52v li-ion battery cutting out at 49v, am I missing 40% of capacity

New to ebikes here but have been doing lots of reading. I have searched for similar problems but cannot seem to find anyone with the exact same issue. My 52v 20ah li-ion battery was purchased about a month ago. On my first discharge cycle the bike cut out earlier than I anticipated and measured 49.0v through the charge port with my multimeter and my luna charger showed the same reading. I re-read the specs on my battery and I should be able to use my battery between 58.8 and 42v.

I ensured that my controller was not set to cutoff at 49v and my Bafang C695 is only capable of cutting of at up to 43v. After a bit of research I found that I may need to balance the battery, though balancing to make up 7.0v seemed like a huge disparity.

I went about the business of breaking in and balancing the battery with 5 relatively shallow chargers to 58.8v (measured with multimeter and charger) and allowing the balancing to do its thing until I saw the green light. Feeling confident that my battery wasn't spending too much time in the balancing mode, I took it out with the intention of discharging below 49.0v. Again, the bike cutoff at exactly 49.0v.

At this point, I have hit the cutoff 3 times and every time it is at 49.0v on the nose. I feel like perhaps the incorrect bms was installed (49v seems appropriate for a 60v battery, no?) or perhaps I may have to reset it. If there were bad cells I don't believe I would be able to achieve a 58.8v charge, perhaps I am wrong. I have contacted the manufacturer and am in the process of creating a video to demonstrate this issue, I will like here when it is finished. The manufacturer's response led me to believe that I am correct in expecting to be able to discharge this specific battery to 42v.

I am really hoping there is a fix for this that would not require me sending my battery back to china. I live in a city with a few electric bike shops.

When it cuts off, does the display go off?

If the controller is cutting off, then the display should stay on and maybe give you an error code.
If the display goes off and you have to turn everything off and back on again to reset it, then I would suspect the BMS.

To check the balance, you really need to open the pack and get access to the wires so you can check the individual cell voltages and see if one is low.

I recent bought a BMS, 36 volts. Would charge up to approximate 40.7 volts and cut the power at 36 volts.

Bought another BMS and yet to drain the battery far enough down. Expect this BMS, to allow the bike’s controller cut-off the power at 30.5 volts. Time will tell.

Add. the balance wires the connector matched the new BMS. Perhaps some continuity, or standard to each BMS, allow swapping out for different brands?

fechter said:

When it cuts off, does the display go off?

If the controller is cutting off, then the display should stay on and maybe give you an error code.
If the display goes off and you have to turn everything off and back on again to reset it, then I would suspect the BMS.

To check the balance, you really need to open the pack and get access to the wires so you can check the individual cell voltages and see if one is low.

Click to expand...

Yes the display goes off. Im not super excited about opening the pack. I really think I may try and convince the builder to let me buy another one and I will ship this battery back once I have the new one and confirm in operates as advertised.

fechter said:

When it cuts off, does the display go off?

If the controller is cutting off, then the display should stay on and maybe give you an error code.
If the display goes off and you have to turn everything off and back on again to reset it, then I would suspect the BMS.

To check the balance, you really need to open the pack and get access to the wires so you can check the individual cell voltages and see if one is low.

Click to expand...

Also what to you mean "turn everything off and back on again to reset it"? I have a BBSHD with C695 controller and my battery has no switch. So when the battery shuts off, everything is already off and my system won't power up.

Sounds like the BMS then. If you unplug the battery cable going to the motor and plug it back in, that should reset the BMS. Charging will also do it.

Some packs are relatively easy to open and access the wires going to the BMS. Some packs are nearly impossible to open.

I had a problem like this a while back with my BBSHD. One of the balancing shunts on the BMS had fried and drained the cell it was going to. Charging would bring it back up a little but the other cells would reach the high limit and cut off the charger so the pack would never get fully charged. The low cell was also damaged by charging at full rate when it was near zero volts, so the pack was toast.

its your bms i my brother had this problem ..he went around the bms so that he could take as much power as he wanted by the bms still controlled charging and leveling cell ..I think his was only rated at 80a and his controller was trying to pull 100.

acwhitney said:

I went about the business of breaking in and balancing the battery with 5 relatively shallow chargers to 58.8v (measured with multimeter and charger) and allowing the balancing to do its thing until I saw the green light. Feeling confident that my battery wasn't spending too much time in the balancing mode, I took it out with the intention of discharging below 49.0v. Again, the bike cutoff at exactly 49.0v.
...
I re-read the specs on my battery and I should be able to use my battery between 58.8 and 42v

Click to expand...

I suspect there is nothing wrong with your BMS.

According to the reported specs, you can expect the BMS to cutoff at 42V/14groups = 3V per cell. It's cutting off at 49V so in the worst case you have one low group -- at cutoff you have (49V - 3V)/13groups = 3.53V per cell for the other groups. 3V vs 3.53V is a pretty unbalanced pack but if you have several low groups things look a little better.

Your policy of 5 shallow charges and shutting off the charger when the green light goes ON is exactly backwards. You want a few shallow DIScharges (not charges) to avoid hitting the BMS cutoff then you want to leave the charger with the green light ON for some time to allow the battery to balance for each cycle. When the green light goes on, it is just beginning the balance phase, not ending it. Effectively, you have been repeatedly flogging the battery to cutoff and never balancing it. Unfortunately, running a battery to BMS cutoff is really punishing and tends to unbalance the groups making balancing take even longer.

Set your charger for the highest voltage (not a partial charge voltage), hook it up, and let the battery sit on charge for several days. The green light should cycle on and off over that period as the balance shunts discharge the full groups so the BMS can then kick in the charger and bring all the groups up a little bit. A big problem is that you have a 20Ah battery and balance currents of 200ma or less are common. This means it will take a long time to repeatedly bleed down those high capacity 'full' groups so the charger can then add a little bit to all groups - and then repeat, etc. Unless you want to open the pack, measure the individual groups, and manually bleed down the 'high' groups, you are probably in for a bit of a wait.

In any case, I think you need to try to balance your pack properly before you start thinking about more radical alternatives.

teklektik said:

acwhitney said:

I went about the business of breaking in and balancing the battery with 5 relatively shallow chargers to 58.8v (measured with multimeter and charger) and allowing the balancing to do its thing until I saw the green light. Feeling confident that my battery wasn't spending too much time in the balancing mode, I took it out with the intention of discharging below 49.0v. Again, the bike cutoff at exactly 49.0v.
...
I re-read the specs on my battery and I should be able to use my battery between 58.8 and 42v

Click to expand...

I suspect there is nothing wrong with your BMS.

According to the reported specs, you can expect the BMS to cutoff at 42V/14groups = 3V per cell. It's cutting off at 49V so in the worst case you have one low group -- at cutoff you have (49V - 3V)/13groups = 3.53V per cell for the other groups. 3V vs 3.53V is a pretty unbalanced pack but if you have several low groups things look a little better.

Your policy of 5 shallow charges and shutting off the charger when the green light goes ON is exactly backwards. You want a few shallow DIScharges (not charges) to avoid hitting the BMS cutoff then you want to leave the charger with the green light ON for some time to allow the battery to balance for each cycle. When the green light goes on, it is just beginning the balance phase, not ending it. Effectively, you have been repeatedly flogging the battery to cutoff and never balancing it. Unfortunately, running a battery to BMS cutoff is really punishing and tends to unbalance the groups making balancing take even longer.

Set your charger for the highest voltage (not a partial charge voltage), hook it up, and let the battery sit on charge for several days. The green light should cycle on and off over that period as the balance shunts discharge the full groups so the BMS can then kick in the charger and bring all the groups up a little bit. A big problem is that you have a 20Ah battery and balance currents of 200ma or less are common. This means it will take a long time to repeatedly bleed down those high capacity 'full' groups so the charger can then add a little bit to all groups - and then repeat, etc. Unless you want to open the pack, measure the individual groups, and manually bleed down the 'high' groups, you are probably in for a bit of a wait.

In any case, I think you need to try to balance your pack properly before you start thinking about more radical alternatives.

Click to expand...

Thank you for this. I did mis-speak about "shallow charges", i meant discharge. I would charge to 58.8, ride to work and plug it back in when i get home at about 55/54v back up to 58.8. I would unplug as soon i noticed green light. I assumed it was done as i use a luna charger. Once it reaches 58.8, it will start reducing its charge until the current is 0. I assumed that was the end of the balance cycle.

I intend to charge it up full and leave it plugged in for a good 12 hours after the light turns green.

acwhitney said:

I would unplug as soon i noticed green light. I assumed it was done as i use a luna charger.

Click to expand...

I'm not sure what that means - in the end the Luna charger is just another CVCC charger. Also, disregard the illustrations they have about charge cycle voltages - charging doesn't really work that way if your battery needs balancing.

Here's the problem: no two wire charger can have any notion of whether the pack is balanced - it deals entirely with the aggregate 'pack voltage' and just tapers off charge current when the pack reaches terminal voltage. Cell groups always get the same charge current, but they are not guaranteed to have the same state of charge or voltage. The charger is just a resource to be used as needed by the BMS - it's not really in control.

Case 1
If you have balanced cell groups then since they have the same state of charge they all have the same individual voltages. When the pack reaches terminal voltage all the groups still have equal voltages and the charger just rolls off the current and blips on the green LED. In this case the BMS never plays a role and sleeps through the whole endeavor - which is why bulk charging without a BMS works okay for balanced packs.

Case 2
But - if you have unbalanced cell groups then one or more group will have a lower voltage than the others. As the charger does it's job, all the groups take on charge and their voltages all rise - but the 'high' groups reach the desired per-group terminal voltage before the 'low' groups so overall the pack does yet reach the desired pack terminal voltage. So - the charger just keeps on truckin' until the pack has the 'proper' voltage, but by that time the 'high' groups are over-voltage and 'low' groups are still under-voltage. Even though the groups are unbalanced, the pack looks great to the charger.

Here's where the balance part comes in - the BMS per-group shunt resistors get switched in to bleed charge off the over-full groups which reduces the pack voltage allowing current to flow from the charger to slowly charge up the 'low' groups (unfortunately, also the high groups). In the typical BMS, balancing is a matter of bleeding away unwanted charge from 'good' cells that got over-volted because of the low cells. While the BMS is busy doing this, the charger is clueless. Depending on the degree of imbalance and the nuances of design in the BMS and charger, you may see some variations in behavior with charger cycling or not, but from a 30,000ft view, the same basic operation is going on under the covers.

• FWIW: Think of charging a pack as a footrace of 'cell-guys' in a mall finally ending at the second level. The Down escalator is the BMS. In the balanced case above, everybody starts from the same place and finally reaches the Down escalator at once, it never turns on, they run up it and everyone arrives at the second level at once. Charged and Balanced. But if some guys start out ahead (more initial charge), then when the race gets to go up the Down escalator, the escalator turns on and guys in the lead run in place pissing away charge energy until the other guys catch up, then the escalator turns off and everybody gets to the second level at once. Forced balance.

Regarding time to balance: Let's assume a 10% imbalance in a 20Ah pack. That means the high groups need to bleed off about 20Ah*10% = 2Ah of charge. For a 'typical' BMS with a 120ma max balance current, this will take 2Ah/0.12A = 16.6hrs. Sadly, this whole business doesn't work with max efficiency so it will actually take a lot longer. Anyhow, just making the point that this is not a quick undertaking if things are seriously out of wack. Here you can see the thinking behind the recommendation for shallow discharges and long balance periods as a means to be able to begin using a new pack while it gets into a solid balanced state where all this stuff doesn't need to happen (case 1 above).

I am glad I made this post and you have been especially helpful tek. The key thing I did not understand was that I needed to leave the battery on the charger for hours even days if possible in order to balance a badly out-of-whack pack. I was always in a hurry to discharge the pack after reading that it was not advisable to leave a pack 100% charged any longer than necessary.

After reading your previous posts, I completely disconnected my battery from the motor and hooked it up to my Lunacycle charger (has display indicating Voltage and current) and set it for a 100% at 1 amp to ensure I didn't pummel the drained cells with too much current. When I got up this morning, I found the charger had stopped around 55.6V and was starting to think I already did irreparable damage to one or more parallel group.

I switched to the 4amp charger provided with the battery and it began to charge again. It seems to have started the balance cycle maybe two hours ago, I hear the charger come on for a few minutes a couple of times every hour. I am hoping the first <1 amp charge got a little juice into any low groups and they were topped off at 4 amps.

My plan is to leave it on until I go to work in three hours (total of 5 hours balancing), where I can top it off and keep it charging/balancing for a good 6 more hours. Ride home and do it again.

acwhitney said:

I hear the charger come on for a few minutes a couple of times every hour.

My plan is to leave it on until I go to work in three hours (total of 5 hours balancing), where I can top it off and keep it charging/balancing for a good 6 more hours. Ride home and do it again.

Click to expand...

This seems to be behaving as expected and sounds very promising. Keep up this policy for a few cycles of modest use and the BMS will likely get things sorted out for you.

• 
  It's not clear how the Luna charger works at the lower voltage charge (80% vs 100%). The EM3EV chargers on which the Luna chargers are modeled do not balance at the lower settings (they just cutout permanently) so you need to use the 100% charge point once in a while to ensure the pack is balanced. Packs with matched good quality cells stay in balance for many many cycles without balancing, so just use the 100% charge level occasionally to keep things in shape.

Anyhow, you started out on the right plan - just a little knowledge gap about charging

Good luck with your battery!

As a side note to those who already know this subject well, EM3EV is now making battery packs with a "smart BMS " . EM3EV has been recommending & selling chargers with a high/low switch that = 100% / 90% . The " smart BMS " will balance at either setting. There is even a charger that has a third choice of 50% ( I don't think that choice includes balancing ).