QuoteBottom balancing has become the method of choice. You can read more about the rationale and concept HERE and HERE.
I was intrigued to see what 'bottom balancing' is.
Turns out that the guy is destroying cells because his system is NOT balancing, ie trying to keep all the cells in a series stack all at the same voltage (ie all at the same SOC) and therefore some cells would get reverse biased and be destroyed.
Firstly, all cells in a large series stack should be monitored and the pack shut down if any cell reaches a minimum cell voltage. Yes, you do not get the max capacity but it stops cells from being destroyed.
His system is using the full capacity range of the cells. In EV systems, this is unusual because EV application normally require a minimum sevice life. The main strategy to acheive long life is to limit the max and min SOC to 20% for min and 80% for max. By doing this, again it avoids bringing cells down to the point where they get reverse biased.
Bottom balancing is bring all the cells in a series stack to the voltage that is 0% SOC
It is the experience of many eBike and EV builders that charging Lithium batteries below freezing temperature causes irreversible damage. I personally damaged my eBike LiFePO4 pack that way. This does not happen to lead acid batteries.
The fact -based on many decades of experience - is that deep cycle flooded lead acid batteries can withstand abuse in both extremes of temperature and charging/discharging without suffering irreversible damage. Period. Full stop. The same simply cannot be said of lithium batteries. Why else all the concern about cell monitoring, balancing, and use of BMS for lithium and not for LA?
Your are not understanding the concept of bottom balancing correctly.
It is the experience of many eBike and EV builders that charging Lithium batteries below freezing temperature causes irreversible damage. I personally damaged my eBike LiFePO4 pack that way. This does not happen to lead acid batteries.A BMS of any Li-ion battery pack should prevent charging the pack at too low temperatures. One of the problems however is that most of these packs are made in China and have low quality when it comes to the BMS functions.
QuoteThe fact -based on many decades of experience - is that deep cycle flooded lead acid batteries can withstand abuse in both extremes of temperature and charging/discharging without suffering irreversible damage. Period. Full stop. The same simply cannot be said of lithium batteries. Why else all the concern about cell monitoring, balancing, and use of BMS for lithium and not for LA?Lead-Acid batteries need some care during charge and discharge. If you get the charging and discharging right then an LA battery will last very long and don't get problems due to sulphation.
QuoteYour are not understanding the concept of bottom balancing correctly.
I think I understand what is meant by 'bottom balancing'.
I am just pointing out that the circumstances that drove the inventor to come up with the idea are dubious.
Basically, it sounds like he was driving around an EV without any BMS to protect the cells.
The good news is that it turns out that contrary to conventional wisdom, for these system a BMS is not needed and may actually make things more prone to failure. The answer is to bottom balance the cells.
Uhmm. Read the first link. I don't get it.
Correct me if I'm wrong.
Bottom balancing has become the method of choice. You can read more about the rationale and concept HERE and HERE.
That's about balancing. Then, even the cell-level monitoring is not actually a hard requirement; definitely not a generic "must". Companies such as BOSCH do not do it on many of their 6s and smaller li-ion packs.
What the heck am I missing?
I was expecting some fancy system of taking power from good cells and giving it to lower voltage cells
balancing is a must for safety and longevity reasons;
You MUST balance a lithium pack in series in order to charge it safely.
What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series. When a cell in the stack reaches man V, charging MUST be stopped. So yes, high S packs must be balanced during charging. Or be shut down when a cell reaches max V and the stack remians unbalanced, and gets more unbalanced over time.
You MUST balance a lithium pack in series in order to charge it safely. You can do that with individual cell protection with resistive dischargers per cell, but this will limit you charging current.
What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series.
I think that some are missing that battery voltage is not equal to charge.
QuoteI think that some are missing that battery voltage is not equal to charge.Not exactly. But cell voltage of a Li ion battery is a function of charge, and the amount of stored charge is a function of voltage.
Definitely not. You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.
Any current in or out of battery changes voltage you can monitor.
You have a choice of allowing for Max-Min resistance change in measured voltage
Or
Measuring internal resistance.
To compute internal resistance requires measuring Current & Voltage over a short time with different current load.
The internal battery charge will change little over short time, so internal voltage will change little.
There is a clear lack of understanding compounded by dumbasses.
In every field there are dumbasses who are deemed experts.
You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.
Your link explains nothing. Explanations that are incomplete.
It’s always revealing when someone exhibits such strong emotional responses to something new to them.
Why the rage?
The rage is at you, specifically, repeatedly stating things without backing them up.
You have used this link as your support, but this emporer is wearing no clothes.
Please explain what the hell you are claiming as facts
You have disclosed your work in the industry.
You have not thought this all the way through, C. This a red herring. Computing the internal resistance allows you to calculate the energy available at a given load. But it is not necessary to include that into cutoff limits. On the low end, the cell with higher internal resistance will sag in output more than the others. And you will still safely cut off the output when it reaches the lower limit. Maybe it cuts out a little earlier than it could have, but at least it's not a safety issue.
You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.
Yes, absolutely right. I believe this is part of the reason why traditional “ top balancing” of series strings of large LiFePO4 so often fails.