diy solar

diy solar

Started my top balancing

@FilterGuy - your thought experiment is correct to a point. You are so close!! but take it a step further ...

Btw, my 40A Tekpower rockes. So light and quiet. Efficient enough not to blow out an ac outlet. Dig it.

So, as you mentioned, you drop down to 0.013C tail current in your thought experiment with a 0.1C rate to begin with. Ok, not the end of the world.

What I'm saying is that if you START OUT at "tail current" levels, like 0.013C (1.3A for your 100ah batt) then your battery will reach full charge at 3.45V. Even though your TekPower is set to 3.65v, a stall seems to occur near 3.45v because you are simply done. Anything else is overcharge due to starting out so low in current. Here, the clock is simply ticking too long.

If you have a spare cell laying around, and have the time, try it!
 
@FilterGuy - your thought experiment is correct to a point. You are so close!! but take it a step further ...

Btw, my 40A Tekpower rockes. So light and quiet. Efficient enough not to blow out an ac outlet. Dig it.

So, as you mentioned, you drop down to 0.013C tail current in your thought experiment with a 0.1C rate to begin with. Ok, not the end of the world.

What I'm saying is that if you START OUT at "tail current" levels, like 0.013C (1.3A for your 100ah batt) then your battery will reach full charge at 3.45V. Even though your TekPower is set to 3.65v, a stall seems to occur near 3.45v because you are simply done. Anything else is overcharge due to starting out so low in current. Here, the clock is simply ticking too long.

If you have a spare cell laying around, and have the time, try it!
My experience, once you hit 3.4v it takes off like a rocket (comparatively speaking).
Maybe I have different Eve 280AH cells.
 
This is silly advice. 4 280Ah cells in parallel is 1120Ah. 0.2C is 224A. Most are using, successfully, 10A bench top supplies to balance. That is 0.01C - or 1/20th what you are recommending.

Giving this sort of advice is really bad for the community.

Actually the opposite. If you watch Will's video, you will note he is starting out from cells that are already high in SOC so you can get away with using such low rates. (4:23 into it)

What I am saying is to NOT use such low rates, OR, because for most it is impractical, it might be better to do smaller parallel sets at a time, instead of one long string of heavily discharged cells right at the start which Will clearly did not start out with.
 
Hey, we're supposed to be having fun on Memorial Day weekend! I'll drop out because this shouldn't be a contentious issue...
 
Actually the opposite. If you watch Will's video, you will note he is starting out from cells that are already high in SOC so you can get away with using such low rates. (4:23 into it)

What I am saying is to NOT use such low rates, OR, because for most it is impractical, it might be better to do smaller parallel sets at a time, instead of one long string of heavily discharged cells right at the start which Will clearly did not start out with.
I would certainly agree that doing them in smaller groups is a better approach. Anything over 16 would just take forever even starting from a high state of charge.

I charge them in series with a BMS to get them to the high state of charge first, then parallel to top balance.
 
Hey, we're supposed to be having fun on Memorial Day weekend! I'll drop out because this shouldn't be a contentious issue...

Not contentious at all - sorry if I came across dicky.

I am genuinely curious on this as I have never had issues charging at lower rates for long period as long as voltage was set well below max. Then once that is achieved to step up to final voltage desired.

If that is a bad or flawed approach I would like to know this as have built many packs this way always with good success.

But in another thread I am seeing resting voltages fall off quicker and perhaps this is related.

I thought the entire justification for top balancing was to get all the cells to the "same" state as a starting point.
 
Wait - I was chilling out because it was ME that was going dicky. :) So I apologize if I came off like that myself.

What I do may simply be too impractical, and bordering on an industrial-application like concern, when one-off setups where it probably won't matter to most would be more disruptive than informative with casual sub-c storage.

I'm the one always harping on making things fit the application - I need to take my own medicine! Have a great rest of the holiday!
 
Well, they are all finished... They didn't all end up exactly the same at the 0.00A mark for some reason, even though the setting was the same verified by DMM. 3.589, 3.586, 3.585, 3.583 from the first set of 3 to the last. Guess the cheap power supply was getting tired... I'm calling them good and the REC active can do its thing and balance them. Waiting on a couple more items then I can get this all installed!
 
Well, they are all finished... They didn't all end up exactly the same at the 0.00A mark for some reason, even though the setting was the same verified by DMM. 3.589, 3.586, 3.585, 3.583 from the first set of 3 to the last. Guess the cheap power supply was getting tired... I'm calling them good and the REC active can do its thing and balance them. Waiting on a couple more items then I can get this all installed!
Measure again after the cells have rested overnight.
 
Interesting conversation and absolutely appreciate the detailed journey. This helps all of us learn.

Not my intent to hijack but am curious about the serial then parallel approach. Does one use the same “cheap” 10a power supply and rely on the BMS to cutoff or do they behave in the same way as parallel charging, dropping to low amperage draw when complete?
 
Interesting conversation and absolutely appreciate the detailed journey. This helps all of us learn.

Not my intent to hijack but am curious about the serial then parallel approach. Does one use the same “cheap” 10a power supply and rely on the BMS to cutoff or do they behave in the same way as parallel charging, dropping to low amperage draw when complete?

First, test your BMS and verify the high cell voltage disconnect works. How? Easy, set a value lower than one of your cells and verify it turns off charging.

Once you have set your high cell voltage disconnect, you can then charge (using your BMS) the entire pack.
If you have 4 cells in parallel, a 10 amp supply theoretically will supply 2.5 amps to each cell (parallel). When in series, each cell in the string gets 10 amps (series) and thus you would charge 4 times as fast (the full 10 amps in each cell).

Once one of the cells hits 3.65v, the BMS should stop charging. Why always use a BMS? between 3.4 and 3.65 volts on a cell is proximately 5 amp hours of capacity. On a 10 amp power supply, that's not so bad, it means you have 30 minutes to catch it before it over charges your cells. If you use a 40 amp supply (like I do), it means you have far less time, 6 or 7 minutes to catch it. Let the BMS "watch it like a hawk", I can assure you that the vast majority of people will get tired and let their attention wander, which when charging in series, means USE A BMS. Thus charging in series with a BMS means you get to a high state of charge 4 to 8 times faster than when charging in parallel. Only then do you switch to the slower parallel charging to get the last little bit (or maybe a lot, depends on how balanced your cells were). Since this can take days, the charge in series is a great way to speed things up a bit safely.

Yes, assuming your power supply can attain the voltage needed for your pack (in general practice, 24 volt packs are max for series on these cheap 30 volt supplies) you can use the same power supply for both series and parallel. To be honest, if you are building a battery pack, I would use whatever you plan to use to charge the pack (most likely a solar charge controller). Likely it has a higher amp rating to charge with. They just can't be set to 3.65v.

BTW, the objective of parallel top balancing is to get them all (cells) at 3.4v or 3.45v simultaneously. Then you can charge to 13.4 or 13.6v (3.4 or 3.45v/cell) and use the drop in charge current to tell you that indeed, your pack is full (more accurately, 98% full). So in the long run, yes you can use the drop in current from series charging to signal when the charge is done, but first you have to parallel top balance them.
 
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Sorry, I was long winded. Basically, one of the best things about this forum is the resources section. I highly recommend this, I used it, and most beginners should as well:

 
I've spent a ton of time on this great site and this is the first time I've seen the resource section.

Thanks!
 
Well, it would be nice for beginners if that point was made somewhere in the tutorials. When I went to buy a power supply, they didn't have the one that Will recommended at the time, so I went with another 30V, 10A thinking it was equivalent. I even posted about what I was using and how I knew it would take a long time, and not one person said that was not good. And now I'm concerned about the cells sitting there at 3.585V waiting on the rest to get done. I won't be able to discharge them until I get the entire system installed in my 5th wheel. I'm working on it, but not going to be able to get it operational for probably another week.

I plan to set my absorption to Victron's custom LiFePO4 charge profile recommendation from Will's sticky: 12V Battery: 14.1V
Do I also use his recommendation for Inverter Cut-off: 10.7V for a 12V??
Also, I need to determine correct settings for my REC active balancer...
I'm not an expert, but I more or less did the same thing you did and I believe you're probably fine:
  1. 4S setup with10A charger, 280Ah cells, charging at 14.5V and 8.5A, which got it to about 3.55V/cell within ~15-18 hours.
  2. Convert to 4P setup and charged the cells at 3.65V/8.5A until they all reached 3.63V +/-7mV. I was pushing 2A initially and trickled down to about 0.5A before I cut it off
  3. Rewired 4S, battery was at 14.53V, called it a day
I'm sure someone is cringing. I would've preferred a bigger power supply but couldn't really justify a 50A version. I believe from my research the only difference with low C rates is that you're holding the battery at 99.x% SoC for a long time while trying to push the last from mA into it, and LiFePO4 doesn't really like to be floated. So maybe instead of 2000 cycles you'll get 1950 now.

First, I believe you're better off doing the top-balance and holding the voltage up there for a few hours or even a day while you really "stretch" the battery bank. I'm basing this on the fact I saw a LOT of cell divergence above 3.45V (>100mV), despite the cells being within 2mV of each other out of the box and all the way up to ~3.4V. I think if you top-balance to ~3.45V some of those cells are not really at >99% SoC because voltage is a terrible indicator of SoC for LiFePO4! Unless I had set my BMS to shut off at 3.45V, the first time I applied a charge which would have pushed the cells past 3.45V they would've all diverged, been unbalanced, and would've *stayed* unbalanced as they discharged.

Second, I believe you're better off top-balancing at a low current than at a very high current. Why? Because high current increases the likelihood of battery swelling. I'm not saying you can't swell a battery by applying 3.64V and 1A to it and leaving it on a charger indefinitely, but in my (limited) experience using a 0.03C charge rate I saw no swelling, and my battery life seems just fine.
 
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