Top Balancing "How to"

418ImATeapot

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So at the PS I see about a .03V drop when connected to the ps from the expected voltage. There's about a .3v drop at the battery.
 

Hedges

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I read on here that you should run 3.4 wait for amps to drop to near 0 then progress up to 3.5 then 3.65

If you enjoy watching paint dry and grass grow.
Maybe just wait until it transitions from CC to CV, then progress higher? (disconnect supply from battery when setting for next higher voltage.)

So just did some testing with my volt meter. At the connection on the PS (3.6V, 0A), at the end of the leads not connected to battery (3.6V, 0A), and at the connections with the leads connected to battery (3.3V, ~5A)
I'm surprised the voltage at the leads with them on the battery doesn't show 3.6V?

Is current limit set to 5A or to 10A?
If you disconnect leads from battery (or from supply) and short supply, it should show current limit.

Measure voltage across cells, and voltage across supply terminals.
Measure voltage drop from terminal of supply to terminal of cell.


You have both wires at one end of the batch of parallel cells. That will cause some voltage drop to the far end.
Could be improved (at least marginally) by connecting one wire to far end.
Of greater concern is contact resistance between busbars, and busbars to cell terminals.
Measure voltage between each busbar and the next.
If you can poke a probe in, measure voltage between busbar and cell terminal. At least the positive on one end seems accessible.
One guy parallel charged his cells, and once separated one of them settled lower. Charging it separately brought it up.
Contact resistance will matter more discharging the battery at 100A than charging each cell at 1.5A, but it can still affect results.
Waiting for a reasonably low tail current at 3.65V should help.
 

Gazoo

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@GazzooI think my wires are 14ga
Your power supply is not putting out the full amps it's capable of. I read some reviews of your power supply and there were complaints about the size of the cables.

The photo you posted shows the PS is in CV mode. It should be in CC mode. I bet it's in CV mode when putting out 5 amps.

The plastic that covers the banana plug sockets on your power supply should come off by unscrewing them counter clockwise. If you can take them off then you will be able to use ring terminals instead of the banana plugs.

There is a bit too much voltage drop between the power supply and the connection at your cells because of the banana plugs, the wire awg, or both.
 

Just John

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I think someone mentioned here about using 10ga wire? Should I replace the entire wire, or just put lugs on the wire that came with it? (Yes, using alligator clamps) I got this power supply (https://www.amazon.com/gp/product/B082HXVRNF/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1)
I am not sure what size wire you get with that supply. I started recommending the 10 gauge wire and 1/4 inch crimp ring terminals because they are so widely available. Technically 12 gauge wire is adequate for 10 amps, but bigger is better in this instance (less resistance).
 

Just John

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Also worth noting... The bench power supplies are junk. Yeah, they work. They're 70 dollars, large, and the one I bought immediately blew up when I powered it on.

I ended up top balancing with an ISDT Q8 (70 dollars, 20 amps!) (They also have a 30 amp version for 100 bucks)


Much more current from this device, and it has other features that are handy if you have DIY packs.
I might be wrong, but don't the ISDT chargers also require a power supply to operate? An active balancer (basically what the ISDT accomplishes) might be cheaper. Either solution allows you to charge in series and accomplish a top balance faster, but cost more.
 

MisterSandals

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I might be wrong, but don't the ISDT chargers also require a power supply to operate? An active balancer (basically what the ISDT accomplishes) might be cheaper. Either solution allows you to charge in series and accomplish a top balance faster, but cost more.
Yes the ISDT needs an external power source. I used a 12.8V LiFePO4 battery (which was being solar charged) as the power source.
The ISDT allow you to charge in parallel at 3.65V. I think the charge range is 3V-30V but thats a foggy memory at this point. I have the 30A ISDT, that's pretty good i thought.
 

Horsefly

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The problem with the iSDTs (or probably any hobby charger) is that they seem to charge up to a voltage and stop. My iSDT Q8 would happily charge with only one cell attached (parallel connected for top balance), but as soon as the voltage got to 3.6 (where I was top balancing at the time), it turned off, rather than holding that voltage until the current drops off to some point.
 

Just John

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Yes the ISDT needs an external power source. I used a 12.8V LiFePO4 battery (which was being solar charged) as the power source.
The ISDT allow you to charge in parallel at 3.65V. I think the charge range is 3V-30V but thats a foggy memory at this point. I have the 30A ISDT, that's pretty good i thought.

Yes, after watching @Off-Grid-Garage , I am sold on using an active balancer for top balancing. They are usually cheaper than an ISDT. I had already discovered that leaving them running full time was counterproductive, I am happy he explained it to everyone else. For 24v or less, an active balancer (or ISDT) makes a top balance much easier using the cheap power supply.
 

TWG

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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:

Explanation for Beginners of Top and Bottom Balance

If you're asking yourself if you should top balance your cells, the answer is always: "YES!" If you don't, it's almost certain that even a small imbalance in the cell SoC will cause "runners" at near full charge and will be VERY frustrating.
In Will's video I thought I remembered, he mentioned you can also put four 3.2v cells in series for 12v unit and balance that pack... Did I understand that correctly? That this will is also a way to top balance cells if you do not have a 3.2v source to do the cells in parallel?
 

Just John

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In Will's video I thought I remembered, he mentioned you can also put four 3.2v cells in series for 12v unit and balance that pack... Did I understand that correctly? That this will is also a way to top balance cells if you do not have a 3.2v source to do the cells in parallel?
Did you read the document on top balancing? In particular the explanation of top and bottom balancing?

In series, you need an active balancer or a balancing charger.
The objective of top balancing is to get all cells to the same state of charge. Charging in series charges all cells equally, so if one is at 40%, and the rest are at 60%, when charged in series, you wind up with 3 cells at 100%, and one cell at 80%. It's not a difficult concept.

The documents were created to explain this.
 

TWG

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Did you read the document on top balancing? In particular the explanation of top and bottom balancing?

In series, you need an active balancer or a balancing charger.
The objective of top balancing is to get all cells to the same state of charge. Charging in series charges all cells equally, so if one is at 40%, and the rest are at 60%, when charged in series, you wind up with 3 cells at 100%, and one cell at 80%. It's not a difficult concept.

The documents were created to explain this.
Thank you... I was at work and did not have time to read this.. Looks like I have to get one of those variable DC power supplies... Thank you for your reply...
 

Just John

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Thank you... I was at work and did not have time to read this.. Looks like I have to get one of those variable DC power supplies... Thank you for your reply...
Or you can get an active balancer, or a balance charger. If you are doing a 12v or 24v pack, the power supply does make an excellent 10 amp charger.
 

Horsefly

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Or you can get an active balancer, or a balance charger. If you are doing a 12v or 24v pack, the power supply does make an excellent 10 amp charger.
Hey @Just John - My experience is that a balance charger (mine is an iSDT Q8) does not do a very good job of actually top balancing. Once it thinks the cells are at 3.65V, it just stops. Doesn't maintain that voltage until the current drops down to 2-5% like I think you would want.
 

Just John

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Hey @Just John - My experience is that a balance charger (mine is an iSDT Q8) does not do a very good job of actually top balancing. Once it thinks the cells are at 3.65V, it just stops. Doesn't maintain that voltage until the current drops down to 2-5% like I think you would want.
Well that is too bad. I think the easiest is to use an active balancer, once you get above 3.4v on a cell, just limit current to 2 amps and let it charge. Probably cheaper than the balance charger, since the balance charger requires a power supply as well. I don’t have a balance charger, but was under the impression they would do a good job.

If you have quality cells, you don't need an active balancer. For grade B cells they can actually get you out of balance if they are not turned off. The better active balancers will actually turn themselves off in the flat part of the curve. For grade A cells, it doesn't really matter, they stay very much in balance with or without. I think Heltec makes some better quality ones (but they cost more).
 

Johannlog

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I purchased 16 Cells from @Amy, they read 3.299 -3.300v, also a Chargery BMS for 24v system.
Do I need to top balance or am I good to go and connect it to the BMS?
 

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MisterSandals

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Do I need to top balance
It really helps.
Your Chargery will do some balancing along the way but a top balance has proven to be a real difference maker in getting cells to behave at the higher voltages where they tend to diverge significantly. At the mid voltages any capacity/Soc issues are hard to detect.
 

smoothJoey

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I purchased 16 Cells from @Amy, they read 3.299 -3.300v, also a Chargery BMS for 24v system.
Do I need to top balance or am I good to go and connect it to the BMS?
Are these "matched and batched" cells?
You mention 16 cells, a bms and 24 volts so I guess you are going to make a 2p8s battery, confirm?

Historically I would have always said yes regardless of configuration but now for simple serial packs I would say make a battery with a bms and see what delta you get at the top.

Your battery will have cells in parallel so I think you should top balance.

UPDATE: fixed thinko
 
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