Top balancing, which is the 100% correct and definitive way? are we doing it wrong?

juanmijm

Solar Enthusiast
Joined
Nov 2, 2020
Messages
73
Hello guys, I've been thinking for a long time that I was doing things correctly, placing the cells in parallel and charging them at 3.65v, advising my community colleagues on how to do it, following everything I have read about it, and seen in many videos from different channels, including Will's video:


but today a colleague has put this link:


As they say here, charging the cells beyond 3.40v for a long time can damage them, I would like to know what the experts think here, I think we need to have a definitive and very well explained guide on how to do it correctly.

Thanks friends.
 

time2roll

Photon Sorcerer
Joined
Mar 20, 2021
Messages
2,446
5 amps into 800 Ah probably is a bit slow. I recommend two at a time or get a bigger charger if you are concerned about time above 3.4 volts.
 

juanmijm

Solar Enthusiast
Joined
Nov 2, 2020
Messages
73
5 amps into 800 Ah probably is a bit slow. I recommend two at a time or get a bigger charger if you are concerned about time above 3.4 volts.
it does not matter if you use a power supply or a charger that is capable of giving 200a, when the voltage is close to the established one the amps will go down, and it stays like this for many hours, according to what they say in the forum we would be damaging the cells.
 

Supervstech

Administrator
Staff member
Moderator
Joined
Sep 21, 2019
Messages
6,545
Location
Belmont, NC
it does not matter if you use a power supply or a charger that is capable of giving 200a, when the voltage is close to the established one the amps will go down, and it stays like this for many hours, according to what they say in the forum we would be damaging the cells.
This.

amperage below 3.4V will talker all the amps you can supply, but above 3.4V the cells will trickle the amperage.
 

toms

Photon Sorcerer
Joined
Jul 24, 2020
Messages
811
I think we need to have a definitive and very well explained guide on how to do it correctly.

Speak directly to your battery manufacturer and ask them how they balance cells for their major clients. Then balance your cells like this.

Low current into a LiFePO4 cell that is in its upper voltage knee will damage the cell.

I don’t recommend charging/discharging a LiFePO4 cell without using a monitor capable of disconnecting the cell if it goes out of its safe temp/current/voltage range.

For that reason alone i wouldn’t parallel top balance a group of cells. Overcharging/lithium plating is another reason i wouldn’t parallel top balance.
 

Substrate

Solar Addict
Joined
Apr 28, 2021
Messages
675
Location
SoCal
@juanmijm - that discussion link is good, but quite frequently gets confusing to some when voltage, SOC, and TIME get mixed up. What triggers the confusion is the amount of charge current you are using initially.

Here is the major rule you are looking for starting with some facts:

1) ANY cv voltage from 3.45v to 3.65v for LFP and allowed to continue charging as the "tail current" (aka absorb sometimes known) falls will result in a cell that is 100% charged.

2) That 100% SOC will be achieved faster at 3.65v. It will take much longer at 3.45v, but it will happen.

3) Using a charge source that is < 0.1C will result in cells becoming 100% charged, albeit slowly by the time they reach 3.45v. Essentially, at <0.1C, you are already in low "tail current" charge levels starting out at the beginning of the process! This sometimes manifests itself as an inability to go much higher than 3.5v no matter how long your charge is. (bad)

4) If you use a charge current on a relatively discharged bank of < 0.1C, and you let the charger reach 3.65v, then you are actually exceeding 100% SOC with an overcharge!

(In my old EV days when doing this at slightly higher voltages and low currents, we were actually trying to balance based on secondary-reactions! [overcharge voltages] Really bad!)

So there is your rule:
If you use a commonly recommended CV for top balancing of 3.6 to 3.65v, then your charge source should be at least 0.1C capable when starting from a relatively normal discharged state.

Ie, a 10ah programmable bench charger, should not be used on a battery larger than 100ah.

This is best practice. For those that DO care about spending the least amount of TIME when charging or balancing, then the 0.1C minimum charge rate for a 3.65 CV to get the charge over and done with quickly, is prudent to follow.

(What I and they are saying is essentially you can do more harm spent at low voltages, and long times, than you can at a higher cv voltage, and a short time spent getting there. To get to 3.65v without actually overcharging past 100% SOC and entering secondary-reaction territory, you need a minimum of 0.1C to start)

The BIG ISSUE!
So what happens when you try to balance 8 paralelled 280ah cells (now 2240ah !). Not many of us have our minimum 224ah charger handy.

So you simply do the best you can. And limit your initial balance to a one-time only event. Larger current is better, but may not be practical to have. Just do your best, but don't do it often with a 5ah charger! :)

A solution to < 0.1C chargers - stepped balance.
The solution when using < 0.1C is to do a "stepped balance". For some this is too much, but if you bought yourself a programmable bench supply, why not make the most of it?

* You START your CV set to 3.4v. Don't touch it again! Take a reasonably long time if you like until tail current falls way off, since overcharge generally doesn't happen at 3.4v.
* DISCONNECT the bench charger, reset the CV voltage to 3.5v (and leave it alone!) until tail current falls off again.
* DISCONNECT the bench charger, reset the CV to 3.65v (and leave that alone!) until the tail current falls off. This gets you high in the knee, where balancing does the most good.

If you are impatient, and just start swinging and changing your voltages around to push more current because you are impatient, then all it can take is one distracting phone call when you have set to 3.65v, and return to find the actual value MUCH higher! Ie, once you set your CV, don't touch it!
 
Last edited:

Gazoo

Dumb Dumb
Joined
May 12, 2020
Messages
2,811
Location
Texas
Parallel top balancing should be a one time event. I do not believe the amount of damage to a cell is measurable, if there is any damage at all, no matter what method is used. There is a lot of data missing and much of the data that's avaiable is old. That's my opinion. Others have, will, and are welcome to disagree with me.

I do not like the step method. The common 5 or 10 amp power supplies will not supply their rated output and will remain in CV mode if set at 3.4 volts, even if using beefy cables and terminals. In my case when I parallel top balanced my 8 280ah cells, I set my PS to 3.65 volts and when the PS switched to CV mode it only took about 4 hours to finish the charge when the tail current reached 100ma's. It doesn't take much time to go from 3.4 volts to 3.65 volts.

Many of us have parallel top balanced the traditional or old fashioned way and have had zero issues. Having said that I do believe damage could result if holding a cell at 3.65 volts for a number of days. Also it's always important to remind everyone to set the voltage of the power supply before connecting to the cells.
 

Substrate

Solar Addict
Joined
Apr 28, 2021
Messages
675
Location
SoCal
Sure enough - not everyone's cup of tea and you bring up some good points which I agree with.

Since part of my background is Pb, I guess I can't let that go - by watching people destroy expensive banks with week-long charging with auto-parts store maintainers, and other simplistic solutions which do more harm than good.

I've just seen many good batts tickled-to-death, so my inclination is to hit what I paid for hard and earn their keep. :)
 
Top