Top balancing with Longwei power supply

AlexanderKristiansen

Solar Enthusiast
I'm top balancing 8 brand new 3,2V 202Ah cells with a Longwei power supply.

I have set the voltage for the time being to 3,5. But have a couple of questions.

1. When the charging starts, the voltage on the power supply dropped to 3,40. But the voltage on the cells is about 3,1. Should they not be the same?

2. After a couple of days of balancing the voltage on the cells are not the same. Lowest 3,31, highest 3,34. Should they not be the same? I thought this was the whole idea of the power supply, to make the cells equal?

The voltage on the Longwei is now 3,48.
 

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Gazoo

Dumb Dumb
No, I didn't have enough of them so I made my own, see attached picture. 2mm thick.
It might help if you connect your power supply leads on opposite ends of the calls so the voltage will be distributed more evenly.

I still believe you have a connection problem between the busbar and the cells that have the lower voltage. Did you clean your cells terminals? Having said that the voltages will even out as you approach the top of the charge. Keep an eye on things.

I always say this and it doesn't account for the voltage differences, but the voltage of the power supply must be set before connecting it to the cells.
 

AlexanderKristiansen

Solar Enthusiast
It might help if you connect your power supply leads on opposite ends of the calls so the voltage will be distributed more evenly.

I still believe you have a connection problem between the busbar and the cells that have the lower voltage. Did you clean your cells terminals? Having said that the voltages will even out as you approach the top of the charge. Keep an eye on things.

I always say this and it doesn't account for the voltage differences, but the voltage of the power supply must be set before connecting it to the cells.

Yes, I set the voltage on the power supply before I connected it to the cells. Haven't touched it after that.

Could any of the cells go above the 3,5 limit I set on the power supply? Any danger of leaving it on during the night?
 

Gazoo

Dumb Dumb
Yes, I set the voltage on the power supply before I connected it to the cells. Haven't touched it after that.

Could any of the cells go above the 3,5 limit I set on the power supply? Any danger of leaving it on during the night?
Personally I wouldn't. Your highest cell is 3.38 volts and the majority are 3.37. So you don't have much longer to wait and you want to be keeping an eye on the voltages as you are getting close to done. :)
 

Gazoo

Dumb Dumb
Ok, thanks a lot!
You have your supply set to 3.5 volts. I am sure you know you will want to reset it to 3.65 volts once it reaches 3.5 volts. Things will go very quickly at this point. Please update what you see once your cells reach 3.65 volts. The voltage of the power supply and the cells voltages will be the same but the current will drop. And as you approach the voltage set on the power supply it will switch from CC to CV mode.
 

AlexanderKristiansen

Solar Enthusiast
You have your supply set to 3.5 volts. I am sure you know you will want to reset it to 3.65 volts once it reaches 3.5 volts. Things will go very quickly at this point. Please update what you see once your cells reach 3.65 volts. The voltage of the power supply and the cells voltages will be the same but the current will drop. And as you approach the voltage set on the power supply it will switch from CC to CV mode.

Yes, I know I will have to crank it up to 3,65 to do a top balance. I will post result, but I'm going away for a few days so will be some time next week :) Thanks a lot for your help!
 

halfwave

Electron Sorcerer
Gazoo has given great advice Alexander however one thing I notice from your picture above is the cells do not seem to be clamped together. Is that true? There is no doubt they have/will swell a little and may affect contact as Gazoo suggested above.

In the future I would temporarily clamp a set of cells you are charging with some bar or pipe clamps.
 

AlexanderKristiansen

Solar Enthusiast
Gazoo has given great advice Alexander however one thing I notice from your picture above is the cells do not seem to be clamped together. Is that true? There is no doubt they have/will swell a little and may affect contact as Gazoo suggested above.

In the future I would temporarily clamp a set of cells you are charging with some bar or pipe clamps.

Yes, it's true @halfwave , I haven't clamped the cells together. Didn't know about this actually and the cells are swelling a bit now as they are fully charged/top balanced. I think I saw a video with Will where he just puts some tape around the cells so I did the same...

I have however swapped out the lone bar in aluminium and just use the fabric bus bars in copper. I charge balance 4 at a time, on my last top balancing of the last 4 cells now. Will assemble them this weekend.

I will not have time to come up with a clamp solution at the moment but plan to do this in about a month. As I have understood it, clamping the cells will prolong the life span, but its, not something that is critical. Correct in your opinion, @halfwave?
 

halfwave

Electron Sorcerer
Yes, it's true @halfwave , I haven't clamped the cells together. Didn't know about this actually and the cells are swelling a bit now as they are fully charged/top balanced. I think I saw a video with Will where he just puts some tape around the cells so I did the same...

I have however swapped out the lone bar in aluminium and just use the fabric bus bars in copper. I charge balance 4 at a time, on my last top balancing of the last 4 cells now. Will assemble them this weekend.

I will not have time to come up with a clamp solution at the moment but plan to do this in about a month. As I have understood it, clamping the cells will prolong the life span, but its, not something that is critical. Correct in your opinion, @halfwave?

I believe its not critical outside of that one factor: without clamping the cell expansion can put stress on the terminals (threads) which are certainly weak. If you have a more flexible busbar than solid stock I think this alleviates the primary issue with cell expansion.

As to any cell longevity from clamping I can not speak to that as I'm not an expert on this by any means. It seems some here have gone to rather extreme measures in designing cell clamping systems. I just use the tried and true threaded rod solution. One day I might obtain some of the flexible stranded copper bus bars I've seen posted here before which seem an ideal solution to this problem.
 

GSXR1000

Solar Addict
mine didn't swell when first top balanced.. then bad weather hit so they sat for 3 weeks fully charged and the swelled enough the bus bars didn't want to go on ....
next time I will build the battery first the zip zag wires to parallel charge them
 

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
I use the longwei that you have and after MANY uses I trust it.
One thing of course is to set it to the voltage you want. Turn it off. Then turn it back on and NEVER touch the voltage dial again.
Every time you touch the voltage control, turn it off and on. Otherwise it will never stop properly. There was someone here that did that and their cell looked like a balloon and of course they killed the cell.

One other thing I noticed is that while the wires get warm, they do not get very hot. But it is best to replace them with thicker wires as you have done.

I have looked at my spec sheet for my cells and they say they are 71 mm thick when depleted. So for the four cells per battery I made an adjustable box for them that is 284 mm. That is how I did my "clamping". They finally work great now [after replacing 3 bad cells].

But the longwei is dependable. Just don't drop it. The insides are fragile. :)

As for the voltages. You can never really rely on them until after they have rested say 4 hours or so. But they will never go above what the longwei says IF you have turned it on and off as mentioned.

Once top balanced mine went down but they all read the exact same voltage after a few hours. At the top they were close to within 0.001 volts to each other. And the longwei was putting out less than 10 milliamps. So I stopped it

Have fun. :) I hope this helps.
 

Just John

Photon Sorcerer
1. When the charging starts, the voltage on the power supply dropped to 3,40. But the voltage on the cells is about 3,1. Should they not be the same?
Yes, this indicates a high resistance connection between the power supply and the cells. Don't use the alligator clip leads that come with the supply, make some nice 10 gauge copper wire with 1/4 inch crimp ring connectors. 10 gauge wire and 1/4 inch ring connectors because they are very common and easy to find (likely Home Depot or Lowe's have them in stock if you're in the USA). 8 gauge and M6 would be better, but much harder to find.
2. After a couple of days of balancing the voltage on the cells are not the same. Lowest 3,31, highest 3,34. Should they not be the same? I thought this was the whole idea of the power supply, to make the cells equal?
This indicates a bad connection as well (one of the busbar to cell terminals).
Usually a .001 difference is the most you will see, but certainly it should be below .005.
Clean the cell terminals and the busbar ends that connect to the cell terminals. Pretty common problem, actually.
 
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