Active balancer, make it smart!?

[Ampster]

So a difference of (3.4V to 3.456V = 56mV) of 56mV has a different significance depending where you are on the charging curve.

My experience is that the difference between 3.45 and 3.56 is less than 10 percent SOC because that is at the knee of the charge curve. The difference between 3.2 and 3.4 could be 80 percent SOC because it is away from the knee on the flat part of the curve.

 

[Hans Kroeger]

Thanks, yes i know the job of a balancer has nothing to do with capacity but concerns only voltage.

Why i measured and mentioned it has following reason.
As you know for especially lifepo4s voltage and capacity doesn't behave linear. For example a difference of 56mV (3.4V to 3.456V = 56mV) has a different significance depending where you are on the charging curve (at 0.07C discharge current, it is 50%capacity in some curve areas according to my testing). So i wanted to put my deviated (from what i ordered and was confirmed before ordering) switch ON and OFF voltage points in perspective. Is the difference from 3.4V (or 3.43V since that is the number they tested in their lab and shared with me before ordering) to 3.456V significant or not was my question. Should i be concerned or not since the deviation is higher than the 50mV they stated before? 3.43V (what they said they tested in the lab) would have been within the promised 50mV tolerance. 3.456V (56mV) was already outside but let's say that is still fine because my voltmeter has an 0.5% error at a scale of 0.001V. 3.311V (89mV) was way out of tolerance. So after testing of how much difference it makes in relation to capacity i thought, well never mind, i can live with a +-2%capacity tolerance (it will be of course more or less if i would reduce or increase the charge or discharge rating), not worth the trouble of complaining. Overall although not perfect this customized balancer is already way better, more suitable for me than the stock version. Of course i would also have preferred if it would have switched on at 3.4v or even 3.43v. 3.456V is indeed borderlining but still usable since i know it and can adapt to it.
So I just shared the numbers of whatever i tested, the 4% was just an information for those who also have questions like me above.

Sure everyone has a different system, different charging end points, different demands etc so i understand judging from your final charge voltage of 13.9V, 3.45V is too high since there could be instances where the balancer wouldn't even turn on which would render it a useless accessory.

After all this customized version is their first try, a prototype, maybe they will improve it and then bring it eventually on the market. If they want to commercialize it, switch ON should be at 3.4V (or little bit less), switch off at 3.3V (or little bit more). that would cater most demands and charging profiles. I already gave them a feedback and a recommendation for a commercial special version for Lifepo4 batteries in solar systems. Switch on at 3.4V (or little bit lower), switch off at 3.3V (or little bit higher).

Hi Max, I am 100% with you!
Taking into account that a high charging current has a strong impact on the SOC over Voltage curve, makes it even more difficult to select a fixed voltage level for turning on or off a smart active balancer.......but as always, we must live with compromises. I believe your proposed voltage level for the On/Off control is a good compromise.
Would be great if Hankzor modifies the balancer following your proposal. Would be a big improvement compared to the actual design.

Can you ask Hankzor to think about a "next generation" smart active balancer which is monitoring the total voltage of the 4 cells in order to switch on at 13.5 V, and switch off at 13.3 V, with an accuracy of 0.1 V or better.

Regards Hans

 

[Hans Kroeger]

My experience is that the difference between 3.45 and 3.56 is less than 10 percent SOC because that is at the knee of the charge curve.

Very true, unfortunately...... At 3.45 V an LifePO4 cell may be slightly overcharged with a very low current. The same cell may be well below 80% SOC at a high charging current.

In case the charger goes to a floating mode the voltage must be at or below 4 x 3.4 V = 13.6 V. Otherwise the cells are continuously overcharged which reduces their lifetime.

 

[Vi s]

Very true, unfortunately...... At 3.45 V an LifePO4 cell may be slightly overcharged with a very low current. The same cell may be well below 80% SOC at a high charging current.

In case the charger goes to a floating mode the voltage must be at or below 4 x 3.4 V = 13.6 V. Otherwise the cells are continuously overcharged which reduces their lifetime.

Hi Hans,
According to your testing and experience, if you never charge above 0.16C (peak, maybe 1h long) but most of the time around 0.04C, what voltage would you recommend as a charge stop voltage (no Absorption, no floating, just a charge resume voltage at 13.3V) in order to not reduce the lifespan?

 

[Vi s]

Hi Max, I am 100% with you!
Taking into account that a high charging current has a strong impact on the SOC over Voltage curve, makes it even more difficult to select a fixed voltage level for turning on or off a smart active balancer.......but as always, we must live with compromises. I believe your proposed voltage level for the On/Off control is a good compromise.
Would be great if Hankzor modifies the balancer following your proposal. Would be a big improvement compared to the actual design.

Can you ask Hankzor to think about a "next generation" smart active balancer which is monitoring the total voltage of the 4 cells in order to switch on at 13.5 V, and switch off at 13.3 V, with an accuracy of 0.1 V or better.

Regards Hans

Hi Hans

Sure i can. Could you explain to me why you think monitoring total voltage would be significantly more advantageous or better than just the B1 voltage? I mean i should also give them a good reason why they should go through the trouble to change the design.

 

[Vi s]

Hi Hans
Wouldn't 13.6V and 13.3V be better?
13.6V would be, with a accuracy of 0.1V, 3.375-3.425.
13.5V would be 3.35-3.4V.

 

[Hans Kroeger]

Hi Hans,
According to your testing and experience, if you never charge above 0.16C (peak, maybe 1h long) but most of the time around 0.04C, what voltage would you recommend as a charge stop voltage (no Absorption, no floating, just a charge resume voltage at 13.3V) in order to not reduce the lifespan?

Max,
From my experience an end of charge voltage of 13.9... 14.0 V is a good compromise. 3.5 V max cell voltage keeps the stress low, gives you 100% SOC after the current has tapered off, guarantees ample margin to the 3.65 V max cell voltage and a 3.7 V overvoltage disconnect.
13.9 V to 14.0 V is too low for the normal passive balancer. But this drawback is compensated by the usage of "our smart active balancer" .
I am glad you brought up and continue to pursue this important issue.
Cheers Hans

 

[Hans Kroeger]

Hi Hans
Wouldn't 13.6V and 13.3V be better?
13.6V would be, with a accuracy of 0.1V, 3.375-3.425.
13.5V would be 3.35-3.4V.

You may be Wright, I will think about it.

 

[Hans Kroeger]

Hi Hans

Sure i can. Could you explain to me why you think monitoring total voltage would be significantly more advantageous or better than just the B1 voltage? I mean i should also give them a good reason why they should go through the trouble to change the design.

I must run, will get back to you later

 

[Vi s]

You may be Wright, I will think about it.

I mean you want the balancer active when the cells start to drift apart (knee). Before that it wouldn't be necessary. Of course since we talk about total voltage at 13.7V the chances are higher than at 13.6V that you already have one cell way ahead. To also think about people with unmatched cells, to be safe, i also think 13.5V is better.

 

[Vi s]

I must run, will get back to you later

Sure there is no rush since it's weekend. Nami hasn't read my last message yet.

 

[Vi s]

Max,
From my experience an end of charge voltage of 13.9... 14.0 V is a good compromise. 3.5 V max cell voltage keeps the stress low, gives you 100% SOC after the current has tapered off, guarantees ample margin to the 3.65 V max cell voltage and a 3.7 V overvoltage disconnect.
13.9 V to 14.0 V is too low for the normal passive balancer. But this drawback is compensated by the usage of "our smart active balancer" .
I am glad you brought up and continue to pursue this important issue.
Cheers Hans

Hi Hans
When I read that even at 3.45V one could slightly overcharge ones battery with a slow charge speed i thought maybe i should lower my charge end voltage from 13.8V to 13.7V or even lower... During most day hours of the day the charge power is only 2-25w and about 2 hours up to 96W (320wh yield at most per day, 680wh battery. My battery cycles usually between 70 and 100%soc. Hardly ever goes above 13.41V even at 100%soc. Maybe they sold me higher capacity cells than they stated ? but often I feel my cells are bottomless. They stay quite a long time at 100% (coloumb counter) before the voltage goes above 13.41V (at a slow slow charge speed). Voltage is really no good indicator for lifepo4-SOC under certain circumstances.

 

[Ellcon123]

Voltage is a good indicator, 2.5V is 0% and 3.65V is 100%. Everything else is a bit of guesswork. ? I would say at 13.41V you are not 100%.

 

[Vi s]

Voltage is a good indicator, 2.5V is 0% and 3.65V is 100%. Everything else is a bit of guesswork. ? I would say at 13.41V you are not 100%.

I thought so too. I am truly wondering because i checked the coloumb counter and it seems to be precise. I will have to check it again. It even counts below 1w loads or charges unlike the BMS coloumb counter.

 

[Ellcon123]

SOC or coulomb counter are only precise when reset. This can only occur at 0% or 100% and in between they will drift. Tesla telks owners using LFP batteries to charge to 100% once a week to reset range/SOC. Why do we expect our batteries to work differently? LFP is safe and great for our purposes but you can't operate 20-80%, voltage curve is to flat. Cycle to near 100% and you will have no problems.

 

[Hans Kroeger]

Hi Hans

Sure i can. Could you explain to me why you think monitoring total voltage would be significantly more advantageous or better than just the B1 voltage? I mean i should also give them a good reason why they should go through the trouble to change the design.

Max,
With single cell monitoring the risk is much higher that the balancer is not turned on at the end of charging with a low voltage.
Example: charging with 13.9 V, cells 1 to 4 may be at 3.35 + 3.50 + 3.50 + 3.55 V. The balancer will stay off.
You can play around with theses numbers but you will end up with the same result.
Arguing from a different point of view: when charging wth 13.9 Volts it is safe to turn on the balancer if the total battery voltage goes above 13.5 Volt (or 13.6 V) no matter what the individual cell voltages are. Turning the balancer off at 13.4 V (or 13.3 V) leaves ample margin for the balancer to do its job.
The change of the circuit should be easy. The monitoring input of the comparator would have to be disconnected from the +B1 terminal and be connected to the +B4 terminal. The actual comparator may have to be exchanged against another type which can accept 16 V or more.

Summary: the actual design of the active balancer invites you to charge at voltages far below the normal 3.6 V +/- cell voltage, required by normal passive balancers.
Low end of charge voltages like 13.9 V total increase the life expectancy of the cells.
Unfortunately with a low charging voltage, single cell monitoring for on/off control of the balancer is not reliable to turn on the balancer when needed.
Battery voltage monitoring however will resolve this shortcoming.
Hope that is reason enough for Hankzor to think about su h an improved design.
Regards Hans

 

[Hans Kroeger]

I guess, many of the above questions and comments will be answered by the following chart:

 

[Horsefly]

I guess, many of the above questions and comments will be answered by the following chart:

Where is this from?

 

[Vi s]

I guess, many of the above questions and comments will be answered by the following chart:

Many thanks for this chart!
If i read this chart correctly then even at 0.04C i could charge it till 3.65V without overcharging it. Since i stay below 3.65V my cells should be fine.

Also another takeaway is (if i understood correctly) if i would charge CC with say 0.01C i could overcharge the cells given i do this long enough. If i would charge CC till 3.45V with 0.5C and then switch to CV and stop at 0.1C i could never overcharge them but would come close to if not hit 100%soc.

 

[Vi s]

Max,
With single cell monitoring the risk is much higher that the balancer is not turned on at the end of charging with a low voltage.
Example: charging with 13.9 V, cells 1 to 4 may be at 3.35 + 3.50 + 3.50 + 3.55 V. The balancer will stay off.
You can play around with theses numbers but you will end up with the same result.
Arguing from a different point of view: when charging wth 13.9 Volts it is safe to turn on the balancer if the total battery voltage goes above 13.5 Volt (or 13.6 V) no matter what the individual cell voltages are. Turning the balancer off at 13.4 V (or 13.3 V) leaves ample margin for the balancer to do its job.
The change of the circuit should be easy. The monitoring input of the comparator would have to be disconnected from the +B1 terminal and be connected to the +B4 terminal. The actual comparator may have to be exchanged against another type which can accept 16 V or more.

Summary: the actual design of the active balancer invites you to charge at voltages far below the normal 3.6 V +/- cell voltage, required by normal passive balancers.
Low end of charge voltages like 13.9 V total increase the life expectancy of the cells.
Unfortunately with a low charging voltage, single cell monitoring for on/off control of the balancer is not reliable to turn on the balancer when needed.
Battery voltage monitoring however will resolve this shortcoming.
Hope that is reason enough for Hankzor to think about su h an improved design.
Regards Hans

Got your point, will convey your suggestion and reasons to her.
Regards Max