Since I am going to use CV (I think) this is now a potential issue. I will need to make sure that voltage imbalance on the op-amp input doesn't create a problem. Some series resistance is almost never a bad idea.
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Electronic Loads
- Thread starter HaldorEE
- Start date
This does help. I will definitely need to monitor charge current not cell voltage to decide when to stop charging.
Since CV power supplies are absolutely dirt cheap and my circuit will be providing current limiting, I think I will just get a Mean Well 4.2V supply. Voltage is adjustable from 3.6V to 4.4V and a max current output of 60A is perfect for charging large lithium cells. Can't complain about the price either.
LRS-350-4.2 MEAN WELL USA Inc. | Power Supplies - External/Internal (Off-Board) | DigiKey
Order today, ships today. LRS-350-4.2 – Enclosed AC DC Converters 1 Output 4.2V - - - 60A 90 ~ 132 VAC, 180 ~ 264 VAC, 240 ~ 370 VDC Input from MEAN WELL USA Inc.. Pricing and Availability on millions of electronic components from Digi-Key Electronics.
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Based on this, it doesn't sound as though my middle charge stage is needed. I should be able to use a CV/CC charge source of 0.2C then simply monitor for when the charge current drops below 0.01C.
Is this an accurate summation?
In that case I think I will use two FETs in parallel and a bigger heat sink. That way I can take advantage of the full charge acceptance capability of the cells to speed up the process.
Do you have any experience measuring IR of a cell? All of my cells came with a vendor applied label claiming the IR was 0.14 milliohms. I am a bit suspicious at them all have exactly the same IR. Makes me wonder if they just have a role of pre-printed labels that they slap on the cells before putting them in the box.
I assume my load will also make testing IR pretty trivial. Just use Thevenin to determine IR from the open circuit and loaded cell voltage. Does cell IR have a reasonably linear function? Is there a standard load (C rate) or SOC point to use when measuring IR? How long should I let a cell settle after charging before measuring IR?
-Post heavily edited, as usual-
You can't overcharge a cell at 3.65V.
You don't need any stages really. Charge at the chargers max current until you hit 3.65V. You don't even need current feedback if do a bit of test and check to set the "absorb" timer. Depending on the current limit of the charger, you may need 5-15 minutes at 3.65V.
Even if you decide to leave the cell sitting for hours at 3.65V, you won't cause any damage to the cell. Now if you left it that way for weeks/months, the cell would experience capacity loss for various reasons. Mostly just high SOC related. Hence why lithium cells should be stored below 70% SOC.
Its hard coming from lead/nickel, where speedy chargers will push the voltage way up until a delta-V rate is achieved, then taper down as the SOC climbs. LFP has no need of anything that complicated, as it has no real memory effect, and accepts current easily due to low internal resistance.
You don't need any stages really. Charge at the chargers max current until you hit 3.65V. You don't even need current feedback if do a bit of test and check to set the "absorb" timer. Depending on the current limit of the charger, you may need 5-15 minutes at 3.65V.
Even if you decide to leave the cell sitting for hours at 3.65V, you won't cause any damage to the cell. Now if you left it that way for weeks/months, the cell would experience capacity loss for various reasons. Mostly just high SOC related. Hence why lithium cells should be stored below 70% SOC.
Its hard coming from lead/nickel, where speedy chargers will push the voltage way up until a delta-V rate is achieved, then taper down as the SOC climbs. LFP has no need of anything that complicated, as it has no real memory effect, and accepts current easily due to low internal resistance.
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
For my IR testing, I use a Yaorea YR1035+ tester.
EDIT:
Often Vendors will use a YR1030 or YR1035 variant when testing & packaging. They often call this "Matching" it isn't. It is done at Storage Voltage (more or less what you received your cells at, 3.200 ish)
EDIT:
Often Vendors will use a YR1030 or YR1035 variant when testing & packaging. They often call this "Matching" it isn't. It is done at Storage Voltage (more or less what you received your cells at, 3.200 ish)
I have read exactly the opposite here many times.
Leaving a cell disconnected at 3.65V is completely different from maintaining a float voltage of 3.65V on a cell. Every single competent lithium charge vendor makes a point of stating that the float voltage MUST NOT be 3.65V.
Ampster
Renewable Energy Hobbyist
I have been using them with success for a long time. I recently learned of some subtleties regarding current protection. Some have hiccup mode and others actually have constant current mode. There is a big difference. I looked at the spec sheet and could not tell about the one you cited. I suspect you probably know the difference so my comments are for the benefit of others.
There is a big difference from thousands of hours at 3.65V, versus a few hours after a one-off capacity test. If you are talking about a full time charger VS the testing rig we were discussing earlier, then thats my mistake. Obviously a full time charger should have a float or silent stage after reaching the desired setpoint. If thats the case, I suggest 3.5V as the cell target for longevity.
Again this isn't overcharging per se (which happens around 4-4.2V+). Its degradation due to natural capacity loss, which occurs faster at high SOC. 4V+ the electrolyte starts to break down, and other nasty stuff happens. That is true overcharging. At 3.65V all the active lithium has been removed from the anode, and charging basically drops to the 10s of mA, as you just counter self discharge.
Thanks for this suggestion. I found a thread here on DIYSolar about that very tester with a copy of the manual.
Yaorea YR1035+ Battery Internal Resistance Meter Tester, Manual
I Purchased an Original Yaorea YR1035+ to replace my YR1030 and it came with this "document" as a Manual, which on paper is really not easy to read. I, therefore, scanned it and turned it into a PDF file to make it easier for me and I felt I...
diysolarforum.com
The YR1035+ manual recommends measuring the IR on a full charge with new batteries. For old batteries they recommend measuring IR after the cell is drained. The manual states that the difference between full charge IR and drained IR on new batteries should be very small, but that in general the IR is more stable at full charge.
The manual also emphasized the importance of making 4 wire measurements (Kelvin connection). I will certainly build that into my design. I wasn't concerned about that for measuring battery capacity since all I really need to measure for that is charge current.
Unfortunately the YR1035+ is unable to accurately measure IR below 0.3 milliohms. The manufacturer specifically states it is for cells of 100 AH capacity or less.
This was interesting and informative.
I am torn. It would simplify my setup to use the version with CC as well as CV, but I was thinking that being able to dynamically adjust the CC value on the fly might be useful. Based on Steve's latest information, I am not sure that is still an advantage. The version with CC plus CV is 3 times as expensive, but it also provide a heck of a lot more charge current. That eliminates a lot of high current switching circuitry I would otherwise have to add to use the same CC load for both charging and discharging cells.
The HRPG-450-3.3 can be adjusted up to 3.8V output so it will work just fine to charge LiFePO4 cells. It also does CC up to 90A which is more than I need. It would make capacity testing of cells a lot faster and I could parallel my cells for top balancing instead of having to individually charge each cell for top balancing.
Hum, paralleling 3 FETS and water cooling would get me up into the 100A discharge range. And I happen to have some water cooling equipment from another project that is currently stalled...
90A charge/discharge capability means I will be able to capacity test a single 280 AH cell every 6 hours. So basically 2 per day, 4 days total for my 8S battery pack. Then another 2 days to top balance the cells.
I am going to have a couple of hundred dollars invested in this thing once I am done. And I am probably only going to use it at most once a year after that.
I wonder if anybody might be interested in renting the setup after I am done with it? I am in Phoenix, AZ. How many members are in the area? Would being able to rent a system like this for say $50 a week be unreasonable? I would certainly pay that if someone had something similar available.
Or I could provide a cell characterization/top balancing service. How much would something like that be worth? When you buy new cells from the manufacturer that have had this done they more than double the cost of the cells. You also get matching which is not something I could do unless someone has purchased a large quantity of cells and wanted them sorted by capacity (to make several individual battery packs). Cost of shipping would probably make that not feasible.
I should get with the member that is importing and reselling 280AH cells and see if he is interested in this. To be honest I would be happy if I could just use it now for building my battery pack, then send it on to somebody that could really use it. My garage is already full of stuff I hardly ever use.
I am going to have a couple of hundred dollars invested in this thing once I am done. And I am probably only going to use it at most once a year after that.
I wonder if anybody might be interested in renting the setup after I am done with it? I am in Phoenix, AZ. How many members are in the area? Would being able to rent a system like this for say $50 a week be unreasonable? I would certainly pay that if someone had something similar available.
Or I could provide a cell characterization/top balancing service. How much would something like that be worth? When you buy new cells from the manufacturer that have had this done they more than double the cost of the cells. You also get matching which is not something I could do unless someone has purchased a large quantity of cells and wanted them sorted by capacity (to make several individual battery packs). Cost of shipping would probably make that not feasible.
I should get with the member that is importing and reselling 280AH cells and see if he is interested in this. To be honest I would be happy if I could just use it now for building my battery pack, then send it on to somebody that could really use it. My garage is already full of stuff I hardly ever use.
Ampster
Renewable Energy Hobbyist
Me too. I bought an inexpensive one on Ebay before I realized that it would go into hiccup mode. My workaround is to try to fire it up at a lower voltage so the Amps do not max out and force it into hiccup mode. I am doing that just do see if I can get some value from the $20 I spent.
Never mind!!!
Meanwell makes a 150W 3.3V at 30A version with CC and Mouser has it in stock for $65.
HRPG-150-33.
I just bought one.
It is a bit small for doing 8 cells in parallel, but is just fine for charging individual cells. A complete capacity measurement cycle on a single cell will take about 18 hours at 30A. I can live with that, especially if I don't have to baby sit it. I will use the same discharge current of 30A to simplify the electronic load design. I didn't really want to have to deal with water cooling.
Meanwell makes a 150W 3.3V at 30A version with CC and Mouser has it in stock for $65.
HRPG-150-33.
I just bought one.
It is a bit small for doing 8 cells in parallel, but is just fine for charging individual cells. A complete capacity measurement cycle on a single cell will take about 18 hours at 30A. I can live with that, especially if I don't have to baby sit it. I will use the same discharge current of 30A to simplify the electronic load design. I didn't really want to have to deal with water cooling.
I could be wrong. Not unusual. My thinking is the lower the tail current when the charger cuts off, the lower the delta will be at the top when the cells are connected in series and fully charged. I used a Riden 12 amp charger to top balance my 8 EVE's. I let them charge until the charger cut off at 100ma's. After parallel top balancing and connecting my cells in series, my delta using the same charger is is 80mv's. I don't think that's bad considering the cells are not matched. At the bottom my delta is .5 volts.
It's also been said leaving the cells in parallel after top balancing helps with the balancing. I left mine in parallel for around 24 hours before I connected them is series.
It's also been said leaving the cells in parallel after top balancing helps with the balancing. I left mine in parallel for around 24 hours before I connected them is series.
I believe that as charging process continues, the current naturally tails off. That certainly agrees with what Steve reported.
"I'll chime in, a tad if I may. I'm using the TekPower 1540 switching power supply.
Observed with EVE 280s. Other cells may be slightly different from these observations.
It starts off at 35-38A depending on the cell voltage. At high amp, it is CC, below 30, the supply switches to CV.
The lower the voltage, the higher the amperage and it will only be about 1V above the cell actual voltage.
The cells will reach 3.600-3.650 the amperage drops after 30-45 mins of reaching target voltage fairly quick and slow at 2.5A ish and usually stop @ 1.8-2.0A and just sit there... This is fairly consistent with the last four cells I did."
I can believe if you use the 0.05C rate as the point to stop charging then you in fact have not yet reached the tail of the charge cycle (which is what snoobler so graciously acknowledged in his thread). I am not convinced however, that there is any real reason to use an interim charge stage at a lower CC rate like I was previously suggesting. If the charge current has reduced itself to 2A, then I believe the cell is charged. Key takeaway is to use a threshold that will actually be satisfied so the thing doesn't sit there charging forever at 2A.
I agree with leaving them connected in parallel for a while after charging to let them top balance. Heck they have been sitting that way on my bench for weeks now while I get my stuff together. At about 3.2V so it isn't meaningful, but at least it looks like I am doing something with them.
"I'll chime in, a tad if I may. I'm using the TekPower 1540 switching power supply.
Observed with EVE 280s. Other cells may be slightly different from these observations.
It starts off at 35-38A depending on the cell voltage. At high amp, it is CC, below 30, the supply switches to CV.
The lower the voltage, the higher the amperage and it will only be about 1V above the cell actual voltage.
The cells will reach 3.600-3.650 the amperage drops after 30-45 mins of reaching target voltage fairly quick and slow at 2.5A ish and usually stop @ 1.8-2.0A and just sit there... This is fairly consistent with the last four cells I did."
I can believe if you use the 0.05C rate as the point to stop charging then you in fact have not yet reached the tail of the charge cycle (which is what snoobler so graciously acknowledged in his thread). I am not convinced however, that there is any real reason to use an interim charge stage at a lower CC rate like I was previously suggesting. If the charge current has reduced itself to 2A, then I believe the cell is charged. Key takeaway is to use a threshold that will actually be satisfied so the thing doesn't sit there charging forever at 2A.
I agree with leaving them connected in parallel for a while after charging to let them top balance. Heck they have been sitting that way on my bench for weeks now while I get my stuff together. At about 3.2V so it isn't meaningful, but at least it looks like I am doing something with them.
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Just to give you an idea, once my charger switched from CC to CV mode it took 4 hours to finish my top balancing. That Meanwell charger you bought should work well. As @Luthj indicated there is no magic involved whether parallel top balancing or charging a pack. Just don't go over 3.65 volts.
As you know floating at high voltages is not good. And leaving cells unused with a high SOC isn't good either. My opinion for top balancing is to pack the cells with as much current as possible without going over 3.65 volts. It should only need to be done once. That doesn't necessarily mean to cut off the power supply or charger with a zero amp reading at the tail end of charge. But I think the closer to zero the better. I don't have any scientific data to support this and I am starting to babble.
As you know floating at high voltages is not good. And leaving cells unused with a high SOC isn't good either. My opinion for top balancing is to pack the cells with as much current as possible without going over 3.65 volts. It should only need to be done once. That doesn't necessarily mean to cut off the power supply or charger with a zero amp reading at the tail end of charge. But I think the closer to zero the better. I don't have any scientific data to support this and I am starting to babble.
What you are saying makes sense.Just to give you an idea, once my charger switched from CC to CV mode it took 4 hours to finish my top balancing. That Meanwell charger you bought should work well. As @Luthj indicated there is no magic involved whether parallel top balancing or charging a pack. Just don't go over 3.65 volts.
As you know floating at high voltages is not good. And leaving cells unused with a high SOC isn't good either. My opinion for top balancing is to pack the cells with as much current as possible without going over 3.65 volts. It should only need to be done once. That doesn't necessarily mean to cut off the power supply or charger with a zero amp reading at the tail end of charge. But I think the closer to zero the better. I don't have any scientific data to support this and I am starting to babble.
I plan on repeating the process every year, just to see how the cells are holding up. Consider it research.
That's entirely up to you but you may not have to. I suggest you record your deltas as reported by the BMS when you finish your pack. Then look at them in a year. If the deltas are close there will be no need to top balance. I think taking delta readings every so often will tell us what we need to know. That and doing a full capacity test every so often.
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