Top Balancing "How to"

[Hedges]

Capacity test drained individual cells each to low voltage cutoff? Varying number of Ah?
Yeah, I'd say top balance them again.
But charging in parallel will take forever, longer than when you first got them.
You could charge them in series with BMS protecting, until the first cell high voltage disconnect. Then top them off individually or in parallel (different SoC, use some method to get them to same voltage before paralleling, like individual charge.)

 

[hardtop]

I have 4 EVE LF304 Grade A Cells - 3.2V and a JK BMS 4-8S. Each battery measures 3.307v, 3.308v, 3.308v, 3.307 volts. What are the consequences if I don't top balance?

 

[MisterSandals]

What are the consequences if I don't top balance?

If they are matched well enough to charge to your top charge voltage without hitting a cell over volt, then you are good to go.

If you find that you cannot charge to that voltage, you can lower your charge voltage or try to top balance them to see if that helps.

 

[hardtop]

If they are matched well enough to charge to your top charge voltage without hitting a cell over volt, then you are good to go.

If you find that you cannot charge to that voltage, you can lower your charge voltage or try to top balance them to see if that helps.

It sounds like there is a possibility top balancing isn't needed. I'm planning to get the Victron Quattro-II https://www.currentconnected.com/product/victron-quattro-ii-12v-3kva-2x-120v-inverter/ charger/inverter and I have the JK BMS 4-8S. How will I know if I have a cell over volt condition?

 

[FilterGuy]

How will I know if I have a cell over volt condition?

Your BMS app will tell you there is a Cell overvoltage.

If you have one of the JK BMSs with an active balancer, it will balance your cells for you, but you need to have the system set correctly.

(This was corrected in an edit to the original post)
* Set the inverter bluk charge to 13.8V for a 12V system.
* Set the inverter accumulation time to an hour or two.
* Set the BMS cell overvoltage to 3.65V
* Set the BMS to start balancing at 3.45V

 

[42OhmsPA]

Your BMS app will tell you there is a Cell overvoltage.

If you have one of the JK BMSs with an active balancer, it will balance your cells for you, but you need to have the system set correctly.

Please check your listed voltages...
Excellent advice on a BMS with an active balancer or secondary active balancer if your BMS only has passive balancing.
If you don't plan to use an active balancer I would say top balancing is a must, unless you want to let the cells float for days.

I'm running 3 JBDs. 2 packs were top balanced, 1 wasn't. I see a high deviation at the top of the knee on the pack I didn't top balance.

Regardless what BMS you use you must use one!

 

[FilterGuy]

Please check your listed voltages...

OOPS. It should be:
* Set the inverter bluk charge to 18V for a 12V system.
* Set the inverter accumulation time to an hour or two.
* Set the BMS cell overvoltage to 3.65V
* Set the BMS to start balancing at 3.45V

It is corrected above as well.

 

[HRTKD]

18 volt bulk charge seems a bit high.

 

[FilterGuy]

18 volt bulk charge seems a bit high.

crap.... my mind is out to lunch. One more try:

* Set the inverter bluk charge to 13.8 V for a 12V system.
* Set the inverter accumulation time to an hour or two.
* Set the BMS cell overvoltage to 3.65V
* Set the BMS to start balancing at 3.45V

 

[HRTKD]

crap.... my mind is out to lunch. One more try:

* Set the inverter bluk charge to 13.8 V for a 12V system.
* Set the inverter accumulation time to an hour or two.
* Set the BMS cell overvoltage to 3.65V
* Set the BMS to start balancing at 3.45V

I was going to ask who you were and what you did with FilterGuy.

 

[alexcl]

I have been pre-charging some EVE cells in a 2p8s setup with this power supply: https://www.amazon.com/dp/B08DJ1FDXV?psc=1&ref=ppx_yo2ov_dt_b_product_details and a JK BMS (to prevent over charging any of the 8 pairs).

Unfortunately at the upper ends of the voltage spectrum (~27V) the batteries stop charging. From what I've read, this is likely due to smallish/resistive stock leads dropping the voltage. So I made some leads from 12 AWG wire with ring terminals on the end.

These new leads seem much "sparkier" - presumably owing to the lower resistance. As such I'd prefer to attach all of the leads before turning the power on. Up until this point, I've been turning on the power supply and the using the alligator clips of the crappy leads to clip terminals.

Note: Both the multimeter and the cell balance leads from the BMS show even voltages across the cells (~3.37V).

Assuming I have the voltage and amperage set correctly on the PSU, is it okay to turn on the power supply with the leads already connected? Is there a ramp up/down of some kind that could be harmful to the cells/bms?

If it's not okay, what is the best way to solve this? I was thinking about sticking a circuit breaker in the loop opening it once the PSU is at the correct voltage.

 

[42OhmsPA]

With the leads disconnected turn on the power supply and use a trusted multimeter to dial in the voltage, don't rely on the power supply display.
Turn off the power supply and connect the leads.
Turn on the power and don't touch the adjustment knobs regardless what the voltage in the display is.
Disconnect the power supply when amperage drops.

 

[FilterGuy]

The only issue I can think of is that if the power supply allows backfeeding from the battery when it is not turned on. I have a similar power supply that I turn on after I connect the batteries and have seen no issues.

1) With the leads disconnected, set the voltage.
2) With the power supply off, short the leads. Then turn on the supply and set the current. (With a 10A power supply this will almost always be all the way on).
3) With the power supply off. Hook up the leads to the battery.
4) Turn on the power supply.
5) Be patient.... it will seem like nothing is happening for a very long time. The voltage will seem to just sit there and not change. The voltage at the power supply will be slightly higher than the voltage at the battery. This is normal. Don't try to adjust the power supply. Just be patient and let it go.
6) Be more patient.
7) Be even more patient.
8) Eventually the voltage will start rising. When the voltage starts rising things will start happening quickly. The current will start going down and the voltage difference between the supply and the battery will start going down. This is all normal.
9) At some point, one of two things will happen.

A) The current goes to near zero without the BMS kicking off due to a cell going above the voltage protection point. If this happens, you are pretty much done and the battery is close to top-ballanced. I would not bother trying to top balance it further.​

B) One of the cells goes above the voltage limit and the BMS turns off charging. At this point, the battery needs to be top-balanced using one of the many ways described on this and other threads.
​

 

[alexcl]

The only issue I can think of is that if the power supply allows backfeeding from the battery when it is not turned on. I have a similar power supply that I turn on after I connect the batteries and have seen no issues.

1) With the leads disconnected, set the voltage.
2) With the power supply off, short the leads. Then turn on the supply and set the current. (With a 10A power supply this will almost always be all the way on).
3) With the power supply off. Hook up the leads to the battery.
4) Turn on the power supply.
5) Be patient.... it will seem like nothing is happening for a very long time. The voltage will seem to just sit there and not change. The voltage at the power supply will be slightly higher than the voltage at the battery. This is normal. Don't try to adjust the power supply. Just be patient and let it go.
6) Be more patient.
7) Be even more patient.
8) Eventually the voltage will start rising. When the voltage starts rising things will start happening quickly. The current will start going down and the voltage difference between the supply and the battery will start going down. This is all normal.
9) At some point, one of two things will happen.

A) The current goes to near zero without the BMS kicking off due to a cell going above the voltage protection point. If this happens, you are pretty much done and the battery is close to top-ballanced. I would not bother trying to top balance it further.​

B) One of the cells goes above the voltage limit and the BMS turns off charging. At this point, the battery needs to be top-balanced using one of the many ways described on this and other threads.​

​

Great thanks for the info. I will give this a shot. I have disallowed discharge in the BMS so hopefully that would also stop any backfeeding.

Is there a reason I can't start with a low current and dial it up? E.g. follow your directions above but substitute the following steps:
2) Set the current to 1A
4) Turn on the power supply. Turn up the amperage to desired current (i.e 10A in this case)

Side note: I noticed the voltage rising for a while before they stopped charging (e.g. cells have gone from ~3.2V to ~3.34V already). When I just tested the pack voltage it was at 26.7V (BMS reading confirmed by multimeter). The output voltage of the PSU was 27.34 (PSU reading confirmed by multimeter). Each cell was at ~3.34V (BMS confirmed by multimeter). When the cells appear to stop charging, the PSU voltage is equal to the battery voltage. This makes me think that the resistance in the leads is dropping the voltage.

 

[FilterGuy]

Is there a reason I can't start with a low current and dial it up? E.g. follow your directions above but substitute the following steps:
2) Set the current to 1A
4) Turn on the power supply. Turn up the amperage to desired current (i.e 10A in this case)

Yes, that works

 

[alexcl]

Okay I think something else might be going on. The pack climbed from 26.76V to 26.92V in ~1.5 hours and now just sits there. This is the same voltage that the pack sat at before (for roughly 8 or 9 hours). Below is the BMS status page looks like (these numbers were corroborated with multimeter readings):

Again the voltage at the end of the leads is exactly the same as the battery voltage which makes me think the cells are not being charged. Any thoughts?

 

[sunshine_eggo]

The screen capture you provided indicates 5.6A of charge current.

LFP voltage climbs VERY slowly.

 

[Tomthumb62]

Okay I think something else might be going on. The pack climbed from 26.76V to 26.92V in ~1.5 hours and now just sits there. This is the same voltage that the pack sat at before (for roughly 8 or 9 hours). Below is the BMS status page looks like (these numbers were corroborated with multimeter readings):

Again the voltage at the end of the leads is exactly the same as the battery voltage which makes me think the cells are not being charged. Any thoughts?

View attachment 182218

The battery controls the charge current. So if there is current flowing (5.6A in your case), then the battery is receiving a charge.

If you haven’t read the excellent “how to top balance” PDF, it says that when the charge cycle hits the flat parts of the curve, it can be hours or days without the voltage going up. But if the amps are not near or at zero, then you know it IS still charging.

 

[alexcl]

The screen capture you provided indicates 5.6A of charge current.

LFP voltage climbs VERY slowly.

Not sure why I was thinking that would be anything other than net current ?‍ (i.e. I thought maybe the BMS was measuring only discharge on the way back to the PSU).
Okay, I'll wait overnight. Previously this is the voltage I charged to and then continued to sink in 6A (into 608Ah pack) for ~8 hours with only a 0.01V change in the entire pack (~1mV per individual cell).

Will update tomorrow morning.

The battery controls the charge current. So if there is current flowing (5.6A in your case), then the battery is receiving a charge.

If you haven’t read the excellent “how to top balance” PDF, it says that when the charge cycle hits the flat parts of the curve, it can be hours or days without the voltage going up. But if the amps are not near or at zero, then you know it IS still charging.

Makes sense. I did read it, and was trying to pre-charge in order to avoid a long top balance. Tried to calculate the percentage of the entire battery I charged and then line up the current voltage with LiFePO4 voltage-SoC charts to see how much the voltage should have changed wrt the SoC delta I calculated. Possibly I did some faulty arithmetic or the curve is particularly flat at this voltage for these cells. Will replicate the calculations if nothing has changed tomorrow. Thanks for the intel.

 

[sunshine_eggo]

most cells ship around 30%, so you need to supply 70% of a 608Ah pack

0.7 * 608Ah / 6A = 71 hours.