Bottom Balancing? Top Balancing? Charging and BMS

Every cell is different not just within lifepo4 or lico ect. The big ah cells live different than there lower ah brother's of same type ect. So there is no golden number but mine to me. As I live in the range. Cal. Boy. Living in the range or out of range simple aussie AAh not ah.I think more important is starting with the true balance of cells to pack building to start with. I'm not there yet but have many questions ???
 
I do intend to have the monitoring system disable charging if a single cell is going overboard (by controlling a switch that does not make part of the charger).
As for discharge, I'm not sure yet if I'll just issue an alarm sound warning or if I issue an alarm sound warning and cut power. Not sure yet if I want the bike to cut power on me suddenly, even if I know the bat is low. So I will be riding in total piece of mind knowing I'll be warned if something is going out-of-range. Probably have 2 or 3 warning levels so that I won't get caught by surprise (I'll also be coulomb counting). I still have to think about the exact behavior I want to see.
 
I have a 20ah take out 12ah or as much as 15ah 25 miles plus and don't over charge just carry a little extra wieght. Why push the top voltage ?
 
Njay said:
According to "experts" you can take 16Ah (80%) and have a long lived battery.
Click to expand...

Certainly there is a non-linear relationship between regular depth of discharge and cycle life. This is well documented in some of the cell testing done for hybrid vehicles, where, unlike many applications, cycle life is more important than calendar life. The problem is that this relationship is highly non linear. For example, you might get 500 cycles from a cell discharged by 80%, or 5000 cycles from one discharged by 60%, or even 20,000 cycles from one discharged by 20%.

This non-linearity is one cause of problems with so-called "bottom balancing" technique, as all the cells in the pack will always be running at a different state of charge, all the time, due to natural small variations in capacity and charge efficiency between cells. The pack will never ever be balanced so all cells are at the same state of charge, as you can only be certain of state of charge under one conditions, and that's with all cells fully charged and held at the same terminal voltage.

The net result is that some cells in the pack will be operating over a slightly greater depth of discharge range from others, so they will drift further and further away from balance with every charge, and those cells will suffer premature ageing. What's worse, the further they drift away, the worse the problem gets.

If you always charge cells to a notional 100%, then you can at least be sure that when discharging by, say, 80% you're only getting the pack slightly out of balance in one charge cycle, a situation that will be corrected on the next charge cycle. The weakest cell will still be the limit on pack life, but at least you can be sure that there is no cumulative out-of-balance situation developing in the pack over several charge-discharge cycles.
 
Jeremy You said there is no cumulative of out of balance of cells some cells tail off farer each time there are charged or is this just damage cells ( weak ) lower I.R. ? I think if left to cycle at 80% they will drift without bms or balnce charger ? Different type cells different behavior. I'm talking lifepo4 and thinking konion's. behavior. ? I'm triing to bounce back some .038 konions that where slow drained after 5mos. as we sit. Self drain and the 4 single ping pouches in the garage for 2yrs 3.3v ?
 
What I meant was that, if you always charge to cell full state of charge cut off (i.e. use a balance charge, either with a BMS or with a balancing charger) then there won't be any cumulative drift over several charge-discharge cycles. Sure there will be very slight differences in state of charge between cells when the pack is being discharged, but these will be corrected every time the pack is charged.

This is where the so-called "bottom balancing" method falls over, as there will be no correction of this natural very slight imbalance when the pack is charged, so over time the state of charge difference between cells in the pack will cumulatively increase.
 
I bet that particular method of bottom balancing works really good, since you would be top balancing every cycle in my opinion.

You can top balance at any voltage you choose of course, 3.45,3.5,3.65, etc.
 
I've seen "evidence" from all sides so, since I have no hands-on experience, as far I'm concerned all odds are still out. There are too many parameters and conditions (what I call "the context") and I think those have not yet been all identified by people who claim A or B or C.
 
Njay said:
I've seen "evidence" from all sides so, since I have no hands-on experience, as far I'm concerned all odds are still out. There are too many parameters and conditions (what I call "the context") and I think those have not yet been all identified by people who claim A or B or C.
Click to expand...

You're right, evidence is key. Rickard has no evidence, just his word as someone who's clearly not very technically knowledgeable. His suggestion has been examined by people with greater technical knowledge than he and has been found to be flawed. On the other hand, there is a great wealth of real evidence from battery manufacturers and well-respected labs (Sandia, for example) that shows beyond doubt that charging to a set cell terminal voltage gives a known state of charge.

If the "bottom balancing" technique was viable in the long term, then one has to ask why no commercial product with lithium chemistry cells uses it. Pretty much everything from cell phones to large electric vehicles uses cell terminal voltage monitoring to control charge, despite the slightly greater cost of using this method. As a general rule, manufacturers don't add cost to a product without a good reason.
 
Well, I'm still not convinced on any side yet. This "Rickard bottom balance" has a very specific context; and I do see people loosing cells on top balancing too.
 
I checked this out.

http://jackrickard.blogspot.com/2009/11/get-rid-of-those-shunt-balancing.html

I wonder if he knows he can save his batteries from going too low with these ?

http://www.hobbyking.com/hobbyking/store/__18588__Hobbyking_2_8S_Cell_Checker_with_Low_Voltage_Alarm.html

Loud and keeps my cells alive.

( I already got the warning about spamming but I did it anyway) :evil:
 
Yep, if I took every suggestion or assessment I read out here as gospel, I would certainly end up mighty confused. Sometimes even the overwhelming consensus can leave one under served at times... one case in point for my needs being the recommended use of a full featured BMS.

But in this case where I actually tried it (i.e., bottom balancing), even with the understanding it was not likely to be proven useful (kind of stubborn in that regard sometimes), I personally just can't see any advantage what so ever to "bottom balance" in my particular application. It just strikes me as a royal PITA to manually bottom balance even if just occasionally... and then I still have to worry about using a HVC device that actually STOPS the charging... otherwise it would just undo the original bottom balancing effort. And since I usually want to avoid deep discharging my packs anyway, well then it seems even less useful of a strategy for me.

Although, I am glad I took the time to follow this discussion even if it is redundant for me. That's because of that nifty little LVC alarm from Hobby King link that was posted... I think want some for my packs to use when I am on some of my longer journeys. I have just been keeping track of total amp-hours used on my CA to avoid over-discharging up until now. But that has its own risks too.

So good luck with your efforts... there's no substitution for actually trying something and proving or disproving something to yourself as long as it isn't dangerous to life and limb and your willing to cheerfully accept the outcome and/or consequences from trying. :wink:
 
Jeremy Harris said:
... as all the cells in the pack will always be running at a different state of charge, all the time, due to natural small variations in capacity and charge efficiency between cells. The pack will never ever be balanced so all cells are at the same state of charge, as you can only be certain of state of charge under one conditions, and that's with all cells fully charged and held at the same terminal voltage. The net result is that some cells in the pack will be operating over a slightly greater depth of discharge range from others, so they will drift further and further away from balance with every charge, and those cells will suffer premature ageing. What's worse, the further they drift away, the worse the problem gets.... The weakest cell will still be the limit on pack life, but at least you can be sure that there is no cumulative out-of-balance situation developing in the pack over several charge-discharge cycles..

What I meant was that, if you always charge to cell full state of charge cut off (i.e. use a balance charge, either with a BMS or with a balancing charger) then there won't be any cumulative drift over several charge-discharge cycles. Sure there will be very slight differences in state of charge between cells when the pack is being discharged, but these will be corrected every time the pack is charged..
Click to expand...
Again, many thanks in helping me understand the charge/discharge characteristics of my pack, as it is my intention to keep it alive for as long as possible. Mind you, I'm taking this is as corroboration of my basic "simple" approach of using a really good smart charger matched to my 12S AMP20 pack - charge after every use. Charger cuts off when the pack is fully charged. Cells will charge everywher between about 3.4 and 3.7V. Average is 3.65V, as would be expected. I periodically monitor each individual cell, and if unusually low (3.2-3.3V), single cell charge it. Seems to be working for the moment.
 
Bump-Bottom balancing is a confusing term to me, beside the connotation :) .
A bms that balances cells at lvc is that bottom balancing, getting cells to deliver a fixed Ah is that coloumb counting
Soooo confusing.
 
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