diy solar

diy solar

Top Balancing "How to"

This is normal.
Okay. Even if there's nothing plugged into it? In this case it becomes very difficult to balance them !! ?? When I think that when I received them they had 0.01v difference ... I should have connected in series !!! At what voltage can we estimate that it is normal 3.55v? Thank you all for the help you give me, I'm so afraid of doing anything stupid !!!!!
 
Ben, ease up on the exclamation points!!! :)

No, just because the voltage differential was miniscule, that doesn't mean the cells were closely balanced already. The Top Balance method is the safe way to go.
 
Even if there's nothing plugged into it? I

Yes.

In this case it becomes very difficult to balance them !!

No, at that stage they are balanced. You can do this for each cell, charge to 3.6V, wait for the current to settle, repeat with other cells. They are all balanced after that even if the voltage drops.
 
When I think that when I received them they had 0.01v difference ... I should have connected in series !!!

Stop looking at the voltage. Again, repeat after me: stop looking at the voltage. Voltage is not an indicator for anything outside the knees.

Look at the graph (for a 12V battery, but doesn't matter):

xLiFePO4-charging.gif.pagespeed.ic.ZYJX_3h8SJ.webp


You can not determine the state of charge by looking at the voltage when this sits at 3.3V per cell. It's flat. Once cell could be at 30%, the other at 60% and you wouldn't know. This is what balancing is about: bring them to the same state of charge. The only way to do that is to either completely charge them to within the upper knee (top balance), or discharge to the lower knee (bottom balance).

In our case we opt to top balance because cells will spend most of their time there, but I digress. In any case, you can see in the graph that the only place where you can use voltage and current to determine state of charge is in those knees. Now, if you charge each cell to 3.6V or thereabout, you can see that the current will taper off and the cell is 'full'. Self discharge is negligible. Do this with each cell (or all at once in parallel) and you can know for sure each cell is 'full' - i.e., balanced. Just because the voltage drops a little (which is normal for the chemistry) does not mean that the cell is somehow 'less full'.
 

Hi I just received my 32 eve cells and want to top balance. I have read the pdf and watched wills video, I just have one question. If I am using the step by step method and only using a power supply could I set the power supply voltage to the highest voltage (in my case 32v) which will allow for the PS max current of 10.10amps. I would obviously keep a close eye and monitor cells with multimeter at the terminal heads. Once I reach 3.4 at the cells I would then reduce the voltage to a safer 3.65. The reason being even if I set this ps to 3.65v it is only charging at 3.2amps. Currently my cells are 3.304 and at that rate for 32 cells it may never finish. I do have the ability to set up with in series, with a 16s Daly bms as stated in the pdf where I get up to almost full charge. If this sounds crazy to you all please let me know, I just had my reason as to why this would be simpler but can definitely go the pdf route as posted by snooker.

 

Hi I just received my 32 eve cells and want to top balance. I have read the pdf and watched wills video, I just have one question. If I am using the step by step method and only using a power supply could I set the power supply voltage to the highest voltage (in my case 32v) which will allow for the PS max current of 10.10amps. I would obviously keep a close eye and monitor cells with multimeter at the terminal heads. Once I reach 3.4 at the cells I would then reduce the voltage to a safer 3.65. The reason being even if I set this ps to 3.65v it is only charging at 3.2amps. Currently my cells are 3.304 and at that rate for 32 cells it may never finish. I do have the ability to set up with in series, with a 16s Daly bms as stated in the pdf where I get up to almost full charge. If this sounds crazy to you all please let me know, I just had my reason as to why this would be simpler but can definitely go the pdf route as posted by snooker.


Yeah, that's crazy. Don't do it. Patience is critical. 32v would likely damage 3.2v cells in parallel.
 

Hi I just received my 32 eve cells and want to top balance. I have read the pdf and watched wills video, I just have one question. If I am using the step by step method and only using a power supply could I set the power supply voltage to the highest voltage (in my case 32v) which will allow for the PS max current of 10.10amps. I would obviously keep a close eye and monitor cells with multimeter at the terminal heads. Once I reach 3.4 at the cells I would then reduce the voltage to a safer 3.65. The reason being even if I set this ps to 3.65v it is only charging at 3.2amps. Currently my cells are 3.304 and at that rate for 32 cells it may never finish. I do have the ability to set up with in series, with a 16s Daly bms as stated in the pdf where I get up to almost full charge. If this sounds crazy to you all please let me know, I just had my reason as to why this would be simpler but can definitely go the pdf route as posted by snooker.

Sounds like you might want to invest in some real cables instead of the alligator clips that come with the supply. I don't have any problems pushing full amps, but I use 10 gauge wire and crimp ring terminals and never have a problem. Charging in series with a BMS is a good idea, what will you charge them with in "production". A 30v power supply won't charge 16 cells in series.
 

Hi I just received my 32 eve cells and want to top balance. I have read the pdf and watched wills video, I just have one question. If I am using the step by step method and only using a power supply could I set the power supply voltage to the highest voltage (in my case 32v) which will allow for the PS max current of 10.10amps. I would obviously keep a close eye and monitor cells with multimeter at the terminal heads. Once I reach 3.4 at the cells I would then reduce the voltage to a safer 3.65. The reason being even if I set this ps to 3.65v it is only charging at 3.2amps. Currently my cells are 3.304 and at that rate for 32 cells it may never finish. I do have the ability to set up with in series, with a 16s Daly bms as stated in the pdf where I get up to almost full charge. If this sounds crazy to you all please let me know, I just had my reason as to why this would be simpler but can definitely go the pdf route as posted by snooker.

I think the cables that came with your PS might be crap and there are many posts confirming the same problem you have.

Get some bigger cables (at least 12 AWG) and ring terminals. Your power supply should be able to charge with the voltage set to 3.65 volts and be able to output the full 10 amps, or very close to it. Is it in CC or CV mode? It should be in CC mode. Also it's important to set the voltage of the PS before connecting it to your cells.

I tried the step method and it didn't work for me. Any voltage under 3.5 volts and the current was too low. So I went straight to 3.65 volts and waited 4+ days for my 8 EVE cells to parallel top balance using 12 amps.

Since you have 32 cells it would take 16+ days to parallel top balance them.
 
Thanks for the replies, I am just about to make my own cords for my ps with 10 gauge and ring terminals I will let you know.
 
I came across this post which had me concerned about all of us using low-current (~10A, typically) chargers:


Specifically: "I can't stress this enough: you cannot determine state of charge (SOG) by measuring the charging voltage alone. At low charging current (like the current you are using) you can seriously overcharge a LiFePO4 battery by applying any voltage over 3.4 volts per cell for too long."

I don't know if this person is correct or if any battery manufacturer even specifies these sorts of things at low charge currents. The battery manufacturers are probably assuming industrial users will have bigger chargers that can deliver hundreds of amps to a battery pack like the 4x or 8x 280Ah ones most people are assembling on this forum. That other forum's post also talks about "springback", basically saying the higher the charge current the larger the spring back, so to reach an equivalent resting voltage you need to set a higher stopping voltage if you're using a higher charge current. For a lower charge current you would not want to go all the way to 3.65V, because there is less spring back by that logic, if it risks damaging the cells.

Does anyone have any evidence on whether this is true and if we should either:
1) use higher-current chargers (well obviously we should ideally try to use 0.1C like EVE recommends)
or
2) use 3.35V resting voltage as a gauge of fullness rather than charge current diminishing at 3.65V?

Figure 1 reproduced here of the EVE documentation (http://www.dcmax.com.tw/LF280(3.2V280Ah).pdf) suggests any resting voltage over about 3.31V is on the "vertical" part of the curve and is essentially fully charged. Granted, that plot only goes down to 0.1C discharge, not 0.0C, but we can extrapolate and guess that at 0.0C (open circuit), it's just another step higher like 0.1C to 0.2C is a step down. I'm leaning towards the "charge then rest" method as I don't have a particularly high-current charger (6A).

1618470150000.png
Table 1 of the EVE documentation says for room temperatures (10-45degC) "charge to 3.65V cutoff with the current of 0.5C", but of course for an 8x280Ah battery, this would be 1120 Amps (right?) which is absurdly high.
 
Last edited by a moderator:
Does anyone have any evidence on whether this is true and if we should either:
This has been discussed on this forum and maybe this thread. Parallel top balancing is usually done only one time and that's it. In my opinion there is no damage to the battery charging at low C rates and there has been no reported damage providing it's done properly. I used a 12 amp charger and charged to 3.65 volts and the charge terminated with a tail current cut off of 100ma's. Once the cells reach 3.4 volts or so, the voltage rises rapidly so the cells are at a high SOC for a very short time.

The poster, Cpt Pat, links to Nordkyn. Nordkyn states: The tell-tale sign of a fully charged (or overcharged) battery is that it is no longer able of absorbing any significant current, or even any current at all.

That statement is good enough for me. Also Cpt Pat did not link to Nordkyn's material relating to parallel top balancing.


And following are his steps to parallel top balance although I personally don't care for the step method:
  1. With the output disconnected, set the voltage regulation limit at 3.40-3.45V and preset the current limit (if any) to a value that won’t overload the PSU. Refer to the manual as required. In doubt, always start with a low current limit and never exceed 80% of the rated output.
  2. With all the cells wired in parallel, connect the PSU, bulk charge and absorb until no current flows any more. The voltage will stay around 3.3V for a very long time before starting to rise. Charging this way can take several days. This will near-fully charge the cells without stressing them unduly, but don’t hold them at that voltage indefinitely. Keep checking up on them at least a couple of times each day. Briefly disconnect the cells and recheck the voltage limit setting on the PSU: better safe than sorry. Avoid charging the cells individually, or in batches; the whole process would take just as long, but would also result in some fully charged cells lying around for several days.
  3. Once the voltage has reached the PSU output regulation limit and there is no apparent charging current any more, disconnect the cells from the PSU and increase the output voltage regulation limit to 3.60V.
  4. Then, while standing by only, reconnect the cells and allow the voltage to rise up to 3.60V and stabilise for a few minutes; this normally takes little time and additional current, provided the cells were fully absorbed at the lower voltage. Whether you target 3.60V, 3.65V or even 3.70V is of no consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart only.
  5. Disconnect the PSU from the cells again and wait. The cells should hold above 3.50V for at least 30 minutes. If not, bring them up again and hold them for a little longer until they do. At 3.60V, you may need to insist a little more than if using 3.65 or 3.70 volts; that’s all.
NOTE: He does not give any recommendation for a power supply. He only says you need an adjustable, regulated power supply unit to follow this process.

I would not get into the weeds concerning everything posted regarding these cells. We know charging above 3.65 and discharging below 2.50 volts is bad. We know using higher C rates than the manufacturers recommend is bad. We know temps can effect the cells. There are a lot of things we know. But there is also a lot of data missing and much of the data that's out there is old. That's my own personal opinion. Others will have their opinions and that's fine.
 
I came across this post which had me concerned about all of us using low-current (~10A, typically) chargers:


Specifically: "I can't stress this enough: you cannot determine state of charge (SOG) by measuring the charging voltage alone. At low charging current (like the current you are using) you can seriously overcharge a LiFePO4 battery by applying any voltage over 3.4 volts per cell for too long."

I don't know if this person is correct or if any battery manufacturer even specifies these sorts of things at low charge currents. The battery manufacturers are probably assuming industrial users will have bigger chargers that can deliver hundreds of amps to a battery pack like the 4x or 8x 280Ah ones most people are assembling on this forum. That other forum's post also talks about "springback", basically saying the higher the charge current the larger the spring back, so to reach an equivalent resting voltage you need to set a higher stopping voltage if you're using a higher charge current. For a lower charge current you would not want to go all the way to 3.65V, because there is less spring back by that logic, if it risks damaging the cells.

Does anyone have any evidence on whether this is true and if we should either:
1) use higher-current chargers (well obviously we should ideally try to use 0.1C like EVE recommends)
or
2) use 3.35V resting voltage as a gauge of fullness rather than charge current diminishing at 3.65V?

Figure 1 reproduced here of the EVE documentation (http://www.dcmax.com.tw/LF280(3.2V280Ah).pdf) suggests any resting voltage over about 3.31V is on the "vertical" part of the curve and is essentially fully charged. Granted, that plot only goes down to 0.1C discharge, not 0.0C, but we can extrapolate and guess that at 0.0C (open circuit), it's just another step higher like 0.1C to 0.2C is a step down. I'm leaning towards the "charge then rest" method as I don't have a particularly high-current charger (6A).

View attachment 45198
Table 1 of the EVE documentation says for room temperatures (10-45degC) "charge to 3.65V cutoff with the current of 0.5C", but of course for an 8x280Ah battery, this would be 1120 Amps (right?) which is absurdly high.
Tekpower makes a nice 40 amp power supply, and I can point you to a 60 amp supply if you like.
 
Can someone recommend a solid power supply for top & bottom balancing? Ive ordered 32 of the catl/310 cells from Dongguan New Lightning.
Any other items I should get placed on order?
 
@ Just John: yes please recommend a 60amp PSU, and also any other items/equipment/cables/connecters Id need to do a proper top or bottom balancing of 32 310ah cells? I currently have a fluke meter, thats it.
 
Last edited:
Can someone recommend a solid power supply for top & bottom balancing? Ive ordered 32 of the catl/310 cells from Dongguan New Lightning.
Any other items I should get placed on order?

Bottom balancing is rarely done.

You can capacity test the cells to see how well they stay together at the end of your discharge cycle, but almost everyone sets their BMS to balance on charge, not discharge.
 
Can someone recommend a solid power supply for top & bottom balancing?
Most power supplies are not designed to absorb large amounts of power like you would need for bottom balancing. For that you should get a dedicated battery tester or a DC load tester. Rigol comes to mind for the latter.
I think you could effectively bottom balance with any load and a shunt monitor though, although I’ve never seen anyone do that for purposes of balancing cells.
 
Back
Top