I don't know how clear I can be about this: If you don't bottom balance your batteries you risk loosing the whole pack. When one battery goes dead at the bottom of it's capacity it takes all the others with it eventually.
But be my guest whoever does not understand or follow this procedure eventually will pay the price of the whole pack.
-First Bottom Balancing the cells is only done once in the life of the pack unless you add more cells to the pack in which case you want put all the cells at the same level from the bottom of their capacity.
-Second I never said completely discharge the cells. You said that which is wrong. I said and still do: 2.75 volts per cell to be safe. You bottom balance at that voltage NOT "0 volts".
-Third I am sorry if I am blunt but I have been doing this since the 80's with radio controlled airplanes. When you loose an aircraft at 80mph over the people,s heads because of batteries failure you start investigating the reasons why.
-Fourth When you say and I quote: " it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime" you are speaking about preferences not facts. People preferred SUVs, war and politics and think they are safe yet the figures show the contrary.
-Fifth on the HVC side you are right you should cut off once the first cell reaches that state. Yet you'll be surprised what you find if you measure them individually 24hrs later. Do the test and verify my words. In any case the Cubans have a saying around here that goes like this: :It's not one thing or the other but the opposite of both". That goes only to say that both systems combine will do the trick. I say Both System Combine... just in case.
-Sixth and probably this is the most important since there is a danger factor. Cells will not take more energy than what their chemistry allows on HVC but will go downhill in a hurry if one goes through their nominal low bottom threshold. When that happens it happens in a steep curve. Look at the manufacturers specifications it's there.
Last this is really a controversy and I am not here to antagonize but I am just looking at the facts take this blog for example I think you'll find it amusing how with the same facts they arrive to totally opposite conclusions:
Endless-sphere.com • View topic - Bottom balancing?
There is an endless discussion about how arrogant Jack Rickard is about this matters. Who cares? Basically people who care more about personalities than science. Yup he might arrogant but I think he is right. Furthermore I see he is right. And it is kinda convincing when you can see things don't ya?
So look at the graphs on that blog and tell me what you notice on the graph.
There's top balancing and bottom balancing. Now this is math. Numbers no bull. Tell me where the curve is flat and I'll tell you where you have the most average energy.
Rest my case.
Quote:
Originally Posted by nlc
I done some search about "bottom balancing". All that can be read on the various debates confirms what I though : I really don't see any advantage of bottom balancing !! On the contrary, there are severe disadvantages :
- You need to discharge all cell at the LVC, it's a waste of energy and it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime
- You also need to cutoff the charge when the first cell reach the HVC, because they will not reach the same full voltage in same time if cell are not completely identical (capacity, internal resistance, temperature ...), and they are not.
- When discharging the pack, you also need to check the LVC on each cell, because cell cannot be identical, even if same amount of Ah have been entered in all cell, the cell with higher internal resistance will reach the LVC before the other.
- With bottom balancing, you completely discharge all cell, thus at the next charge, you cut off the charge when the lower capacity cell reach the HVC, but at this time the higher capacity cell are not completely charged and their voltage is not at full voltage, thus your pack is not at full voltage. Thus when discharging, for a given power you will need higher current...
In fact, I cannot understand why there is a debate between top or bottom balancing, because I cannot see any advantage of the bottom balancing method  If anyone has a link to an explanation about the advantage of bottom balancing I am interested !!
Anyway, in my opinion, balancing is not really mandatory, the global pack capacity will alway be in any case the capacity of the lower cell. Just cut charge when the first cell reach the HVC and cut the discharge when the first cell reach the configured LVC and your pack will alway be healty, because no cell can be over discharged or overcharged.
But an intelligent top balancing (balancing during charge and not at the end) can add an advantage, have the maximum of cells at their top voltage at the end of charge, to maximize the global voltage of the pack. |