Ideal Top Balancing

[D. Abineri]

Definite newbie here!

I have a 16 cell lifp4 battery and am wondering what is the best/safest(for the cells0/fastest way to top balance these cells?

I have a small power supply on which I can set the voltage (ie 3.6v) and the current with the cells in parallel but it takes a very long time to balance the cells. The typical charging current might be about 4A to start with.

What would be an equivalent, but more powerful power supply for this job? I assume that the more current used to top balance, the faster the process will be. What is considered the maximum charging current for such cells?

Thanks for any assistance with these questions.

 

[FilterGuy]

Definite newbie here!

I have a 16 cell lifp4 battery and am wondering what is the best/safest(for the cells0/fastest way to top balance these cells?

I have a small power supply on which I can set the voltage (ie 3.6v) and the current with the cells in parallel but it takes a very long time to balance the cells. The typical charging current might be about 4A to start with.

What would be an equivalent, but more powerful power supply for this job? I assume that the more current used to top balance, the faster the process will be. What is considered the maximum charging current for such cells?

Thanks for any assistance with these questions.

Have you reviewed this:

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[D. Abineri]

Thanks for all this FilterGuy. I just have one question further. When charging in series with the BMS as a guard again over doing it, do I just turn off the battery to inverter connection so that the battery load is an open circuit?

Thanks again

 

[FilterGuy]

Thanks for all this FilterGuy. I just have one question further. When charging in series with the BMS as a guard again over doing it, do I just turn off the battery to inverter connection so that the battery load is an open circuit?

Thanks again

You have a couple of options.

1) Use the inverter to do the pre-charge.
The inverter probably has a much higher charge rate than the small power supply you have. You could use that to make the pre-charge step much faster. Once the in-series pre-charge is complete, disconnect the inverter and use the power supply to do the in-parallel top balance.

2) Completely disconnect the inverter and use the small power supply for both.

 

[D. Abineri]

When you say completely disconnect the inverter, does that mean turn off the switch I have on the positive lead from the battery to the inverter?

And when you say to use the power supply for both, what are the "both"?

Thanks again

 

[FilterGuy]

When you say completely disconnect the inverter, does that mean turn off the switch I have on the positive lead from the battery to the inverter?

Yes. That would work.

Note: I generally do all the top balancing before I hook up the inverter.... but there is no magic that requires that.

And when you say to use the power supply for both, what are the "both"?

In the document I linked to, there are two steps.
1) Precharge with the cells in series with the BMS as a safety.
2) Top balance with the cells in parallel and no BMS

You could use the small power supply for both steps, but step 1 might take a long time if the Power supply is small.

 

[D. Abineri]

Thanks again!

 

[time2roll]

I would skip the manual top balance and get a NEEY active balancer. Just assemble and immediately put into service.

https://www.ebay.com/itm/314241662802

 

[D. Abineri]

Does one use passive and active at the same time with both connected or just use the active?

 

[time2roll]

I use both but not sure it matters much. Any real balancing is only going to take place when held near the top or very bottom of charge.
Both should have an app to select what you prefer.

 

[D. Abineri]

I use both but not sure it matters much. Any real balancing is only going to take place when held near the top or very bottom of charge.
Both should have an app to select what you prefer.

In looking at the NEEY, it seems that it balances the VOLTAGES of the cells but this is not neccessarily a TOP Balance is it?

 

[FilterGuy]

In looking at the NEEY, it seems that it balances the VOLTAGES of the cells but this is not neccessarily a TOP Balance is it?

Top balancing is a process of matching the voltages of the cells at the top end of the charging.

 

[D. Abineri]

Top balancing is a process of matching the voltages of the cells at the top end of the charging.

I understand this. But if the balance is just balancing the voltages, how do we know this is being done at the TOP of the range? Does the NEEY do this at the TOP?

 

[FilterGuy]

I understand this. But if the balance is just balancing the voltages, how do we know this is being done at the TOP of the range? Does the NEEY do this at the TOP?

There is a version of the Neely active balancer that does not turn on till the cell voltage is 3.4V.

 

[D. Abineri]

There is a version of the Neely active balancer that does not turn on till the cell voltage is 3.4V.

But couldn't that 3.4V be near the lower end of the state of charge? The graph of voltage is nearly flat. Could it not be anywhere along that charging curve?

 

[time2roll]

No. 3.4 is the start of the upper curve. 2.9 volts is approaching the bottom of charge. Usually balancing is looking for a differential of 15+ mV to even start. In the mid capacity range all s/b within 0 to 10 mV and nothing moves. As the battery approaches top charge (3.4+ vpc) the cell voltages may diverge and will be equalized by the balancer. Max cell voltage is about 3.650

This is how it works regardless of the balancer. Yes some trigger points are programmable.

 

[D. Abineri]

No. 3.4 is the start of the upper curve. 2.9 volts is approaching the bottom of charge. Usually balancing is looking for a differential of 15+ mV to even start. In the mid capacity range all s/b within 0 to 10 mV and nothing moves. As the battery approaches top charge (3.4+ vpc) the cell voltages may diverge and will be equalized by the balancer. Max cell voltage is about 3.650

This is how it works regardless of the balancer. Yes some trigger points are programmable.

So, you are saying that the NEEY is in fact top balancing it set for 3.4V or so?

By the way, why is the NEEY 4 only $11 on AliExpress? Is there any problem getting one from them?

Thanks.

 

[time2roll]

I assume $11 is a fraud. Good luck. Shop around.

 

[FilterGuy]

Note: Out on the wild-wild-web you can find a lot of images of LiFePO4 charge curves that show 3.4V at the low end of the curve. That is not what we see in real life. 3.4V is where we see the voltage start to rapidly rise during charging.

So, you are saying that the NEEY is in fact top balancing it set for 3.4V or so?

The Neey will start to attempt to balance starting at 3.4V. As long as the cell difference is large enough it will keep trying to balance the cells till the delta is small enough or the cell voltages drop back down to the cut-off voltage. (I do not remember the cut-off voltage, but it will be lower than 3.4V).

Note that even if the BMS detects an overvoltage and cuts off charging, the NEEY will continue to balance.

The net result is that the cells will get top-balanced to the voltage the charger(s) are set to. (Assuming the charger is set to something greater than 3.4V/cell)


Just as a side note: One flaw in some BMSs is that they only balance while the charge current is present. Since the charge current is typically multiple times larger than the balance current, the balancer never gets a chance to balance the cells. If the balance is allowed to continue after the charge current goes away, it has a chance to catch up and actually balance the cells. When done this way, even passive balancers with a very low balance current can keep a fairly large bank of cells balanced.

Note: A lot of BMSs have a setting that determines if the balancer only activates during charging. For most applications, this should be off

 

[D. Abineri]

Note: Out on the wild-wild-web you can find a lot of images of LiFePO4 charge curves that show 3.4V at the low end of the curve. That is not what we see in real life. 3.4V is where we see the voltage start to rapidly rise during charging.


The Neey will start to attempt to balance starting at 3.4V. As long as the cell difference is large enough it will keep trying to balance the cells till the delta is small enough or the cell voltages drop back down to the cut-off voltage. (I do not remember the cut-off voltage, but it will be lower than 3.4V).

Note that even if the BMS detects an overvoltage and cuts off charging, the NEEY will continue to balance.

The net result is that the cells will get top-balanced to the voltage the charger(s) are set to. (Assuming the charger is set to something greater than 3.4V/cell)


Just as a side note: One flaw in some BMSs is that they only balance while the charge current is present. Since the charge current is typically multiple times larger than the balance current, the balancer never gets a chance to balance the cells. If the balance is allowed to continue after the charge current goes away, it has a chance to catch up and actually balance the cells. When done this way, even passive balancers with a very low balance current can keep a fairly large bank of cells balanced.

Note: A lot of BMSs have a setting that determines if the balancer only activates during charging. For most applications, this should be off

Thanks for all this. Good information!