Top Balance Question

MKalman

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Okay, so I've started top balancing my batteries about a week ago and it's a 16S 48V Eve 230AH battery pack.

I have my bench power supply set to 3.65V (according to meter) and all the cells attached in parallel.

The past two days, the cells have been floating at 3.64V and won't go much higher, the amperage on the power supply has gone down, and little to nothing is happening with the cells, is this common?

Can I just leave them balanced at 3.64V if I am only going to use them up until 3.64V and what capacity would 3.64V be, 95%? What would be around 90% capacity and 10% capacity so I only use 10-90% capacity.

Thanks for reading!
 

TomC4306

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You have fundamental misunderstanding of Lifepo4 charging.
Watch this end to end.
 

MisterSandals

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what capacity would 3.64V be, 95%? What would be around 90% capacity and 10% capacity so I only use 10-90% capacity.
Anything over 3.45V is essentially 100%.
I am a conservative charger and charge to 3.45V which I consider a gentle 100%.
I do not like to go much below 3V discharge.

Note that 3.65V is the max voltage before they become damaged, not where you want to charge them daily.

1652016752587.jpeg
 

MKalman

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Okay, to confirm, I have not damaged my cells in having the voltages at 3.64V, correct?
 

Rossman

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Okay, to confirm, I have not damaged my cells in having the voltages at 3.64V, correct?
It sounds like they are fine, maybe your power supply is just a bit inaccurate. You're prob top balanced if there is little current going in.
 

AntronX

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Discharge them to 3.4V or below asap. I would not go beyond 3.5V to top balance. There is very little capacity left above 3.5V and degradation is fastest. In normal operation I would limit voltage to 3.4 - 3.45V max. Similarly avoid discharging below 3.15V.

Here is good video explaining that charging above 3.45V is not necessary. Relevant info at 8min 26sec time. (Ignore his 3.8V typo, it should be 3.38V)
 
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740GLE

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How are you measuring voltage of the cells?

From the power supply display? or DMM directly on the cells?
 

MisterSandals

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Okay, to confirm, I have not damaged my cells in having the voltages at 3.64V, correct?
If you are at 3.64V with all in parallel, you are done top balancing.
Label your cells, take voltage readings after an hour and write this in your notes.

You are now ready to assemble your cells into a battery. Well done, that's a successful top balance, all you needed to do.

Keep us posted how it goes.
 

AntronX

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Day to day charging, yes, this 3.45Vpc is spot on. But for initial top balance, 3.60V to 3.65V is the goal.
That does not make sense. The goal of balancing is to have all cells arrive at same voltage at the top of desired state of charge. They will do the same at either 3.5 or 3.65v once all cells absorb to < 0.02C. Now if you are using pre-built 12v battery with internal balancer set to shunt at 3.6V then yes, but if you are balancing your cells manually then you can pick any voltage where cells start to drop charge current as long as you let cells sit at that voltage for some time to equalize cell currents.
 

MKalman

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Day to day charging, yes, this 3.45Vpc is spot on. But for initial top balance, 3.60V to 3.65V is the goal.
Thank you all for your input! I'll follow your advice!

A quick question, as I am making a 48V battery pack, how do I power some 12V fans that are cooling the case? Is it okay to connect to 4 cells only or will this throw the cells out of balance?
 

sunshine_eggo

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Thank you all for your input! I'll follow your advice!

I didn't see this presented.

Everything you need to know, and how to do it right:


A quick question, as I am making a 48V battery pack, how do I power some 12V fans that are cooling the case? Is it okay to connect to 4 cells only or will this throw the cells out of balance?


If you want to completely destroy your battery's top balance and create problems, yes. Do that. Seriously though, the ONLY place you attach a load to a battery is the main terminals. Ever. Period.

If you want fans, get a 48V-12V converter.
 

Trkarl

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Thank you all for your input! I'll follow your advice!

A quick question, as I am making a 48V battery pack, how do I power some 12V fans that are cooling the case? Is it okay to connect to 4 cells only or will this throw the cells out of balance?
You will throw your cells way out of balance. Never do that.
 

Bossrox

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If you are at 3.64V with all in parallel, you are done top balancing.
Label your cells, take voltage readings after an hour and write this in your notes.

You are now ready to assemble your cells into a battery. Well done, that's a successful top balance, all you needed to do.

Keep us posted how it goes.
I agree, that's close enough.
 

Substrate

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Have any of you watched your tail-current (aka absorb) when held at a specific CV voltage as the key to stop and then performed a discharge capacity test rather than letting it go to zero amps?

What I'm saying is that you if you hold your tail-current too long, you can invoke needlessly small cell imbalances that are basically caused by trying to push cells beyond full capacity.

Here's a generic chart. Charge and hold at the CV voltages indicated. Stop at the C/rate indicated and measure capacity.

3.6v (14.4v) = end amps tail current C/20
3.55v (14.2v) = end amps tail current C/60
3.5v (14.0v) = end amp tail current C/100
3.45v (13.8v) = end amps tail current = zero (does not mean forever!)


You may find that when you do your actual test, that the C/n rate might differ *slightly*, like C/30.

The overarching point is not to try and drive your cells to zero tail current, where secondary reactions can cause an abnormal rise in terminal voltage, batteries suddenly going well out of balance, when in fact, if you measured balance at the C/20 rate, (according to the voltage chart) they are IN balance.
 
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