph depending on wind direction. Results are better but I expect 1 mile per ah as a rough expectation. I don't think that is asking a lot. Is 51 v shut off reasonable? If voltage means little, then this is a bad pack as suggested. I just don't understand how some can take their batteries so low down.
I just swapped out throttles. The replacement does not start out yellow. This throttle thing is my Hail Mary. Otherwise, pack needs surgery or will sell, donate etc.
 
A healthy 48v 12ah battery should get you about 20 miles of moderate riding, give or take. Cutting out at 51.5v usually means one of more cell groups has gone bad. Or it could be a loose wire.

2 volts of self discharge in 80 minutes is a lot.

It sounds to me like one of the cell/cell groups is triggering the low voltage cutoff in the BMS.

Are you sure you don't want to open it up to test the cells?

:D :bolt:
 
It sounds like one of two primary possibilities:

The first and most common is that it is just very badly imbalanced (though this is usually caused by the second problem below). To at least temporarily fix this, it may take days, or even weeks on the charger, to allow the BMS to drain off the high cells enough to allow the charger to restart and refill all the cells, with each repeat of this process bringing the low cells closer and closer to the high cells. It takes so long because the BMS can only drain a tiny amount off each high cell at a time, and then the charger can then only bring each low cell up by a similar tiny amount. If your charger does not automatically restart after the BMS reallows charge, then you have to manually do that each time.


The second is that it is a poorly made pack, and is either made of very mismatched (or even damaged) cells, or they are poorly connected, so that the different groups all have different capacities and can never be balanced.


Since the lowest capacity group determines the maximum capacity of the whole pack, then either of the above problems could cause what you see.


The best way to find out the problem is to first check the voltages in the two different stages as I'd noted previously. Once you see what the voltages are, it may be possible to charge up the low group(s) if there are few of them, or drain down the high groups if there are less of those than the low ones. This is a relatively quick process.

The second best way is to do the temporary fix outlined above (though this usually takes a lot longer because you're depending on the BMS to just drain high cells, and if there are mostly high cells and there is a large difference between those adn the low ones, it can take a very very long time).
 
Yea, but 58.1v to 56.1v in 80 minutes simply by disconnecting it from the charger?

Sounds like a bad cell or cell group to me.

However, I agree that leaving it on the charger for long periods for a few weeks wouldn't hurt.

It depends on the OP's need to gain range while riding.

:D :bolt:
 
e-beach said:
Yea, but 58.1v to 56.1v in 80 minutes simply by disconnecting it from the charger?
For a 16s LiFePO4 battery, if there were say, 14 normal (high) groups, and a couple low ones, the BMS could probably drain 0.15ish volts off each high group in that long, also allowing for the normal voltage drop of non-new LiFePO4 cells (the latter could even be the entire cause of that drop).

It's not uncommon for non-new LiFePO4 cells that are cahrged to 3.65v to drop as low as 3.4v and still be as "full" as they are going to get, with very little actual capacity (a few dozen mAh or less) available in that 0.15v difference. This drop would happen even if they werent' connected to anything, after being disconnected from the charge current.


I don't know how much of the data is in there, but there are some threads I have for repairing some old Vpower / CammyCC packs, and there's a lot of old Ping pack threads by various people, that should show some of that sort of thing happening.


The V/C packs were poorly built (badly connected groups, etc) as well as being made of not-very-good cells (calling them "2C capable" would be stretching it), so if this pack is like those, I wouldn't be surprised to see similar problems.
 
Yes 3.33 to 3.36V is 100% Full at rest.

Anything higher is "surface voltage" from a charge setpoint unnecessarily higher than 3.45V, not yet "settled" down after charging, can take 48 hours.

Or just remove say 0.1% of Ah capacity
 
thanks for all the input. First and foremost, I have practically no battery disassembly skill. I can, of course, charge it for a week or 2 straight. I don't understand why a drop from 58.1 to 56.1 is so bad. There is suppose to be a drop when the charger is removed.
I will leave charger on it and wait to decide what's next. I have 2 other packs so I can still ride.
This pack was bought brand new and I believe that it was such. And it been treated well, so maybe the long charge can correct imbalances.
As of 5:20 AM, pack is on charger w/ green light on. This is a different charger and voltage is 58.5. Don't know if that will help.
 
amberwolf said:
It sounds like one of two primary possibilities:

The first and most common is that it is just very badly imbalanced (though this is usually caused by the second problem below). To at least temporarily fix this, it may take days, or even weeks on the charger, to allow the BMS to drain off the high cells enough to allow the charger to restart and refill all the cells, with each repeat of this process bringing the low cells closer and closer to the high cells. It takes so long because the BMS can only drain a tiny amount off each high cell at a time, and then the charger can then only bring each low cell up by a similar tiny amount. If your charger does not automatically restart after the BMS reallows charge, then you have to manually do that each time.


The second is that it is a poorly made pack, and is either made of very mismatched (or even damaged) cells, or they are poorly connected, so that the different groups all have different capacities and can never be balanced.


Since the lowest capacity group determines the maximum capacity of the whole pack, then either of the above problems could cause what you see.


The best way to find out the problem is to first check the voltages in the two different stages as I'd noted previously. Once you see what the voltages are, it may be possible to charge up the low group(s) if there are few of them, or drain down the high groups if there are less of those than the low ones. This is a relatively quick process.

The second best way is to do the temporary fix outlined above (though this usually takes a lot longer because you're depending on the BMS to just drain high cells, and if there are mostly high cells and there is a large difference between those adn the low ones, it can take a very very long time).
What kind of 3.6 v charger is needed to individually charge cells? And what do you mean by automatic restart after BMS reallows charge? Is that when the red comes on briefly a few times at end of charge?
 
My LiFePO4 at full charge has always been 3.65v per cell. If I disconnect it from the BMS and the charger, it stays there for weeks. As for surface voltage, I get a mile or a bit more riding on my 15ah pack before the voltage dips to 39.something or lower. So, not entirely surface voltage.

As for the OP's wondering about the voltage drop, IMO your cells should be more stable then that. It could be that he has an aggressive BMS trying to balance his pack. Leaving it on the charger to attempt to balance it might help. If leaving it on the charger for a week and the battery still doesn't keep the full 58.4 voltage, or close to that at rest, then he surly has a pack with a bad cell or cells.

The proof would be to let it charge for a week straight and then ride it to see if it performs differently.

As for the individual 3.65 charger, you would need to get a single or multi cell charger from ebay or Amazon or someplace and some jumper wires. Open you pack and meter the cells, then one at a time attach the jumper wires to the individual low voltage cells and allow them to charge until they are up to equal voltage to all the rest.
Or at least close enough to each other so your BMS can do the rest.

:D :bolt:
 
I understand what you are saying, e-beach. Right now it is going to sit until next Wednesday or maybe more. I'm just curious how you
access individual cells in the middle of the pack without tearing things up. Or do you just try to charge up the weaker groups or series through BMS readings location. Minor soldering is easy, but welding is risky and there is a chance of frying/shorting a small charger due to error in hookup.
I was reading up on bad/low cells and it is reported that the voltage will be higher in the bad cells when charging up fully. When discharging the voltage will be higher in the good ones. So the quick drop from 58 to 56 is not just normal self discharge.
 
To go from 58v to 56 so quickly means either you have a weak cell or cells, or something is draining it down.

To access the cell voltages you will have to open the battery. It cleanest way usually is to open the case and then cut open the shrink wrap at the part of the BMS connects to the since wires and then meter each cell/group through the since wire connectors.

Just be careful not to short anything.... :kff:

:D :bolt:
 
is leaving charger on a week or more harmful to pack?
 
No, just make sure it doesn't get hot.

You should be ok.

:D :bolt:
 
RTLSHIP said:
I don't understand why a drop from 58.1 to 56.1 is so bad.
It isn't necesarily "bad" for a LiFePo4 pack. It would be for other Li chemstry packs. With LFP it just means teh cells aren't holding that "surface charge" anymore.

But it *could* be bad, if you have a cell group with an internally shorted cell that is draining down the entire group very fast (like I had on a Vpower/cammyCC pack).

RTLSHIP said:
So the quick drop from 58 to 56 is not just normal self discharge.

It *can* be. You can lose at least 0.15v per cell to the normal non-new-cell surface-charge loss, whcih in a 16s (48v) pack is around 2v total. But you can't tell if this is teh cause for sure without getting into the pack to monitor voltages on individual cell groups. My bet, is that is all this is in your case, and that it doesnt' have anythi8ng to do with the actual problem you have with the pack.



RTLSHIP said:
What kind of 3.6 v charger is needed to individually charge cells?
They make LiFePO4 single-cell chargers for the purpose; or most RC chargers will do just one cell, or if you are careful with it and keep watch over it with a voltmeter the entire time you can sometimes just use a USB-charger for a phone (which is 5v so you must watch it to make sure it doesnt' overcharge past the 3.65v the BMS is made to deal with), or a number of other charging methods.


And what do you mean by automatic restart after BMS reallows charge? Is that when the red comes on briefly a few times at end of charge?
Typically you'll see the lights change whenever the charger state changes, so if yours is red when charging, then yes, that's what you'll see if it automatically restarts charging. (some chargers don't, and those have to be manually restarted)

So in your case, you should be able to just leave it on the charger anytime you are not using it, and it will help rebalance the pack over time. How long...that depends on the BMS balancing current, which is usualy very low (10-50mA), and the amount of capacity difference between the cells. For instance, if right now some cells are so out of balance that you only get say, a quarter of pack capacity, then with 12Ah pack that'd be about an 8Ah difference. 8Ah is 8000mAh, so 8000mAh / 50mA = 160 hours of difference at that constant charge rate...but it probably won't be constant, and it may not be nearly as high a rate as 50mA. If it was only 10mA, it would be 800 hours. :shock:

So...how much time it takes to rebalance varies very widely. :)
 
It is normal for LiFeP04 cells when charged up to between 3.5 & 3.6VPC to settle in an hour or three to about 3.47 - 3.47 VPC. This is what they do. Completely normal. Not a point of concern
 
Amberwolf, I have an old cell phone charger that 5 v .7A, but it looks like I would have to cut of the USB plug to get 2 separate wires (plus and minus.) I have that old pack that gets a hot BMS when charging is finished. It never jumps back and forth from red to green like other packs. So I'm wondering that this could be why my old pack is out of balance (among other reasons). And I would try to tamper with that one first before tinkering with this newer 48/12.
That old pack still runs and could be rejuvenated some. You have seen a few pics of it Fully charged the first pin(metal behind removed bms plug) reads 3.2,7.0, all the way to 53. Curiously, there is a .3 or .4 volt difference between reading from charging negative spot and nedative discharge. It should be zero difference. Again, this is the older geriatric pack which is still good to ride and learn from as the BMS is right there.
 
RTLSHIP said:
I have an old cell phone charger that 5 v .7A, but it looks like I would have to cut of the USB plug to get 2 separate wires (plus and minus.)
If leaving the pack on the charger long enough doesn't fix the problem, then you can open up the pack and do the voltage tests first, and then if necessary go down that single-cell charger route. If you didn't want to cut the wires you could buy a USB extension cable for that same kind of plug, and cut taht cable in half, and use *it's* 5v and ground wire to make the connections for your cells. That makes it a little safer making the connections first, so you can be sure you have + and - in the right spots before hooking up the charger, too.


I have that old pack that gets a hot BMS when charging is finished. It never jumps back and forth from red to green like other packs.
That just means most of it's cells are close in voltage to each other, so the BMS has most of it's balancers turned on then, so they are shunting current away from the cells and dissipating this as heat. That's normal. If the current never drops below the minimum for the charger to turn off, or the BMS never shuts off the charge input port (or you are charging thru the discharge port) then the charger will never "turn off" so it never changes LED color. Some chargers also don't have the turn-off electronics, so they stay on all the time they are plugged in, and never change color.


So I'm wondering that this could be why my old pack is out of balance (among other reasons).
It's not the cause of the problem, it is a symptom of the problem. ;)


And I would try to tamper with that one first before tinkering with this newer 48/12.
Well, tampering with it won't do you any good unless it's broken, and it wont' do anything at all to fix this one. ;) It might teach you how the packs are built inside, if they are both similar (there are a lot of ways to build packs, some of which are very different).


That old pack still runs and could be rejuvenated some. You have seen a few pics of it Fully charged the first pin(metal behind removed bms plug) reads 3.2,7.0, all the way to 53.
You should note voltages down not as a total from one end, but as the specific voltages between each pair of pins. Start from the most negative cell going up to the most positive. This is a lot easier to "read" and directly see where problems may lie.

For instance, if you had 3.2v on the first cell, and reading across it and the next one is 7.0v, then you have 3.8v on that second cell, which is a lot higher than it should be. Showing the actual individual voltages would help see what is really happening.

Curiously, there is a .3 or .4 volt difference between reading from charging negative spot and nedative discharge. It should be zero difference.

That depends on what is between those spots. If there is current flowing at all, then having electronics like shunt resistors and FETs and things between the two points will have a voltage drop across them. Not much, but some, and could account for what you see.

Connection problems can also account for that, if they are not directly crimped or soldered together and have any connectors in the path.
 
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