Can I top balance a 72v pack in three 24v sections and really be balanced?

[shawnm]

I'm building a 72v system for a vehicle application. I watched the videos and digested the wiki (which is great). I have a 30v charger that can do 8 cells in series. the LiFePo4 cells are new and are all at 3.25 right now.

Rather than buy a 72v charger that I will probably never use again, will do it in the 3 chunks do what I want, then I can wire the 24 in series with a BMS and be done.

I'm new to all of this so any help would be greatly appreciated. thanks

 

[FilterGuy]

Bottom line: Yes. Just make sure to top balance them to the same voltage and wait till the top balance current is zero.

In fact, some folks on the forum advocate top balancing one cell at a time. (I am not in that camp, but it can work).

 

[toms]

How are you going to charge them when they are in service?

If you have access to the cell terminals when the battery is in its final configuration, you are better off only assembling it once.

Then series charge until one cell reaches its high voltage cutoff before top charging each cell individually (without disassembling the pack)

If you have assembled the pack, the available capacity is acceptable, and the self discharge of the cells isn’t greater than the balancing ability of the BMS - it is pointless to top balance.

Scroll through the many threads in this forum to see the pitfalls associated with needlessly reassembling your cells.
(mainly accidental shorts / damaged terminals)

 

[shawnm]

How are you going to charge them when they are in service?

If you have access to the cell terminals when the battery is in its final configuration, you are better off only assembling it once.

Then series charge until one cell reaches its high voltage cutoff before top charging each cell individually (without disassembling the pack)

If you have assembled the pack, the available capacity is acceptable, and the self discharge of the cells isn’t greater than the balancing ability of the BMS - it is pointless to top balance.

Scroll through the many threads in this forum to see the pitfalls associated with needlessly reassembling your cells.
(mainly accidental shorts / damaged terminals)

Thanks! Cells are not together yet so I have a lot of flexibility. Once the pack is assembled I will charge it with an onboard 72v charger (I haven't selected that yet, or the BMS). So you are recommending charging each one to 3.6 or 3.65 with one of the Variable DC power supplies you can pick up on Amazon?

I have a question for your about attaching the interconnects- my battery does not have posts, so my options are to use 8mm threaded connections (like this: https://www.boltdepot.com/Metric_set_screws.aspx?nv=rel) and then nylock nuts, or use an 8mm bolt. What would you suggest? I'm leaning towards the threaded connection and nut since I can snug it tight but not so tight that I need a lock washer.

thanks again

 

[time2roll]

Will your 24s BMS work on 8 batteries? Otherwise I would build the full battery and let the BMS balance or at least initially put an active balancer on it for balancing.

 

[fafrd]

Thanks! Cells are not together yet so I have a lot of flexibility. Once the pack is assembled I will charge it with an onboard 72v charger (I haven't selected that yet, or the BMS). So you are recommending charging each one to 3.6 or 3.65 with one of the Variable DC power supplies you can pick up on Amazon?

I have a question for your about attaching the interconnects- my battery does not have posts, so my options are to use 8mm threaded connections (like this: https://www.boltdepot.com/Metric_set_screws.aspx?nv=rel) and then nylock nuts, or use an 8mm bolt. What would you suggest? I'm leaning towards the threaded connection and nut since I can snug it tight but not so tight that I need a lock washer.

thanks again

You should absolutely get grubscrews into your soft aluminum threads as quickly as you can. They strip exceedingly easily…

Loctite Red is the easiest way to secure the grubscrews so they are permanent.

Too long is a better problem to have than too short (think about the maximum number of lugs / busbars you will ever need to secure on one terminal, including any and all sense & monitor leads).

The easiest way to top balance is the connect common bus wires (2 long pieces of copper ground wire will work) and then attach to a bench power supply set to 3.65V.

Once you get the pack charged past 3.5V the task is largely done (though it’s best to wait until the current declines to well under ~10mA per cell.

The most important task if you purchase d these cells from a budget vendor is to capacity test each cell while still within your ‘guarantee’ or ‘complain’ window.

The cheapo 10A / 150W capacity testers are a good way to do that…

 

[fafrd]

Also, not clear what a ‘30V charger’ is but if it is a 28.8V charger suitable for 8S LiFePO4, yes, you can greatly accelerate the balancing time by making an 8S3P o PPI ack and using the 8S charger to get the cells charged up close to full.

Use a BMS and keep an eye on 3P cell voltages.

When one gets up above 3.5V approaching 3.6V, time to disconnect and switch to parallel 24S1P charging to 3.65V using a bench supply.

 

[toms]

Thanks! Cells are not together yet so I have a lot of flexibility. Once the pack is assembled I will charge it with an onboard 72v charger (I haven't selected that yet, or the BMS). So you are recommending charging each one to 3.6 or 3.65 with one of the Variable DC power supplies you can pick up on Amazon?

Yes, loctite studs into your cells.

What i am saying regarding charging is to fully assemble the pack in its current state of charge, including connecting the BMS and any other cell hardware.

Then charge the battery as you would once it is fully commissioned.

If your battery has acceptable capacity, you are done.

If one cell stops the pack charging due to high voltage, then one by one charge the cells to 3.65V / 0.05C (leave the pack assembled, with load disconnected)

I haven’t had to top balance any of the packs of new cells i have assembled.

For the couple of packs i have recently assembled using 2nd hand cells, i have used my existing HVD contactor to allow the BMS to stop charging of each individual cell at the same SOC.

Prior to that i have used an active balancer (on a 4 cell 2nd hand pack) to get all cells to the same SOC.

Parallel top balancing was a bit of a fad brought about by a few demonstrations by high profile youtubers. Given the risks involved and the added complexity of the process i’m really not sure why people recommend it.

 

[shawnm]

Also, not clear what a ‘30V charger’ is but if it is a 28.8V charger suitable for 8S LiFePO4, yes, you can greatly accelerate the balancing time by making an 8S3P o PPI ack and using the 8S charger to get the cells charged up close to full.

Use a BMS and keep an eye on 3P cell voltages.

When one gets up above 3.5V approaching 3.6V, time to disconnect and switch to parallel 24S1P charging to 3.65V using a bench supply.

Hi: Here's an example of the type of charger I was referring to (excuse the long amazon link https://www.amazon.com/dp/B082HXVRN...0a6676b0ac223&language=en_US&ref_=as_li_ss_tl

I never thought of the 8s3p option. Thanks!

 

[shawnm]

Yes, loctite studs into your cells.

What i am saying regarding charging is to fully assemble the pack in its current state of charge, including connecting the BMS and any other cell hardware.

Then charge the battery as you would once it is fully commissioned.

If your battery has acceptable capacity, you are done.

If one cell stops the pack charging due to high voltage, then one by one charge the cells to 3.65V / 0.05C (leave the pack assembled, with load disconnected)

I haven’t had to top balance any of the packs of new cells i have assembled.

For the couple of packs i have recently assembled using 2nd hand cells, i have used my existing HVD contactor to allow the BMS to stop charging of each individual cell at the same SOC.

Prior to that i have used an active balancer (on a 4 cell 2nd hand pack) to get all cells to the same SOC.

Parallel top balancing was a bit of a fad brought about by a few demonstrations by high profile youtubers. Given the risks involved and the added complexity of the process i’m really not sure why people recommend it.

The batteries are new, from a reputable seller, and I tested the voltage on each cell and found them to be 3.25 with one at 3.26 and a few at 3.24. Since I'm new to this I think I understand what you are saying except for the part of "load disconnected". How would I do that if the interconnects are already in place and screwed down to each battery? I just want to make sure I get it right. thanks

 

[toms]

Cells will be in series, you will be charging each cell individually across it’s positive and negative.

The load is whatever the battery is powering (ie in an off-grid system the inverter).

There will be no current flow between cells in a fully assembled series pack unless the main positive and negative terminals (in your case the ones that complete the 72V circuit) have a load attached.

You can individually charge/discharge any cell without affecting the others. (that is how a BMS balancing circuit works).

 

[shawnm]

Cells will be in series, you will be charging each cell individually across it’s positive and negative.

The load is whatever the battery is powering (ie in an off-grid system the inverter).

There will be no current flow between cells in a fully assembled series pack unless the main positive and negative terminals (in your case the ones that complete the 72V circuit) have a load attached.

You can individually charge/discharge any cell without affecting the others. (that is how a BMS balancing circuit works).

got it thanks

 

[fafrd]

Hi: Here's an example of the type of charger I was referring to (excuse the long amazon link https://www.amazon.com/dp/B082HXVRNF?psc=1&pd_rd_i=B082HXVRNF&pd_rd_w=nE9eH&pf_rd_p=4269e1a0-a218-4fbd-9748-1cd337d2f2a5&pd_rd_wg=cfrUI&pf_rd_r=QJPZG8SCYHKQJWAPR1CG&pd_rd_r=411e6c8d-47f8-4662-a1c3-5982f6df86fa&smid=A31MZA1O81S6VM&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyMkFNV0JVTEpaVUpLJmVuY3J5cHRlZElkPUEwMTUxOTk0MjJHRUlaUVNCNVU4MCZlbmNyeXB0ZWRBZElkPUEwNjc3MDY0MUoxT1FUTVNYWjlBOCZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=&linkCode=sl1&tag=vehicledwelling-20&linkId=00c61981536818f71cd0a6676b0ac223&language=en_US&ref_=as_li_ss_tl

I never thought of the 8s3p option. Thanks!

Oh, so you already have a bench supply then. Do yes, by using that supply set to 28V or 28.4V, you can charge an 8S3P battery (though you should really connect an 8S BMS or you risk damaging a 3P pack by overcharging).

 

[Ampster]

Rather than buy a 72v charger that I will probably never use again, will do it in the 3 chunks do what I want, then I can wire the 24 in series with a BMS and be done.

To do a proper top balance the cells need to be parallel. That would be 24 cells in parallel at 3.65 volts. Eight in series won't accomplish much.

 

[fafrd]

The batteries are new, from a reputable seller, and I tested the voltage on each cell and found them to be 3.25 with one at 3.26 and a few at 3.24. Since I'm new to this I think I understand what you are saying except for the part of "load disconnected". How would I do that if the interconnects are already in place and screwed down to each battery? I just want to make sure I get it right. thanks

Voltages at that part of the charge/discharge curve are pretty much meaningless (in the ‘flats’).

As toms has suggested, you can directly build your 24S battery and test capacity at the finished-battery level, but to do this, you need you 24S BMS as well as a 72V charger (SCC or AC-charger).

I know you believe your vendor is ‘reputable’ but especially with voltages that low, I recommend you test your cells for capacity sooner rather than later.

Without a BMS, the only thing you can safely do is parallel balance all of your cells to 3.5 or 3.6V.

If these are 280Ah cells, that’s going to take a very long time.

3.35V corresponds to only ~40% SOC, meaning ~168Ah to charge a 280Ah cell to 100% SOC.

That’s over 4000Ah for all 24 cells which will take your 19A bench supply over 400 hours or 16 days to accomplish.

If you have your 2S BMS by then, you can assemble your 24S 72V battery and run a capacity test at the full-battery level. What you are really looking for is a cell or cells that ‘runs’ down in voltage much more quickly than the the others and this will generally become apparent well before the battery as a whole has discharged below 10% SOC.

Have you ordered your 24S BMS yet?

 

[fafrd]

To do a proper top balance the cells need to be parallel. That would be 24 cells in parallel at 3.65 volts. Eight in series won't accomplish much.

It’ll speed up the process (especially in 8S3P configuration).

Assuming all cells are at ~40% SOC, it’ll take a bit over 50 hours or 2+ days to charge them all into the knee (or at least above 3.5V / 90%) compared to over 400 hours or 14 days to charge all 24 cells in parallel to 3.6V with a 10A charger.

On the one hand, while it’s risky to do that without an 8S BMS, if you are able to check cell voltages at least hourly while charging at ~28.4V, you will see clear signs of acceleration (greater mV increase from 1 hour of charging at 10A) well below 3.4V to stop charging long before there is any danger of an overshoot between checkpoints…

Parallel charging to 3.65V from 94% SOC will take 1/10th as long as parallel charging to 3.65V from 40% SOC .

 

[toms]

To do a proper top balance the cells need to be parallel.

You should pass this information on to the manufacturers of millions of battery packs that have not been parallel top balanced. I’m certain they will heed your advice and change to a “proper” procedure ?

 

[newbostonconst]

Yes, loctite studs into your cells.

What i am saying regarding charging is to fully assemble the pack in its current state of charge, including connecting the BMS and any other cell hardware.

Then charge the battery as you would once it is fully commissioned.

If your battery has acceptable capacity, you are done.

If one cell stops the pack charging due to high voltage, then one by one charge the cells to 3.65V / 0.05C (leave the pack assembled, with load disconnected)

I haven’t had to top balance any of the packs of new cells i have assembled.

For the couple of packs i have recently assembled using 2nd hand cells, i have used my existing HVD contactor to allow the BMS to stop charging of each individual cell at the same SOC.

Prior to that i have used an active balancer (on a 4 cell 2nd hand pack) to get all cells to the same SOC.

Parallel top balancing was a bit of a fad brought about by a few demonstrations by high profile youtubers. Given the risks involved and the added complexity of the process i’m really not sure why people recommend it.

There is Loctite for electrical connections but I don't think regular Loctite on a electrical connection is the right idea. If you have it torqued and the threads are pushed to contact one side of each individual thread you might get away with it. I see problems when it is hard and is an insulator and you only have contact in small area. When current is high you have a problem.

Without a true BMS(active balancers are not BMS's) and not top balancing is going to cause cells to go over voltage while charging and potentially ruin the cells. Take your time and do it right and top balance them all to just over 3.5 volts.

 

[shawnm]

Voltages at that part of the charge/discharge curve are pretty much meaningless (in the ‘flats’).

As toms has suggested, you can directly build your 24S battery and test capacity at the finished-battery level, but to do this, you need you 24S BMS as well as a 72V charger (SCC or AC-charger).

I know you believe your vendor is ‘reputable’ but especially with voltages that low, I recommend you test your cells for capacity sooner rather than later.

Without a BMS, the only thing you can safely do is parallel balance all of your cells to 3.5 or 3.6V.

If these are 280Ah cells, that’s going to take a very long time.

3.35V corresponds to only ~40% SOC, meaning ~168Ah to charge a 280Ah cell to 100% SOC.

That’s over 4000Ah for all 24 cells which will take your 19A bench supply over 400 hours or 16 days to accomplish.

If you have your 2S BMS by then, you can assemble your 24S 72V battery and run a capacity test at the full-battery level. What you are really looking for is a cell or cells that ‘runs’ down in voltage much more quickly than the the others and this will generally become apparent well before the battery as a whole has discharged below 10% SOC.

Have you ordered your 24S BMS yet?

thanks. I haven't ordered the BMS yet. Do you have any suggestions?

 

[Ampster]

I see problems when it is hard and is an insulator and you only have contact in small area. When current is high you have a problem.

I do not expect any current to flow through the stud. The surface of the terminal top and the lug or bus bar is where the current should flow. Stainless steel is not a good conductor anyway. It's resistance will drive most of the current through the lower resistance of the terminal top interface.