Top Balancing using a PSU: is this exactly the other way around?

powerB

New Member
I've got a simple 4S 100Ah battery sitting on my table, waiting to be top balanced for the first time. I am a novice when it comes to this stuff, but I've done a lot of reading and researching and have downloaded the Tutorial about balancing LiFePO4 with a 10A power supply. I think I have a good grasp of the important concepts but I am not sure if I'm doing everything right, though...??

The individual cells each measured in at 3.30v and I've hooked them up in parallel. I've decided not to do the pre-charging but instead go straight to the balancing. So I've taken the following steps:
- set the max voltage of the PSU to 3,65v (nothing connected)
- attached the positive (+) and the negative (-) clamps together to then dial in the amps (set to 10A)
- only after that is done clamp to and connect the battery terminals

...the PROBLEM (if it is one): the PSU display shows that the PSU is in the CV (constant voltage) mode, the display shows a permanent 3,65v and the amperage has dropped to around 4,0-4,2A and seems to slowly slowly decrease over time.

Now I'm really wondering whether this is correct?? I thought LiFePO4's needed to be charged up to 3,65v with a constant current??
Is charging at a constant voltage bad?
In another thread one person's response to a similar situation was: the PSU leads could be the problem (because they're way too thin)?
Am I misunderstanding?

Here are three pictures to show whats going on:
lifepo4_detail.jpg Step 1: set the max voltage to 3,65v

lifepo4_detail2.jpg Step 2: set the max current to 10A (by clamping the two leads to each other, as per PSU manual)
lifepo4-detail3.jpg Step 3: connect the cell and watch this happen: the PSU is in the CV mode, the amps are down to 4,377A and slowly seem to be dropping
 

FilterGuy

What, me worry?
Being in CV is normal..... nothing wrong with that.

However, it looks like you are only charging a single cell. To ballance the cells, they need to be all charged together (in parallel)
 

Hiesenfig

New Member
Exactly what Filterguy said. If you were to charge in CC, you have to watch the voltage at the battery terminal VERY close, because when the batteries reach a full state of charge, the voltage will spike beyond 3.65 very quickly. It's best to do the parallel top balance in CV 3.65. But will take a long time if the SoC is low. That's where you can do a pre-charge in Seires with a BMS attached using CC mode. When 1 battery reaches the limit of 3.65, then it shuts off. Then rewire to parrallel and do the top balance with CV 3.65. As the battery voltage approches 3.65, the current will approach 0. each sell will stop charging when it matches the CV. So once it hits 0 (or near 0), then the top balance is done.
 

Loadtoad

New Member
I'm on day 3 parallel charging. These 280 amp hour batteries take a long time. Been charting the numbers since yesterday at 110 pm. At that time the amps were flowing at 4.394 and the cell volts were 3.355. It's now 9pm (the next day) and the current is down to 3.968 and the cell voltage is 3.379 volts. Very slow progress. All charging at 3.65 volts.
 

FilterGuy

What, me worry?
I'm on day 3 parallel charging. These 280 amp hour batteries take a long time. Been charting the numbers since yesterday at 110 pm. At that time the amps were flowing at 4.394 and the cell volts were 3.355. It's now 9pm (the next day) and the current is down to 3.968 and the cell voltage is 3.379 volts. Very slow progress. All charging at 3.65 volts.
That all sounds normal.... particularly if you don't do a pre-charge.
 

FilterGuy

What, me worry?
Some quick numbers.

You have 4 100Ah cells. Each Cell has 100Ahx3.4V=340Whrs for a total of 4x340Wh=1360Wh.
Let's assume the cells started half charged. That means they need 1360/2= 680Wh to get charged.

You are charging at 3.36Vx4A=13.44W.

That means you need 380Wh/13.44W=50 hr to charge. (or 100 hours if the cells were totally empty to start)
 

powerB

New Member
Hi everybody,
well I've been making slow but steady progress and reached the end of the process. Is it normal that when you let the battery rest for a while that the voltage drops? The power supply has spent about two hours hovering around the 3,65V and 0.02A mark, very very slowly decreasing. When it got close to 0.01A I stopped the charging, measured the voltage of the cells right then (3.65v) but then this morning the meter reading only gave me 3.59v... Should I keep charging, literally all the way down to zero? Or can I consider the cells balanced even at 3.59 and now move on to the series connection with the BMS?

Thanks!
 

ArthurEld

Photon Sorcerer
You should be ok. They always settle. Don't try to get them to stay at 3.65.
If you do each cell the same way they should be close enough to each other to connect serial when you are done.
 
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