LiFeP04 Batteries for Solar & BMS

[Sunking]

What is the difference?

There is a huge difference as you charts are showing you. In a Bottom Balanced system all batteries arrive at 0% capacity at the same time making it IMPOSSIBLE to over discharge any one cell. Look at the Lower Right Discharge Curves. That is what you are not catching or understanding.

[Sunking]

Pictures Are Worth 1000 Words.

Perhaps this might enlighten some.

[catharsis]

How do you discharge to -5 Ah?

[karrak]

SK, In the "Bottom Balance" example how do we work out the voltage to charge to?

Simon

[Sunking]

How do you discharge to -5 Ah?

That is Reversal. Technically there is no Energy it is to demonstrate the stronger cells driving the depleted cell into Reverse Polarity. Lithium cells have a unique characteristic in that the further you discharge them, the lower their internal resistance goes. Batteries like lead acid do the exact opposite and not possible for stronger cells to drive a depleted battery into Reversal. You would not even know it happens until you either notice low voltage after you charge, or smoke and fire.

[Sunking]

SK, In the "Bottom Balance" example how do we work out the voltage to charge to?

Depends on what the users is comfortable with and how much money you are willing to throw into Automation.

Most are minimalist. Initially perform a Bottom Balance by connecting all cells in parallel. and use something like a Hobby RC Charger or even just an old salvaged Power Resistors. Once they get down to 2.4 to 2.7 volt range, let the batteries set over night while still in parallel to get them all exactly equalized. Anything from 2.4 to 2.7 vpc is good enough as there is no energy left in that voltage range to drive current. It is just rested open circuit voltage.

From there connect the cells in series and charge. First initial charge you set the voltage set point to 90% SOC charge of xS. Example in a 48 volt system x = 16. You would set the voltage to 54.4 volts. If you are like me and most others will have a Coulomb Counter set to Amp Hours. Anyway you monitor cell voltages and Amp Hours. Once you get near set point voltage the weakest battery will have made itself known being the highest voltage of the batteries. Once the pack voltage reaches 54.4 volts and the batteries saturate, current will stop flowing which is a unique characteristic of lithium batteries. You measure the weak cell and see where you are in terms of Amp Hours and voltage. If rested voltage is a little high, you lower charge voltage a bit, and vice versa.

Only other thing you need to do is set your LVD voltage, and monitor things periodically with voltage checks of cell voltages. If something is going to go wrong, it is going to happen within the first month or two. Any battery manufacture defect is going to show up by then which does happen with Chi-Com cells. What you get is two usable calibrated duel gauges so to speak. Pack voltage actually means something and gives you a rough idea of SOC. Coulomb Counter's are pretty darn accurate and can be calibrated to about any unit you want. I use Amp Hours an don a 100 AH battery all it takes is a glance to know what I have in the tank at any time.

[Sunking]

Be careful, the last off grid person got banned for talking about their system and being called a lot of names.

What do you want to bet that was you, back under a different username there Tasman. He got banned for suggesting unsafe installation practices like you have been warned about. Care to have the moderators compare IP Addresses?

[Willy T]

What do you want to bet that was you, back under a different username there Tasman. He got banned for suggesting unsafe installation practices like you have been warned about. Care to have the moderators compare IP Addresses?

lol, wrong hemisphere.

[Sunking]

lol, wrong hemisphere.

Australian. That explains a lot.

[karrak]

Depends on what the users is comfortable with and how much money you are willing to throw into Automation.

Most are minimalist...

Would you recommend this approach and do you use it?

If something is going to go wrong, it is going to happen within the first month or two.

Why should problems occur in just the first few months?

Simon

[karrak]

Any battery manufacture defect is going to show up by then which does happen with Chi-Com cells.

Language likes this reminds me of to 60s and 70s, when the same sort of comments were made about the Japanese and their products. How is the American car industry going these days? Who is the largest car manufacturer in the world.

Simon

[Living Large]

Language likes this reminds me of to 60s and 70s, when the same sort of comments were made about the Japanese and their products. How is the American car industry going these days? Who is the largest car manufacturer in the world.

Simon

I don't think the two industries are comparable, and as a reader I didn't take that comment as an insult as much as a fact from empirical evidence.

What I read is defects usually exhibit themselves early, and Chi-Com cells have their share of defects.

[Sunking]

Why should problems occur in just the first few months?

That holds true for any manufactured product. In the USA we have Lemon Laws. It is just a fact that mechanical and electrical are prone to defects and does not take long for them to surface after being put into service.

[Sunking]

Language likes this reminds me of to 60s and 70s, when the same sort of comments were made about the Japanese and their products.

It was true back then until Japan was fully developed. The Japanese had to go to the USA to learn quality control. They earned their reputation.

[Living Large]

It is just a fact that mechanical and electrical are prone to defects and does not take long for them to surface after being put into service.

Hence the folly, from an economic standpoint, of extended warranties for most products.