Top balancing charger question

[shawnm]

I’m top balancing my 24s pack using my new Longwei DC power supply and my amperage number jumps around and doesn’t drop. All the cells were at 3.2 before I started. Could this be a charger issue? It’s new and I haven’t used it before. Also, can I use a Battery Tender (I use these to charge up an maintain my 12v lead acid batteries) to assemble the cells in 4 packs to get these to a charged state and then top balance? I one of the Will Prowse videos he recommends this but I don’t know if the Battery Tender is compatible with LiFePo4. Thanks

 

[sunshine_eggo]

Please read the top balancing guide in the Resources section from beginning to end, and you will have everything figured out.

 

[Browneye]

You might also read thru the main 'top balance' thread here as well:

Top Balancing "How to"

For current best practices on top balancing cells, please watch the video and download the PDF at the following page: Top Balancing Cells Using a Low Cost Benchtop Power Supply If you are interested in a more general tutorial of what balancing is about and *Why* to balance, check this out...

diysolarforum.com

Not sure why your amperage output would 'jump around'. Current should taper off as the cells reach full charge.

I got the same brand power supply, next model down with 2 display lines and no usb port. 10A - 30V adjustable. Worked just fine. $59

But parallel charging can take a LONG time depending on charge state - whatever your cell capacity is you multiply that by the number of cells in parallel. My four took 4-5 days. Mine sat at that 5A current level for several days - notice I have volts set to 3.4V. I thought I would do this in steps, or stages - takes way too long. I finally reset the voltage to the full 3.65V, with a DMM, not the unit, and let the cells fully charge to zero amps. Set power supply voltage THEN connect and turn on - don't adjust voltage while it's charging.

To speed up the process you can connect them in series to 12V for regular charging, to get them closer to full charge. A BMS is required to make sure you don't over charge any cells. If you can find the specs on the Battery Tender charger then you'll know if it will work - usually those are a fixed 14.4V or so on the absorption charging stage. But I would be surprised if it is more than 10A as well. So yeah, charge them up to 3.5-3.6V per cell, then rewire in series and do your top balance at 3.65V per cell.

Your power supply will also charge them at 12V nominal, so that's another option. You just need to watch them and stop charging when they reach full charge - if any cells reach over-volt your bms will shut down charging. You might shoot for 3.4 or 3.5 with series charging, then re-wire to parallel to top-balance.

It is recommended to make up new charge leads with 10G wire and ring terminals for your power supply and to the battery. The supplied wire with banana plugs and tiny alligator clips creates a lot of resistance at 10A.

Mine sat like this for 3 days and I thought something was wrong. LOL 230A @30% SOC X 4 = 644Ah. At 8A average that's 80 hours, three and a half days.

 

[shawnm]

You might also read thru the main 'top balance' thread here as well:

Top Balancing "How to"

For current best practices on top balancing cells, please watch the video and download the PDF at the following page: Top Balancing Cells Using a Low Cost Benchtop Power Supply If you are interested in a more general tutorial of what balancing is about and *Why* to balance, check this out...

diysolarforum.com

Not sure why your amperage output would 'jump around'. Current should taper off as the cells reach full charge.

I got the same brand power supply, next model down with 2 display lines and no usb port. 10A - 30V adjustable. Worked just fine. $59

But parallel charging can take a LONG time depending on charge state - whatever your cell capacity is you multiply that by the number of cells in parallel. My four took 4-5 days. Mine sat at that 5A current level for several days - notice I have volts set to 3.4V. I thought I would do this in steps, or stages - takes way too long. I finally reset the voltage to the full 3.65V, with a DMM, not the unit, and let the cells fully charge to zero amps. Set power supply voltage THEN connect and turn on - don't adjust voltage while it's charging.

To speed up the process you can connect them in series to 12V for regular charging, to get them closer to full charge. A BMS is required to make sure you don't over charge any cells. If you can find the specs on the Battery Tender charger then you'll know if it will work - usually those are a fixed 14.4V or so on the absorption charging stage. But I would be surprised if it is more than 10A as well. So yeah, charge them up to 3.5-3.6V per cell, then rewire in series and do your top balance at 3.65V per cell.

Your power supply will also charge them at 12V nominal, so that's another option. You just need to watch them and stop charging when they reach full charge - if any cells reach over-volt your bms will shut down charging. You might shoot for 3.4 or 3.5 with series charging, then re-wire to parallel to top-balance.

It is recommended to make up new charge leads with 10G wire and ring terminals for your power supply and to the battery. The supplied wire with banana plugs and tiny alligator clips creates a lot of resistance at 10A.

Mine sat like this for 3 days and I thought something was wrong. LOL 230A @30% SOC X 4 = 644Ah. At 8A average that's 80 hours, three and a half days.

Thanks for this info. I also read the top balancer tutorial. The question I have and the tutorial doesn't answer is can I actually start with 3.25 v cells and get them up to 3.65 without making a 12v DIY battery and buying a BMS to add to it? I don't have a need for a 4 cell BMS so I'd prefer not to get one.

I also learned that my BMS does not perform as advertised! It only puts out 5 amps max. I'm going to get one that does put out 10 amps.

thanks again for your help.

Shawn

 

[HRTKD]

I don't think my current jumped around. I'm used the same power supply when I top balanced a set of eight 280 Ah cells.

One recommendation that is often made is to use bigger cables to improve the current.

 

[sunshine_eggo]

Thanks for this info. I also read the top balancer tutorial. The question I have and the tutorial doesn't answer is can I actually start with 3.25 v cells and get them up to 3.65 without making a 12v DIY battery and buying a BMS to add to it? I don't have a need for a 4 cell BMS so I'd prefer not to get one.

The guide's recommendation for series charging is merely to expedite the charging. Charging the cells in series @ 10A dramatically cuts the total charge time. MANY people have top balanced parallel only.

I also learned that my BMS does not perform as advertised! It only puts out 5 amps max. I'm going to get one that does put out 10 amps.

This makes no sense unless you are constructing the teeniest of batteries. Please clarify.

 

[Browneye]

I think he means his power supply - it's a 5A version, not a 10. The 5A will work, it's just really slow.

I would not want to series-charge expensive lifepo cells without a bms as it is too easy to cook one.
I see you have another thread of the same topic elsewhere. And that these are 78A cells, 24 of them.

My experience is with 4S 12V, so I'm going to defer to the other experts here. With that many cells you have a lot of amp-hours to fill them to where you can do top-balancing.

Why 72V pack? What is the application? What do you plan to charge them with once the bank is assembled? I'm not familiar with a 24S BMS, so you must be going to build smaller groups and parallel them. I would build those packs and charge them with your power supply to get close to full, THEN rewire them parallel - you can leave them in your bank - just wire them to parallel balance. Once they're near full charge they'll top up pretty fast. You don't need bus bars or heavy wire for top-balancing - you're using very low current. You just want to get them all to the same voltage/SOC at 100% full charge.

Even doing this many report they still need to fine tune cells to get them all equal. I had two of my four that took quite a bit of bleeding off to match the lower ones until they all came into balance at various states of charge. So some advocate actually doing that - build your bank and use an active balancer, or charge or bleed off individual cells.

 

[Browneye]

Here's the other thread - looks like everything got answered.

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

You should pass this information on to the manufacturers of millions of battery packs that have not been parallel top balanced. Perhaps "proper" was not the best choice of words. I think it is an optimal way to top balance for DIYers but by no means the only way. Whatever the manufacturers do...

diysolarforum.com

Why a new thread here?

 

[hayesd601]

what i would like to know, is there some reason all cells cannot be charged at same time independently? seems like the idea is to get all cells to same voltage, and going dedicated cell charging would be best. that way each cell accepts what is needed for that cell.

 

[Browneye]

The concept here is 'balance'. Doing them one at a time doesn't balance them. Conceivably they should all be similar by the time you assemble
the battery, but getting them all there together is the key. I was able to get my four all to the same 3.65V to zero current in parallel. It took a few days.

It's not unusual to have to do some top balancing once the battery is built. You can do that with individual charging or discharging in the built pack.

Tell us why you don't want to follow the good practices already established.

Maybe there are some pointers you can use here in the Overkill BMS battery build manual:

Support/Downloads

Downloads User Manual: Download Manual (6.5mb PDF) Now updated for the 16s BMS! iPhone App: xiaoxiang-bms Android App: version 3.1.1026 (allow 3rd party app to install) New Desktop App with passwor…

overkillsolar.com

 

[sunshine_eggo]

I have sufficient chargers to individually charge multiple cells at 20A. I did this on my Eve 280Ah cells - 9 cells, 9 chargers, 20A each.

Worked fine.

 

[hayesd601]

The concept here is 'balance'. Doing them one at a time doesn't balance them. Conceivably they should all be similar by the time you assemble
the battery, but getting them all there together is the key. I was able to get my four all to the same 3.65V to zero current in parallel. It took a few days.

It's not unusual to have to do some top balancing once the battery is built. You can do that with individual charging or discharging in the built pack.

Tell us why you don't want to follow the good practices already established.

Maybe there are some pointers you can use here in the Overkill BMS battery build manual:

Support/Downloads

Downloads User Manual: Download Manual (6.5mb PDF) Now updated for the 16s BMS! iPhone App: xiaoxiang-bms Android App: version 3.1.1026 (allow 3rd party app to install) New Desktop App with passwor…

overkillsolar.com

I see the advantage to balancing when first building. I was referring more to later when in actual usage. I would think charging each cell at its optimum condition would be preferable to an approach like the BMS does. The BMS does provide a simple and economical way of charging a bank, but the idea of shunting and wasting power just rubs me the wrong way. As cells age, their charging curves will vary. By using a dedicated CV/CC charger to each cell eliminates that issue. Of course, isolated chargers would be needed, hence more complicated and spendy power source. However, the voltage drop from source to each cell would be massive so would have plenty of wiggle room. Thinking solar panel here. As sunlight goes down, the voltage from panels goes down below the needed overhead for charging a series configured setup. By going to single cell, the source voltage can get to 5 or so and still be charging, if needed. If enough panels are used, not even a need for complex power source, just have a dedicated 'set' go to a certain cell.

 

[Browneye]

I see the advantage to balancing when first building. I was referring more to later when in actual usage. I would think charging each cell at its optimum condition would be preferable to an approach like the BMS does. The BMS does provide a simple and economical way of charging a bank, but the idea of shunting and wasting power just rubs me the wrong way. As cells age, their charging curves will vary. By using a dedicated CV/CC charger to each cell eliminates that issue. Of course, isolated chargers would be needed, hence more complicated and spendy power source. However, the voltage drop from source to each cell would be massive so would have plenty of wiggle room. Thinking solar panel here. As sunlight goes down, the voltage from panels goes down below the needed overhead for charging a series configured setup. By going to single cell, the source voltage can get to 5 or so and still be charging, if needed. If enough panels are used, not even a need for complex power source, just have a dedicated 'set' go to a certain cell.

I have a sub-$100 solar charge controller that works just fine keeping the battery charged. (Tracer Xtra)
Some minor balancing is done via bms, but none of them have a very powerful tool to do that - most are half an amp or so clipping of one cell if it gets 5 millivolts or so above above the others. If your cells are further out of balance than that then some manual balancing might make sense, or apply an active balancer if you have a lot of cells.

My system is installed in a motorhome, and it's quite basic, so I just haven't experienced this scenario.

And I surely wouldn't buy 9 separate battery chargers to charge individual cells. Not even once.

 

[hayesd601]

I have a sub-$100 solar charge controller that works just fine keeping the battery charged. (Tracer Xtra)
Some minor balancing is done via bms, but none of them have a very powerful tool to do that - most are half an amp or so clipping of one cell if it gets 5 millivolts or so above above the others. If your cells are further out of balance than that then some manual balancing might make sense, or apply an active balancer if you have a lot of cells.

My system is installed in a motorhome, and it's quite basic, so I just haven't experienced this scenario.

And I surely wouldn't buy 9 separate battery chargers to charge individual cells. Not even once.

I would not buy them either. I would design a single unit that would provide isolated power, then run something like an enhanced XC6802 on each.

 

[740GLE]

How much power do you think a BMS or shunt wastes?

 

[timselectric]

Once placed in service as a single battery. It should be charged and discharged as a single battery.
Charging the cells separately, will throw the entire battery out of balance.
Good luck....