diy solar

diy solar

balancing strategies as they relate to BMS

ArthurEld said:
edit: added cell 3.65V. Meaning I want to charge at 12A until the cell reaches 3.65V. Then do the absorb.
I think when you charge larger packs at 12A you can just charge until the pack reaches 3.65V. Then no absorb because the amps are spread so thin it's already like absorb.
Click to expand...
My cells read 3.355 and have been stuck there since I checked them around noon today. The cells voltages are at the very flat part of the curve. I see what you mean about about setting the charger at 3.65 volts and 12 amps. I just realized by setting it too low it was going into CV mode...duh. So I have set it to 3.65 volts at 12 amps. The display on the charger reads 3.56 volts. This sure is taking a long time...lol. But that was expected.
 
Gazoo, it is also very safe to set you voltage to 3.7 or 3.8 but have your OVP set to 3.65.
You'll probably figure that out soon. My cells are so far away from those settings that I set it to 3.9V and OVP to 3.8V and the cell is still below 3.65V when it gets to 3.8V and cuts off. You will get different results with multiple cells.
 
Colonel.lp said:
Also it's a very competitive market, if they had warehouses in Europe and could get cells to you quicker because of that I'm sure they would tell you as a marketing ploy. You just have to look on eBay, there are plenty of Chinese sellers that tell you they have a UK or Europe warehouse, as people want stuff quick.
Click to expand...
I don't know why your cells came at such a high SOC, but without further evidence it seems an isolated incident.
 
ArthurEld said:
Gazoo, it is also very safe to set you voltage to 3.7 or 3.8 but have your OVP set to 3.65.
You'll probably figure that out soon. My cells are so far away from those settings that I set it to 3.9V and OVP to 3.8V and the cell is still below 3.65V when it gets to 3.8V and cuts off. You will get different results with multiple cells.
Click to expand...
I agree. When the charger cuts off there will be a good .2 voltage difference between the displays voltage and the voltage at the terminals based on what I am seeing now. It would take way too long to wait on the absorb when it switches to CV mode. These low voltages are a pain and the cells are still in the flat part of the curve. Right now the display reads 3.57 volts and the voltage at the terminals is 3.357. I am charging 8 cells.
 
Gazoo said:
I agree. When the charger cuts off there will be a good .2 voltage difference between the displays voltage and the voltage at the terminals based on what I am seeing now. It would take way too long to wait on the absorb when it switches to CV mode. These low voltages are a pain and the cells are still in the flat part of the curve. Right now the display reads 3.57 volts and the voltage at the terminals is 3.357. I am charging 8 cells.
Click to expand...
I think once you get past 3.4V at the cells, it speeds up. And if you get past 3.6V it starts moving really fast. I think 3.6 to 3.65 takes about one minute on a single cell.
 
ArthurEld said:
I think once you get past 3.4V at the cells, it speeds up. And if you get past 3.6V it starts moving really fast. I think 3.6 to 3.65 takes about one minute on a single cell.
Click to expand...
On 4 cells with 5 amp charger, 3.75v, 3.4 to 3.6 took about 1hr, 3.6 to 3.65 took a few minutes. For the current to drop to zero about 4 or 5 hours with careful dropping of the voltage back to 3.65, or the voltage will keep going up.
 
I am finding it difficult to get 280Ah out of these cells. I think the only way is to charge to 3.65V cell voltage and use some absorb.
Then discharge it to 2.5V at the cell. I haven't done that yet but I have tried everything else.
And I still haven't proven that I have one weak cell.
 
ArthurEld said:
I am finding it difficult to get 280Ah out of these cells. I think the only way is to charge to 3.65V cell voltage and use some absorb.
Then discharge it to 2.5V at the cell. I haven't done that yet but I have tried everything else.
And I still haven't proven that I have one weak cell.
Click to expand...
What capacities have you got so far?
 
ArthurEld said:
I am finding it difficult to get 280Ah out of these cells. I think the only way is to charge to 3.65V cell voltage and use some absorb.
Then discharge it to 2.5V at the cell. I haven't done that yet but I have tried everything else.
And I still haven't proven that I have one weak cell.
Click to expand...
How much current was going into the cell when you removed the charge? If it was the full 12 amps and you removed the charge at 3.65 volts the cell didn't absorb anything. I have no idea how many amps are lost due to not letting the cell absorb but considering the size of the cell....
 
Colonel.lp said:
What capacities have you got so far?
Click to expand...
The highest I got so for is 278Ah That was taking the cell down to 2.5V at the cell.
I only charged that one to 3.60V though.
I am going to charge one more to 3.65 then I'll let it do the absorb. Then I will discharge it to 2.5V. And I expect to get over 280Ah.
Then I will do the same thing to the one I suspect is weak. I wouldn't be surprised if that one gives me 280Ah too.
This takes a lot of time. But most of it just runs by it self so I don't have to check it much.
 
ArthurEld said:
The highest I got so for is 278Ah That was taking the cell down to 2.5V at the cell.
I only charged that one to 3.60V though.
I am going to charge one more to 3.65 then I'll let it do the absorb. Then I will discharge it to 2.5V. And I expect to get over 280Ah.
Then I will do the same thing to the one I suspect is weak. I wouldn't be surprised if that one gives me 280Ah too.
This takes a lot of time. But most of it just runs by it self so I don't have to check it much.
Click to expand...
I'd be happy with a figure of 278ah, close enough I'd say. As long as you are consistent through the cells, isn't that what's important? The time it took my pack to go from 3.6 to 3.65 and then current to 0, I'd be surprised if it absorbed more than 3ah across the 4 cells.
 
Gazoo said:
How much current was going into the cell when you removed the charge? If it was the full 12 amps and you removed the charge at 3.65 volts the cell didn't absorb anything. I have no idea how many amps are lost due to not letting the cell absorb but considering the size of the cell....
Click to expand...
The cell I just finished was charged to 3.61V then allowed to rest until it was under 3.6V. Then I set the Riden to 3.6V and let it finish the absorb. It takes less than 1/2 hour to finish the absorb.
The next one I charge, I will charge it up to 3.65V then let it rest a while and charge it to 3.65V again. Then I'll let it rest until it's below 3.65V and run the absorb at 3.65V. The absorb will take less than 1/2 hour. Then I will wait one hour and discharge to 2.5V. All voltages are at the cell.
 
Have you tried measuring the voltage right at the terminals of the power supply, and then at the cells, using the same meter? Any difference is the resistance drop in the leads. If the external reading at the power supply terminals does not agree with power supply internal display, it either means the calibration of the internal meter and your meter do not match, there is a substantial resistance between where the power supply is measuring the voltage and the output terminals, OR it could be a combination of the two.

I would also suggest testing the power supply at a low current and see how accurate the output voltage is. What is your desired tail current? Load the supply to that current and adjust for the desired voltage. Then just leave it on the cells until it reaches your tail current and the voltage is then your desired absorb voltage as well. Yes, it may take days with several large cells in parallel.

I used a 600 watt power supply to balance up my NCM cells and that still took me a few days. NCM and LFP do act a bit different, but the goal is similar. You want to get all the cells to full charge. NCM cells don't fall back, they stay at the absorb voltage until you apply a load. LFP cells will slowly fall back a few tenths of a volt when the charge source is removed. I have not personally worked with large LFP cells yet, so I can't say how much this impacts the process, but from following Will and a few others on here, it just looks like you need a lot of patience. When the cells are all connected in parallel, they will share the power and self balance to some degree even without a power source connected. Even in the flattest part of the discharge curve, the cells will pull each other to the same voltage, and if the cells are at the same voltage and temperature, they must actually be at a very similar state of charge. As you pull the voltage into the knee area, the difference in state of charge will certainly close up and bring the balance together even more. When I did a parallel balance, I actually used a 10 inch piece of #14 wire to each cell so that I could measure the drop on that wire to see what was going on. I had a buss bar on all of the negatives, but the light wire on the positives. When I first connected, one of the cells was high and I saw a fair drop on it's wire, and about 1/3 of the drop on the other three as it was giving all of them some charge. These were only 4 amp hour A123 cells, and it was pushing over 3 amps out of the high cell. They all reached the same voltage in under an hour. Obviously, 280 AH cells will take longer. A LOT LONGER. If the currents are high, you can use a clamp meter on the wires, but measuring the millivolts across the lengths of wire works just fine. I used those cells for a 2S2P RC car pack. Went right back to LiPo as they A123 were not even close in performance.
 
Gazoo said:
I don't think so if I am understanding your post. Will is using a traditional power supply. The Riden power supply has a charging feature. So it can be used as a power supply or as a charger.

Parallel top balancing the cells using it as a power supply or using it as a charger will achieve the same result. Same if charging a pack. The benefit of using the Riden's charging feature is there should be no chance of overcharging the cell since it will terminate the charge at the selected voltage. The set current at charge termination is 100ma. There is no way to change the set current but I think at that point the cells would be well saturated.

Having said that I have tested it using it's charging function with a 14.6 volt battery twice. Both times it worked perfectly. I don't know how it's going to behave when I get around to top balancing my 8 280ah cells. I suspect it will be ok and cut off like it's supposed to. I know it will take a long time I am ok with that and I will be keeping an eye on it. I will also make sure to check the voltages of the cells after they have reached their resting voltages. Going to be interesting.
Click to expand...
GXMnow...in case you forgot my previous reply to you.

Yes there is about a .2 voltage difference between the output of the charger and the cell terminals. My cells read 3.357 volts. The voltage measured at the power supplies terminals is 3.486 and the display reads 3.57 volts. This is the unit we are using:

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GXMnow said:
Have you tried measuring the voltage right at the terminals of the power supply, and then at the cells, using the same meter? Any difference is the resistance drop in the leads. If the external reading at the power supply terminals does not agree with power supply internal display, it either means the calibration of the internal meter and your meter do not match, there is a substantial resistance between where the power supply is measuring the voltage and the output terminals, OR it could be a combination of the two.

I would also suggest testing the power supply at a low current and see how accurate the output voltage is. What is your desired tail current? Load the supply to that current and adjust for the desired voltage. Then just leave it on the cells until it reaches your tail current and the voltage is then your desired absorb voltage as well. Yes, it may take days with several large cells in parallel.

I used a 600 watt power supply to balance up my NCM cells and that still took me a few days. NCM and LFP do act a bit different, but the goal is similar. You want to get all the cells to full charge. NCM cells don't fall back, they stay at the absorb voltage until you apply a load. LFP cells will slowly fall back a few tenths of a volt when the charge source is removed. I have not personally worked with large LFP cells yet, so I can't say how much this impacts the process, but from following Will and a few others on here, it just looks like you need a lot of patience. When the cells are all connected in parallel, they will share the power and self balance to some degree even without a power source connected. Even in the flattest part of the discharge curve, the cells will pull each other to the same voltage, and if the cells are at the same voltage and temperature, they must actually be at a very similar state of charge. As you pull the voltage into the knee area, the difference in state of charge will certainly close up and bring the balance together even more. When I did a parallel balance, I actually used a 10 inch piece of #14 wire to each cell so that I could measure the drop on that wire to see what was going on. I had a buss bar on all of the negatives, but the light wire on the positives. When I first connected, one of the cells was high and I saw a fair drop on it's wire, and about 1/3 of the drop on the other three as it was giving all of them some charge. These were only 4 amp hour A123 cells, and it was pushing over 3 amps out of the high cell. They all reached the same voltage in under an hour. Obviously, 280 AH cells will take longer. A LOT LONGER. If the currents are high, you can use a clamp meter on the wires, but measuring the millivolts across the lengths of wire works just fine. I used those cells for a 2S2P RC car pack. Went right back to LiPo as they A123 were not even close in performance.
Click to expand...
The only time the Riden display voltage matches the cell voltage is at the end of the absorb. But I have a foolproof way of charging and discharging. I know what setting to use so the Riden will shut off before over charging. Then I stay with it until I get to the absorb stage.
I do the same with the capacity tester. I don't have to watch it very long.
 
ArthurEld said:
Gazoo, I read that the Riden RD6018 works with the same case and power supply. So, about $70 gets us 18A instead of 12A.
Click to expand...
Thanks but yeah, I saw that about a week after I received mine. 18 amps vs 12 would be nice to have. But I am happy with what I have and besides funds are a little tight right now.
 
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