diy solar

diy solar

How to Parallel Balancing. (YEP 99% of us is doing it wrong)(PART#1)

electric said:
This is nonsense. You certainly can and should just set your supply at 3.6V and walk away. There is no reason to split up in steps, unless your power supply is not current limited and would shutdown. If you have a good CC/CV supply you can certainly do the balance in one step.
Click to expand...

I think that the difference between just putting 3.65 on it and walking away or doing it in steps may not be significant at all in the long run ... all i know is that the Design Engineer for what is probably the largest LFP battery in the world said that "THEIR" preferred way of TOP balancing is doing it in steps ... i am sure that their is a scientific reason - but it may just be the difference between PEPSI and COKE ... BUT he did say to get the PERFECT balanced cell the step method was the fastest and best ... once again -- not sure if there really is any difference
 
Degradation of electrolyte doesn't occur till 4.2v, and the iron phosphate lattice can handle quite a large degree of over charging. And the threshold on some BMS is 3.7, but most are 3.65.

If cells are charged to 3.6 together, and you do a capacity test and reach full capacity, you are good.

Avoiding upper knee? People have their LiFePO4 connected for months with progressive Dynamics chargers that hold the voltage at 14.6v. is progressive Dynamics/battle born recommendation incorrect? I met the engineering team for r and d and they are qualified experts in battery chemistry.

Do you have any data about avoiding time beyond the knee of the curve? For LiFePO4 specifically. I find data regarding this on nmc, but not LiFePO4. Also, the Marine LiFePO4 guys used to bring this up a lot because the studies wouldn't leave them at high soc (cycle studies would discharge right after they were charged), but there was never any data or proof of degradation beside storage/transportation studies. So could someone post some evidence of this? I have been looking for months and can't find it.
 
Will Prowse said:
I agree that we need to balance at high SOC, but I don't understand the purpose of doing it in steps. They can handle the charge rate just fine. Setting power supply to 3.65v and waiting for current to drop will top balance all of the cells.
Click to expand...

"The key with the PSMTB come from the fact that the cells are essentially full when you get to 3.40V and 0A. This 3.40V threshold is a perfectly safe voltage for the cells so no matter how long it takes to get there will not be causing damage to the cells. Once at 3.40V this means our steps to get to 3.5oV and then 3.60V are much, much shorter than the first step getting to 3.40V. The step to 3.50V is longer than the final step to 3.60V, which happens pretty quickly."
marinehowto.com

DIY LiFePO4 Batteries On Boats - Marine How To

DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals
marinehowto.com
 
Last edited:
See exactly, another marine guy talking about this, but I can't find a study to reference. I think they believe that from the nmc and nca studies. I can't find any proof of this anywhere, and LiFePO4 battery experts have told me keeping at high soc with LiFePO4 isn't nearly as bad as other chemistries. What I want to see, is actual data to support the claims of the Marine crowd. I haven't seen them post peer reviewed data on it at all. And it sounds like they might be drawing these conclusions from other battery chemistry studies. Especially the nmc ones.
 
Last edited:
SolarRat said:
If the cells aren't yet balanced (and the obviously aren't if we are now balancing them), one cell may hang at 3.38v while another zooms up quickly to 3.5v. That higher cell may stay in the upper knee for hours waiting for the lower cell to catch up. Equalizing them at 3.4v first insures this doesn't happen. That's the way I read between the lines?
Click to expand...

I'm confused, If they are in parallel, how could one stay at 3.38 and another be at 3.5?
 
ghostwriter66 said:
I think that the difference between just putting 3.65 on it and walking away or doing it in steps may not be significant at all in the long run ... all i know is that the Design Engineer for what is probably the largest LFP battery in the world said that "THEIR" preferred way of TOP balancing is doing it in steps ... i am sure that their is a scientific reason - but it may just be the difference between PEPSI and COKE ... BUT he did say to get the PERFECT balanced cell the step method was the fastest and best ... once again -- not sure if there really is any difference
Click to expand...

Another interesting note he said was that really only the American's seem obsessed with cell balancing. He said that he only uses BMS for High Current cutoff, Low Current Cutoff, and Low/High Temperature cutoff -- he said that if you get 4 batteries from the same batch that them getting enough out of "whack" (my word) to be concerned about would take a long time ... he says he checks the "balance" on his batteries usually every 6 months but really a year would be fine ... Once again - to each his own ... LoFePO4 is so new - and weird - that even the experts find it hard to completely understand why it behaves in these manners...
 
Last edited:
Also, as long as you are pulling your packs full rated capacity, and the BMS you use has top balancing, the cells are fine. You could actually not balance them at all, and let the BMS do it (might take a long time to balance though if cells are not matched. Which is fine, they will just run at reduced capacity for a few weeks). Every time you cycle the battery, it top balances them to .001 at a high soc, so it's good to go.
 
Will Prowse said:
Avoiding upper knee? People have their LiFePO4 connected for months with progressive Dynamics chargers that hold the voltage at 14.6v. is progressive Dynamics/battle born recommendation incorrect? I met the engineering team for r and d and they are qualified experts in battery chemistry.

Do you have any data about avoiding time beyond the knee of the curve? For LiFePO4 specifically. I find data regarding this on nmc, but not LiFePO4. Also, the Marine LiFePO4 guys used to bring this up a lot because the studies wouldn't leave then at high soc, but there was never any data or proof of degradation beside storage/transportation studies. So could someone post some evidence of this? I have been looking for months and can't find it.
Click to expand...


All good questions that I don't think any of us have the answers to YET. This technology is still in it's relative infancy.

Usually, if I hear something from someone who is more knowledgeable than me on a subject, I listen. If it makes sense to me, I try and follow. The difference in time and effort it takes to do the step method is minimal, so I see no reason not to follow in this case...cause it does make some sense to me. Necessary or not? Time will tell. In the meantime, I think that guy knows more than me.
 
Will Prowse said:
Avoiding upper knee? People have their LiFePO4 connected for months with progressive Dynamics chargers that hold the voltage at 14.6v. is progressive Dynamics/battle born recommendation incorrect? I met the engineering team for r and d and they are qualified experts in battery chemistry.

Do you have any data about avoiding time beyond the knee of the curve? For LiFePO4 specifically. I find data regarding this on nmc, but not LiFePO4. Also, the Marine LiFePO4 guys used to bring this up a lot because the studies wouldn't leave them at high soc (cycle studies would discharge right after they were charged), but there was never any data or proof of degradation beside storage/transportation studies. So could someone post some evidence of this? I have been looking for months and can't find it.
Click to expand...

All i know is that we were told that the number #1 killer of LFP was TEMPERATURE followed closely by Keeping a battery charged at 100% ...

I'm not sure why Balltehorn recommends a "FLOAT" charge at 14.6... That seems to go against everything i have ever read ... I understand some peoples affection toward Battlehorn but their BMS is made in China .. and the LFP Cell is made in China -- and from where I sit all Battlehorn is doing is Assembling it in the USA. They are also the same ppl that said their BMS cutoff was 25 degrees while the rest of the world all agrees you cannot charge below 32.... So I'm not sure if their R&D team is truly doing the research themselves or just parroting what they are told.
 
Will Prowse said:
Degradation of electrolyte doesn't occur till 4.2v, and the iron phosphate lattice can handle quite a large degree of over charging. And the threshold on some BMS is 3.7, but most are 3.65.

If cells are charged to 3.6 together, and you do a capacity test and reach full capacity, you are good.

Avoiding upper knee? People have their LiFePO4 connected for months with progressive Dynamics chargers that hold the voltage at 14.6v. is progressive Dynamics/battle born recommendation incorrect? I met the engineering team for r and d and they are qualified experts in battery chemistry.

Do you have any data about avoiding time beyond the knee of the curve? For LiFePO4 specifically. I find data regarding this on nmc, but not LiFePO4. Also, the Marine LiFePO4 guys used to bring this up a lot because the studies wouldn't leave them at high soc (cycle studies would discharge right after they were charged), but there was never any data or proof of degradation beside storage/transportation studies. So could someone post some evidence of this? I have been looking for months and can't find it.
Click to expand...


This is an extremely long but well worth it read: https://uwspace.uwaterloo.ca/bitstream/handle/10012/12177/Catton_John.pdf?sequence=3

For degradation of LFP's for various reasons, go to page 23. For SOC, see bottom of page 26.

"2.10.1.2.2 Effect of State of Charge (SOC) Testing of various cells at different SOCs but under the same temperature storage conditions has led to the determination that SOC plays a major role in cell degradation. From Ohue et al. results of cells stored at equal temperatures but for different SOCs were found to not age in the same manner [61]. The cells stored at elevated SOCs experienced increased battery degradation compared to those stored at lower SOCs [61]. SOC represents the proportion of ions present on either electrode, thus, for high SOC there is a significant number of lithium-ions available at the graphite electrode to partake in potential side reactions with the electrolyte. "
 
ghostwriter66 said:
They are also the same ppl that said their BMS cutoff was 25 degrees while the rest of the world all agrees you cannot charge below 32.... So I'm not sure if their R&D team is truly doing the research themselves or just parroting what they are told.
Click to expand...

A *lot* of cells I have seen spec down to 25 deg.

1579923675898.png
1579923773528.png

-5C = 23F. These are specs from China of the China Cells.
 
All of the research indicates you can charge at much lower temperature if you also have a very low charge rate, but that would be too complicated to spec. I suspect the -5c shown above is what can safely be done at the max recommended charge rate for these batteries. I also suspect that since they all spec 'around freezing' that the rule of thumb became "Don't charge below freezing" and over time that became "Never ever ever charge below freezing or your battery will instantly become useless"
 
FilterGuy said:
All of the research indicates you can charge at much lower temperature if you also have a very low charge rate, but that would be too complicated to spec. I suspect the -5c shown above is what can safely be done at the max recommended charge rate for these batteries. I also suspect that since they all spec 'around freezing' that the rule of thumb became "Don't charge below freezing" and over time that became "Never ever ever charge below freezing or your battery will instantly become useless"
Click to expand...

I do totally agree that you CAN charge under 0C / 32F ... BUT the issue -- as you stated -- is that you have to reduce the charge to soooo small that it would literally take a day or so (I can't remember the exact figures) ... SO YES if you can control your charger enough to literally trickle charge below 32F then you can ... but I'm going to assume that probably NONE of us has THAT charger ... and really I guess we are talking theory and everyone has their opinion - but I have my stuff stop at 35F ... I'm not trusting a damn soul at this point and i know whoever is right or wrong -- at 35 I am OK ...
 
You must log in or register to reply here.

Similar threads

S
GUIDE to properly Top-Balance and Charge a LFP Battery: Part 1
11 12 13
Replies
258
Views
12K
Helles
H
S
Part 2: LFP Top-Balancing in the context of BMS settings
Replies
4
Views
565
shvm
S
S
Almost everyone is charging their LFP wrong!
2 3 4
Replies
77
Views
7K
shvm
S
Mattb4
I must be doing something wrong.
Replies
18
Views
603
FLD
FLD
J
Top balancing tail current? How low?
Replies
7
Views
427
Jet
J
Back
Top