Help pls - New battery Unbalanced after 3 cycles

[alexdelrio7]

Hi

I just built my first battery pack a few days ago. Sanyo GA 14s6p 52V with a BMS. When finished gluing them I tested each individual cell to 3.7V within 0.01V. Then after welding I tested each parallel group at 3.7V. I finished adding the BMS and cables then charged it for a few hours before going to work. I noticed it wasn't full to 58.8V~ but guessed maybe it needed more time. Next time I charged all night and only charged to 56V~. Following that it only charged to 53V. So I took the battery apart to investigate.

I found that the fifth group of parallel cells is full at 4.2V, while the rest are around 3.7V. I believe there must be something wrong with the BMS as the cells are genuine Sanyo GA which were perfectly balanced just a couple cycles ago. I checked the wiring and the BMS is correctly soldered.

Do you think it is likely the BMS is faulty? Surely this much imbalance can't only be the BMS' fault in such a short time??
My plan is to use a 5V headlamp and connect it to the terminals of that one specific parallel group while still part of the pack....? Is this a reasonable idea to get the one group discharged down to ~3.7V so I can then fully charge the entire pack?

Many thanks
Alex

 

[zeccato]

My plan is to use a 5V headlamp and connect it to the terminals of that one specific parallel group while still part of the pack....? Is this a reasonable idea to get the one group discharged down to ~3.7V so I can then fully charge the entire pack?

Many thanks
Alex

You can but it's dangerous, check often that it doesn't go up more than 4.2.
I think at 3.7v you can't see the balance, when it goes up to 4v. and + yes.

with this imbalance, charging it all with bms could overvoltage some parallels.
Don't trust bms, always check.
It's not ideal make a battery without a charger balancer, maybe they weren't balanced from the beginning, you had to check every single cell, if there are any faults, finally charge them, and then the rest.
I wish you good work

 

[jonyjoe303]

can you put a picture of your pack and bms. The bms won't start bleeding cells until they get up to about 4.18 volts, and they only bleed at about 60 ma, which is insignificant, your charger will easily overpower that.

Why would the bms bleed all the surrounding cells and leave one at 4.2 volts? are the cells new? if you have some bad cells in the pack you will have balancing problems.

If the cells are good, you tested that they didn't self discharge before putting together. And all were 3.7 volts when you connected to the charger, And all the series connections are equal (same amount nickle strip on every connection), when you connect to charger it should charge and stay in balance at least until it gets to around 4.18 volts, then the high cells will bleed to get the pack in balance.

To me it sounds like there be a few bad cells in your pack. Some of the red sanyos are known as "heaters", with high internal resistance. Some are good. Just one bad cell in a pack will get the whole pack out of balance.

 

[amberwolf]

to get unbalanced in the first place, something would have to be wrong with the cells in that group. (or with cells in all the other groups, instead, but much less likely).

so the most likely thing is that that group has one or more cells that either are not connected to the group (broken welds/connections?), or are internally not working as they should (high resistance, low capacity, etc).

so that group reaches 4.2v full long before the other groups do, and so the bms turns off charging to prevent overcharge of that group.

what *should* happen is that you should be able to monitor voltage on that group during a charge and see it drain down to a certain point, and then the bms should reenable the charge port, and the charger should then restart charge.

if you see the bms draining that group, but then the charger does not restart, it could be that the charger itself is of the type that requires being disconnected and reconnected, or even power cycled, in order to restart charging. in that case, you would eventually see the problem you're having, (after only a few cycles if that group doesn't have much capacity vs the others).

if you see the bms is *not* draining that group, then my guess is that there is a problem with the bms channel for that row, where it cannot turn on the balancing resistor for it, so it can't drain that group down to allow charging to restart. if you manually drain that group a little with an automotive headlight or other resistance (while the charger and bms are hooked up normally), and then charging automatically restarts, it is the bms channel not balancing that's the most likely issue.

 

[obcd]

Some of the cells of that high voltage group not being connected or being bad is the most likely cause of what you see.
Discharging the cell group with the 5V lamp is a good idea, but you should monitor that cell group voltage all the time.
After discharge, measure the voltage of each individual cell in that group.
If they all have the same voltage, you might have some bad cells.
If some measure different, you have a bad connection. Make sure they all measure equal before you repair that connection. You don't want to connect still fully charged cells in parallel with the partially discharged ones. You can use your 5V lamp to balance the cells before connection.
In general, never discharge a cell (group) below 3V and don't charge it over 4.2V
Never charge in house over night without supervision.

 

[docw009]

Was this pressure contact battery (Vruzend) or spot welded?

 

[dogman dan]

What I would do. Since I have RC chargers around, I would disconnect the bms first. Then I would connect one way or another, to charge each parallel section to full, or full as the RC charger will take them. ( might not be all the way to 4.2v). Record the voltage of each P section after charging full, and then watch them a few days.

This will at least reveal whether any sections are self discharging, before you start tearing apart the pack.

Then if all seems well, plug in the bms, and continue to watch for discharge. You may just have a faulty bms.

And have a look at that charger, still putting out 58v?

 

[999zip999]

Voltage and capacity are not the same. Just knock down the high cells. And don't over charge a parallel group.. I would put a set of sense wire's on there for easy montering is hook up a cellog a task you see what's going on easy. Best to put in big group of parallel cells and charge all to small voltage you got 3.7v charge all to 3.8v before build with same charger. So the BMS doesn't take 24hrs.