diy solar

diy solar

Looking to top balance my new Eve 304ah cells.

imbuere

New Member
Joined
Oct 2, 2020
Messages
59
Newbie here and just looking for some confirmation on my plan and understanding. I recently bought 5 (4 in the pack and one as a backup) Eve lifepo4 304 ah cells from the recommended seller here.

I have a cheap bench power supply (one Will recommended) that only puts out 30v/10a. Right now my cells are at 3.29v… so very low and will take forever to charge with this psu with all the cells in parallel.

So, I’m thinking about hooking it up in series and charging to 17.9v (3.58*5) (again, might seem odd because I’m using 5 batteries… the final battery will be 4 cells of course). Once that’s complete, I’ll take off the bus bars and use short wires to wire all the cells in parallel. I’ll use wires to put it into parallel as I dont want to take the pack out of compression as it approaches full capacity. Then I’ll finish the top balance at 3.65v.

Once it gets to 3.65 I’ll let it sit for a few days, put the bus bars back in in series and can move to a capacity test.

I’m his sound like the right way to approach this? Any suggestions?

I should be able to use 10awg for the parallel wires, right?

To do the first 17.9v charge I’ll simply be going up my bench power supply to the #1 cell positive and the power negative to the #5 cell negative. Right? (Picture of the pack ready for that series charge)image.jpg
 
Newbie here and just looking for some confirmation on my plan and understanding. I recently bought 5 (4 in the pack and one as a backup) Eve lifepo4 304 ah cells from the recommended seller here.

I have a cheap bench power supply (one Will recommended) that only puts out 30v/10a. Right now my cells are at 3.29v… so very low and will take forever to charge with this psu with all the cells in parallel.

So, I’m thinking about hooking it up in series and charging to 17.9v (3.58*5) (again, might seem odd because I’m using 5 batteries… the final battery will be 4 cells of course). Once that’s complete, I’ll take off the bus bars and use short wires to wire all the cells in parallel. I’ll use wires to put it into parallel as I dont want to take the pack out of compression as it approaches full capacity. Then I’ll finish the top balance at 3.65v.

Once it gets to 3.65 I’ll let it sit for a few days, put the bus bars back in in series and can move to a capacity test.

I’m his sound like the right way to approach this? Any suggestions?

I should be able to use 10awg for the parallel wires, right?

To do the first 17.9v charge I’ll simply be going up my bench power supply to the #1 cell positive and the power negative to the #5 cell negative. Right? (Picture of the pack ready for that series charge)View attachment 83608

My concern with your plan is that if the cells are very far out of balance, one of them could race way ahead before the total pack gets to 17.9V.

I assume that once you build your battery, one of the cells will not be used. That cell will not be balanced with the others so adding it to the top balance process does not do much good. Why not just top balance 4 of the cells and use the bms for the initial precharge?
 

My concern with your plan is that if the cells are very far out of balance, one of them could race way ahead before the total pack gets to 17.9V.

I assume that once you build your battery, one of the cells will not be used. That cell will not be balanced with the others so adding it to the top balance process does not do much good. Why not just top balance 4 of the cells and use the bms for the initial precharge?
My thought was that it would be included in the capacity test so that I might identify a case where the spare was bad.

Can you explain what you mean by one could race way ahead?
 
If one of the cells has a significantly higher state of charge, it will reach full and start rising in voltage while the other cells are still in the flat part of the charge curve at about 3.3V. You could end up with 4 of the cells at 3.3V and the 5th cell at 4.7V. (3.3V x 4) + 4.7V = 17.9V

You could try and be a human BMS and keep checking all the voltages, but this is a long process and Murphes law guarantees you won't be watching when one of the cells starts racing ahead.

You might be safe trying to series charge them to about 16.5V, but even then watch them closely. If they get to 16.5V without one of them racing away, then try 16.75V, then 17.0V, 17.25 and 17.5V If at any point, one of the cell voltages starts getting a lot higher than the others, it is time to start the parallel balance. If you make it to 17.5V I would go to parallel balance even if the voltages are still pretty close.
 
I’m thinking about hooking it up in series and charging to 17.9v (3.58*5)
Without a BMS to protect against cell overvolts this is not a practical method. Unless you take upmost care, you are the BMS, there is a good chance one or more cells will be damaged.
Once it gets to 3.65 I’ll let it sit for a few days
I suspect there is no significant advantage in having cells with the same voltage connected in parallel. Keeping/storing cells at full charge may have effects on service life.

This is what I would do.

Use a 4s BMS, charge the cells as a 4 cell battery with the BMS connected, cells in the compression holder. Add or remove charge to/from individual cells as necessary as the cells approach 3.65 volts per cell, whilst remaining connected as a 4 cell battery. Its possible to get to an acceptable balance with the 'manual' balancing and let the BMS look after the remaining few mV. Miss out the all in parallel.
Charge up the spare cell as a single unit, test and discharge to 50% for storage. Its very unlikely it will ever be needed.

Mike
 
Many thanks to all the suggestions. I believe I understand each, and while they are slightly different, I can see they’re all getting to the same spot and what the considerations are.

I put the pack back in parallel, tightened everything up, and hooked up the charger again. For some odd reason it’s decided it can now push about 13w instead of just 3w like before. This is with the horrible included alligator clips. I think that if I make some nice ring terminals, I might be able to get that up a few more watts. Right now it’s only at 3.5a but should be able to go to 10a. I’ll see what improvement the better cables get me. I’d be afraid to push 10a through those… they're probably in 24awg.

Anyways, at 13w (maybe higher), this guy will take maybe a week to charge up… which is ok because I don’t really have any free time to deal with the charged battery anyhow.

So, I think I’m just going to go at it safe, slow, and easy. Thank you all for the advice… while I’m not going to go either suggestion, I spent a lot of time considering what you all we’re saying g and have learned from the suggestion.

Very much appreciated!
 
Quick update, I was going to make some upgraded cables in a few days, but as I was putting stuff away, found a set of old twin wire 6awg cables. The battery side ring terminal was too big, but good enough to work.

I hooked them up and have 10.3a!!

I was hoping to get like 30% more watts but now have like 250% more. I can charge this thing in a few days.

Note to all the other newbies. Replace those shitty alligator clips!!
 
Another option is to connect some DC-DC step downs to your PSU.
You could set your PSU to a higher voltage @10A and set the step downs to 3.65v and higher amps.

I have successfully done this with one step down, but was lazy to connect the other in parallel (and didn't have the time to make sure both were properly calibrated).

I have two of these 900w adjustable step downs. I was able to push around 18-19A (picture was when I tested with around 13A).
20211231_080903.jpg

You just have to make sure you don't exceed the amp rating for the step down (mine are 20A max) and that they don't overheat.

I'm actually thinking about building a proper "top balance" rig with these, after proper wiring, calibration and additional fans.
I suppose that I will be able to push around 30A total from my 300w PSU (30v@10A max) safely.
 
If charging in parallel at 3.650 volts with no BMS... critical that the PSU voltage was set before connecting to the battery. Best to verify the calibration with your voltmeter.
 
If charging in parallel at 3.650 volts with no BMS... critical that the PSU voltage was set before connecting to the battery. Best to verify the calibration with your voltmeter.
I did this without knowing I should just because the psu I have is pretty cheap. Why do you recommend this?
 
I did this without knowing I should just because the psu I have is pretty cheap. Why do you recommend this?
Once power starts flowing near max, the amps are restricted and the voltage sags a bit making the voltage setting incorrect.

Periodically someone posts that the connection was made first and the PSU inadvertently gets set to a much higher voltage. Eventually the whole thing goes over voltage. Potentially damaging the cells.

OK to disconnect once a day and verify the voltage setting.
 
Back
Top