Top Balancing "How to"

[sunshine_eggo]

"equal voltage" is not "equal state of charge." The latter matters. The only thing you can say with any confidence is that cell is at 100% while the others are not.

Want to elaborate on this...

You received multple cells all at different states of charge.

I purchased nine 280Ah EVE cells in late 2020, received in 1/2021. Cell voltages were identical at 3.297.

Having an absurd amount of discharging capability, upon receipt, I simultaneously discharged each one individually to empty and recorded the Ah extracted. I then did a charge/discharge test to measure capacity of each cells. Once the discharge capacity was established for each cell, I compared that to the as-received capacity. The nine cells were received between 28.0 and 40.4% SoC.

If I had wired those cells in series and charged until one cell is full, the lowest cell will only be at about 88% SoC.

The cell that was received at the highest state of charge hit full first. I have personally seen as little as 0.1% SoC imbalance trigger a cell runner that prohibits full charge voltage due to a runner.

You are having an anologous experience.

 

[Skypower]

You mean check that cell voltage? I

No I mean connection. A bad connection under load can do weird things. You may even be able to see a voltage difference if you can get at the terminal base and at the buss however without load you wouldn’t see the difference between the two points. A little oxidation is all it takes

 

[mrmoon]

Thank you everyone. I have successfuly top-balanced my cells, by combination of series and parallel charging methods. Will check voltages in the morning.

 

[Ampster]

Thank you everyone. I have successfuly top-balanced my cells, by combination of series and parallel charging methods. Will check voltages in the morning.

Based on those thin wires and that both charging cables are at the same end, you may see a slight voltage difference at the top when you charge. The BMS may balance it after a few cycles. You won't see it in the morning because they will all settle to the same voltages. I am not suggesting you change anything now, my comment is for other readers to remind them to put the charging cables at oposite ends of the pack so that current flow will not be reduced by the current having to go through all of those small wires to the last cells.

 

[Hedges]

Batteries wired in parallel, and charged from one end.
We would have recommended connecting positive and negative to opposite ends of the 8x parallel cells.

But balanced within 7 mV, so you win.
Charger should stop a bit below that voltage, to avoid having a runner.

 

[mrmoon]

Based on those thin wires and that both charging cables are at the same end, you may see a slight voltage difference at the top when you charge. The BMS may balance it after a few cycles. You won't see it in the morning because they will all settle to the same voltages. I am not suggesting you change anything now, my comment is for other readers to remind them to put the charging cables at oposite ends of the pack so that current flow will not be reduced by the current having to go through all of those small wires to the last cells.

Got it. Thank you

 

[Dedou]

I did top balance for the cells, the cells remain well balanced at rest, and when the battery discharges, but when I charge (14.2v) the battery, 2 cells charge faster until the bms overvoltage protection (3.6v) stops the charging and the battery is not fully charged.

Do I do top balance again, or are there other ways? I don't have an active balancer, just jbd bms (balance start voltage 3.4v).

 

[sunshine_eggo]

I did top balance for the cells, the cells remain well balanced at rest, and when the battery discharges, but when I charge (14.2v) the battery, 2 cells charge faster until the bms overvoltage protection (3.6v) stops the charging and the battery is not fully charged.

Do I do top balance again, or are there other ways? I don't have an active balancer, just jbd bms (balance start voltage 3.4v).

Worth checking the sensing harness for any potential connection issues. If none are found, your cells are slightly imbalanced.

If you can charge to the desired 14.2V without triggering charge protection, you likely don't need to do anything. Regular use should allow the BMS to slowly balance the cells.

If you can't, recommend you hold the battery at 13.8V for 24 hours. That should get all cells closer to 3.45V and may permit normal charging to 14.2V.

 

[Dedou]

Connections are ok. The cells are well balanced, the difference between 2-10mV when discharging the battery and 2-3mV at rest, the problem is only when charging.

Could it be because I made top balance 3.6v, but now I charge them 14.2 V (3.55 V each)?

 

[Hedges]

Top balance higher, charge to a bit lower is the correct thing to do.
Probably some deviation from completely balanced. For instance, if connected in parallel and +/- both connected at one end, not all cells will get the same voltage and reach same Soc.
If one cell had poor contact, it wouldn't reach the same SoC.

After top balance, did you disconnect the cells from each other, let them settle for a few hours, then check resting voltage? That would indicate how well balanced.

Maybe top balance 3.65, charge to 3.50 would have avoided this.

The cells are well balanced, the difference between ... 2-3mV at rest, the problem is only when charging.

Is that at rest, after charging to 3.55V (or to the point of BMS disconnect)?
Or resting after discharge?


Maybe as suggested, reduce charging voltage slightly and hold there for an extended period, letting BMS balance by bleeding off charge.
If that isn't sufficient or fast enough, you can manually bleed off some charge.

 

[sunshine_eggo]

Connections are ok. The cells are well balanced,

No. They are not. Based on the numbers, they're not horribly imbalanced, but they are NOT balanced.

the difference between 2-10mV when discharging the battery and 2-3mV at rest,

The above mean nothing concerning balance. Even imbalanced batteries have nearly identical voltages in the operating range/at rest.

the problem is only when charging.

Because they are imbalanced, which is the only time you get evidence of imbalance.

Could it be because I made top balance 3.6v, but now I charge them 14.2 V (3.55 V each)?

No. It's because they weren't top balanced properly, they've sat too long since the top balance, or they have uneven self-discharge rates and are poorly matched.

 

[Hedges]

No. It's because they've sat too long since the top balance, or they have uneven self-discharge rates and are poorly matched.

Or top-balancing wasn't balanced. That's the best one to hope for, because it is correctable.

One guy parallel top-balanced his cells, and when separated, one settled lower than the rest.
I forget if he ran in series after that and had a runner, or addressed it immediately.
He connected the low cell to the charger again and brought it up to top-balance voltage (or same as others last charged to) and it was good.

We've often see a large voltage drop from power supply to cells when people use the supplied alligator clips. Heavy wires and ring terminals fixes that.
But, if the connections between paralleled cells aren't good, one or more cells can similarly drop lower.
People may scrub terminals to break oxide (and use corrosion inhibitor) for final assembly. But usually not during top balance.

 

[Dedou]

Jos et pysty, suosittelemme pitämään akkua 13,8 V:ssa 24 tuntia. Tämän pitäisi saada kaikki kennot lähemmäksi 3,45 V:tä ja voi mahdollistaa normaalin latauksen 14,2 V:iin.

Do you mean that I keep the charging on for 24 hours at 13.8v voltage? Does it do the same thing as top balance? Can the JBD passive balancer balance as well as when connected in parallel? I don't want to disassemble the battery and charge again in parallel, if I don't have to.

The battery rested at 13.2v voltage for 3 months, then when I charged, the cells were unbalanced.

 

[sunshine_eggo]

Do you mean that I keep the charging on for 24 hours at 13.8v voltage?

Yes. If you still have the power supply that you used to top balance the cells, set it to 13.8V and then connect it to the battery. Let it sit that way for 24 hours.

Does it do the same thing as top balance?

Mostly, just slower. At 3.45V, the cells can be very nearly fully charged and much closer to properly balanced, i.e., if all cells read 3.45V at the same time, it's likely they'll handle 14.2V just fine.

Can the JBD passive balancer balance as well as when connected in parallel? I don't want to disassemble the battery and charge again in parallel, if I don't have to.

The JBD can balance about 0.5-1Ah/24 hours when held above the balance voltage. Make sure the JBD is set to balance at all times, not just during charge, at 3.40V and above and 20mV difference.

The battery rested at 13.2v voltage for 3 months, then when I charged, the cells were unbalanced.

This is why you're experiencing imbalance. Batteries that are not regularly cycled will eventually drift out of balance. Based on the voltage numbers, and the duration of the inactivity, I would conclude your battery is overall in pretty decent shape and only requires a little extra effort to get it back in balance.

 

[time2roll]

I did top balance for the cells, the cells remain well balanced at rest, and when the battery discharges, but when I charge (14.2v) the battery, 2 cells charge faster until the bms overvoltage protection (3.6v) stops the charging and the battery is not fully charged.

Do I do top balance again, or are there other ways? I don't have an active balancer, just jbd bms (balance start voltage 3.4v).

Reduce charging voltage to 14.10 to avoid over volt protection.
Use the top balance power supply set to 3.65 volts to boost charge cells 1 & 4 to 3.500 volts
No need to dissemble. Connect direct to one cell at a time until voltage rises to match the high cell.
When complete reinstate the 14.2 volt charging level.

 

[Dedou]

The BMS can balance about 0.5-1Ah/24 hours when held above the balance voltage. Make sure the JBD is set to balance at all times, not just during charge, at 3.40V and above and 20mV difference.



This is why you're experiencing imbalance. Batteries that are not regularly cycled will eventually drift out of balance. Based on the voltage numbers, and the duration of the inactivity, I would conclude your battery is overall in pretty decent shape and only requires a little extra effort to get it back in balance.

Why only 20mV difference?

The battery is in winter storage for 2-4 months every year, so if I want to keep the battery in balance, should I keep the voltage at least 13.6v in storage, or get an active balancer?

 

[upnorthandpersonal]

Why only 20mV difference?

The battery is in winter storage for 2-4 months every year, so if I want to keep the battery in balance, should I keep the voltage at least 13.6v in storage, or get an active balancer?

You should be just fine. They're slightly out of balance, but if you lower your charge voltage slightly, and let the passive balancer catch up, they should get in sync. Even slightly off-balance at these cell voltages doesn't matter - mine are the same after winter. Once the sun hits, it takes just a few days and all is fine again (I use only 0.6A active balancers).

 

[sunshine_eggo]

Why only 20mV difference?

The battery is in winter storage for 2-4 months every year, so if I want to keep the battery in balance, should I keep the voltage at least 13.6v in storage, or get an active balancer?

No. Just plan to apply a balancing charge before use or start out at reduced charge voltage. You can get nearly 100% of your capacity @ 3.45V/cell, you just have to hold it there for 2-3 hours.

 

[Delmar]

We are getting more power outages this year so plan to implement my golf cart FLA-to-LiPo conversion. Gave-up waiting for the FLA to die.

For my 48V UPSverter I use a standard 120V 60W incandescent light bulb to precharge the caps. Makes a perfect variable-resistance (24-240 ohm) load with visual indication.

Was thinking of a simple method of balancing the new cells. Wire all cells in parallel, but with a 12V incandescent bulb in series with each cell. Would eliminate having to manually charge each battery to a similar voltage. Any flaw with my hypothesis?

 

[time2roll]

Was thinking of a simple method of balancing the cells. Wire all cells in parallel, but with a 12V incandescent bulb in series with each cell. Would eliminate having to manually charge each battery to a similar voltage. Any flaw with my hypothesis?

Not sure the electrical theory here but I think an active balancer is a better solution.