Do I have to top balance if cells are all the same voltage?

[Rob99]

Just had delivery of 32 Eve 304Ah cells to build into two Seplos 280L kits. Have unpacked the cells and checked voltage on each to identify any differences and find that every one is 3.3v
Am I right in thinking that they don't need top balancing as they are all the same voltage and the inverter and BMS will just charge them when connected?
I have been through loads of threads on here and through the top balancing tutorials and videos but they all seem to focus on having cells out of balance.
TIA

 

[MisterSandals]

Am I right in thinking that they don't need top balancing as they are all the same voltage and the inverter and BMS will just charge them when connected?

Some say yes, some say no. If your cells are close enough to each other in capacity and internal resistance, your BMS may balance them out as they pass the upper knee.
If not, then top balancing will help.

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[Rednecktek]

If you have the equipment and time (bench charger, wire, heats) then it's worth it just to "zero out" your cells. It'll go faster than letting the BMS do it, especially when it can only move like 2a on a 300a cell pack which takes forever.

If you don't have the equipment already, just put your pack together and leave it on the charger a looonnngg time and the BMS will eventually top balance the cells for you.

I like to make sure all my cells are balanced before I put the battery into service, but I've done it both ways myself. Doing it via the bench charger took overnight but letting the BMS do it was almost a week just for reference.

 

[Rob99]

If you have the equipment and time (bench charger, wire, heats) then it's worth it just to "zero out" your cells. It'll go faster than letting the BMS do it, especially when it can only move like 2a on a 300a cell pack which takes forever.

If you don't have the equipment already, just put your pack together and leave it on the charger a looonnngg time and the BMS will eventually top balance the cells for you.

I like to make sure all my cells are balanced before I put the battery into service, but I've done it both ways myself. Doing it via the bench charger took overnight but letting the BMS do it was almost a week just for reference.

Not sure what you mean by "zero out" the cells?

 

[houseofancients]

Just had delivery of 32 Eve 304Ah cells to build into two Seplos 280L kits. Have unpacked the cells and checked voltage on each to identify any differences and find that every one is 3.3v
Am I right in thinking that they don't need top balancing as they are all the same voltage and the inverter and BMS will just charge them when connected?
I have been through loads of threads on here and through the top balancing tutorials and videos but they all seem to focus on having cells out of balance.
TIA

Voltage says nothing about state of charge below 3.48v .
Your seplos has a passive balancer, and will not be able to handle huge soc differences within a reasonable timeframe..
Go to the resource section and top balance, or add a neey 4a smart balancer

 

[Quattrohead]

I say build them and run them unless you're looking for the last 5Ah worth of capacity out of them. No point in flogging them between 0 and 100%....20-95% or 50-55v is easier on them.

 

[houseofancients]

I say build them and run them unless you're looking for the last 5Ah worth of capacity out of them. No point in flogging them between 0 and 100%....20-95% or 50-55v is easier on them.

Correct, but the is a self learning BMS...
So if you teach it that, due to imbalanced cells the capacity is only 50-60% of what it should be, the BMS will only allow 10-90 % of that skewed capacity..
Not to mention the extra strain being put on the highest SOC cells

 

[Quattrohead]

True, the Seplos BMS comes with some bad defaults that need to be changed.
Always worth looking at the BMS data again after a few cycles to see how the cells have settled in and spot poor connections etc.

 

[houseofancients]

True, the Seplos BMS comes with some bad defaults that need to be changed.
Always worth looking at the BMS data again after a few cycles to see how the cells have settled in and spot poor connections etc.

Not a matter of bad defaults at all..
Usually on change the charge voltage from 57,6 to 56,8 because I pref it.
Otherwise the defaults are perfectly fine, no matter what some trying and convince you to because the get better kickbacks...

Thing is, it is a all in the proper prep...
So when building these packs, make sure you top balance, and after that calibrate the BMS properly as stated in the manual...
Full charge till BMS cut off right after full discharge untill BMS cut off, then charge for production use

 

[TomC4306]

every one is 3.3v

With this chemistry, voltage is not indicative of state of charge. 3.3 volts could be anywhere from 50 to 80% state of charge.
You will only see imbalance at a higher state of charge, as previously stated, above 3.45 or so.

 

[Quattrohead]

3.3 volts could be anywhere from 50 to 80% state of charge

Right, but you can deduce a general idea.

You will only see imbalance at a higher state of charge, as previously stated, above 3.45 or so

Absolutely or below 3.1/3.2 or so.

 

[JJJJ]

Mine were all 3.32 when I got them. Voltage on these is very flat and misleading. There are those who hook them up to an active balancer and let go to work. Others still top balance to get them on equal footing before proceeding

 

[CoolWill]

I have no practical experience with these batteries but I was looking at the charge/voltage graph. It seems to me that in the flat section of the curve, the voltage would be unchanged across a wide range of charge. So I think I'd balance them just in case.

 

[Rednecktek]

Not sure what you mean by "zero out" the cells?

When I say zero out, it's a mechanic's term. In a car it would be knowing that the oil is new, the bearings are new, the fluid is new, etc. You know when you drive that car away everything is as pristine factory new as possible.

Same thing with batteries. When you zero out your cells you KNOW that they're all balanced, fully charged, everything is perfect. Zero'd out.

 

[Rob99]

If you have the equipment and time (bench charger, wire, heats) then it's worth it just to "zero out" your cells. It'll go faster than letting the BMS do it, especially when it can only move like 2a on a 300a cell pack which takes forever.

I have a 32v/10A CC/CV bench power supply so presume that will be OK to use for top balancing.

Voltage says nothing about state of charge below 3.48v .

With this chemistry, voltage is not indicative of state of charge. 3.3 volts could be anywhere from 50 to 80% state of charge.

Ah, I had no idea of that. Everyday's a school day.

When I say zero out, it's a mechanic's term. In a car it would be knowing that the oil is new, the bearings are new, the fluid is new, etc. You know when you drive that car away everything is as pristine factory new as possible.

Same thing with batteries. When you zero out your cells you KNOW that they're all balanced, fully charged, everything is perfect. Zero'd out.

Yeah, that makes sense to me when you put it like that.

No point in flogging them between 0 and 100%

I'm probably going to run them 20-90% to start with.


For top balancing is it better to do individual cells or to hook them up into batches of 4 or 8?
The top balance tutorial suggests 4 in parallel but I've also read that parallel charging isn't necessarily the best thing.
Could I connect 4 or 8 in series and charge with either 14.6v or 29.2v? Will that risk one or more cells overcharging?
Time wise I guess parallel/series would take the same time to do all 32.

 

[houseofancients]

I have a 32v/10A CC/CV bench power supply so presume that will be OK to use for top balancing.



Ah, I had no idea of that. Everyday's a school day.


Yeah, that makes sense to me when you put it like that.


I'm probably going to run them 20-90% to start with.


For top balancing is it better to do individual cells or to hook them up into batches of 4 or 8?
The top balance tutorial suggests 4 in parallel but I've also read that parallel charging isn't necessarily the best thing.
Could I connect 4 or 8 in series and charge with either 14.6v or 29.2v? Will that risk one or more cells overcharging?
Time wise I guess parallel/series would take the same time to do all 32.

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

 

[Rob99]

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

I read through the tutorial yesterday and seems pretty straightforward.
In section 3 it talks about hooking the cells up to a BMS in series so that it can monitor cell voltage to avoid any cell overcharging.
I don't have a bms I can hook them up to so could I charge them in packs of 8 in series at 28.4v (3.55v per cell) and just keep checking individual cell voltages?

 

[Quattrohead]

No you should not. Once a cell is full the voltage will rise rapidly and you probably will not catch it in time. It will swell and be damaged.
YOU MUST USE A BMS UNDER ALL CIRCUMSTANCES.

 

[Rob99]

No you should not. Once a cell is full the voltage will rise rapidly and you probably will not catch it in time. It will swell and be damaged.
YOU MUST USE A BMS UNDER ALL CIRCUMSTANCES.

Ok, I get that a BMS is essential if charging more than 1 cell.
As I don't have a seperate BMS (or larger power supply) my only option I think at this stage is to build the battery system inc BMS and to then charge the whole pack through my inverter, and then see if there are any imbalances at that stage.

 

[houseofancients]

I read through the tutorial yesterday and seems pretty straightforward.
In section 3 it talks about hooking the cells up to a BMS in series so that it can monitor cell voltage to avoid any cell overcharging.
I don't have a bms I can hook them up to so could I charge them in packs of 8 in series at 28.4v (3.55v per cell) and just keep checking individual cell voltages?

Why not simply use the seplos and leave cells on case ?
Disconnect BMS leads
Charge individual cells until 3.6 until 1a.
Connect BMS after nr 16 is done

The rest of balancing can be handled by the seplos