Why is rotating batteries recommended?

[Mooch]

Hello,
I have heard, from more than one source, that after charging batteries used in series should not be placed back into a vaping device in the same positions. They said that the "energy comsumption" or loading is different for each battery in a series string so you need to rotate them each time after charging in order to equalize this imbalance.

I know a bit about electronics and this doesn't seem possible. By definition, each cell in a series string of cells will output the same amount of current. How can the loading be unequal?

The capacitiy of each cell will be different and that will cause the voltages to be unequal as each cell is discharged. But this isn't dependent on the location of the cells in a circuit. Or, if cells are charged in a multi-channel charger and each channel charges to a slightly different voltage I can see why rotating would be advisable. But, that's for charging and not during use.

Does anyone know why some folks are recommending the rotation of cell positions every time you put them back into your device after charging? This was for series-connected cells, not parallel.

Thanks!

 

[Martnargh]

I dont. When i pull out batts from a series mod theyre discharged exactly the same. Sometimes the difference will be a hundredth of a volt. Proper series should not overpower one over the other i think youd only want to rotate if theyre stacked but im not even sure if it necessarily applies there either.

 

[Mooch]

I dont. When i pull out batts from a series mod theyre discharged exactly the same. Sometimes the difference will be a hundredth of a volt. Proper series should not overpower one over the other i think youd only want to rotate if theyre stacked but im not even sure if it necessarily applies there either.

I'm new to vaping lingo but I thought stacking cells meant that they are in series. Not true?

 

[Unforeseen]

Moved from Simple Solutions to New Members

 

[bwh79]

Not an expert, this is just a guess, but I would think that it's because each battery in the series has a small amount of internal resistance, and so each one is operating under a slightly different load than the one before/after it.

 

[bacc.vap]

I'm not the expert on stacking batteries, but here's a link to an interesting post. They're also other good links for info on the subject in the thread. Where did the battery "rotation" idea come from in dual battery mods? | E-Cigarette Forum

 

[Mooch]

Not an expert, this is just a guess, but I would think that it's because each battery in the series has a small amount of internal resistance, and so each one is operating under a slightly different load than the one before/after it.

Hmm...the internal resistance differences would cause a difference in cell's voltage under load but the load each cell experiences is still the same though. The same amount of current flows through each no matter what.

 

[Robert Cromwell]

Cell rotation in a series setup should not make any difference at all. Whatever minor differences are in the 2 batteries they will still have the same slight differences no matter whether they are first or last in the string.

 

[Mooch]

I'm not the expert on stacking batteries, but here's a link to an interesting post. They're also other good links for info on the subject in the thread. Where did the battery "rotation" idea come from in dual battery mods? | E-Cigarette Forum

Thanks for that link!
I'm embarrassed to say I didn't find it when I did my search before posting.
Seems like there might have been a reason to rotate if using a particular chemistry of Li cells (protected) but I can't think of any reason why those cells would experience a problem based on their physical position in a series string unless uneven heating of the cells was involved (due to proximity to the atty, etc.). I've been working with series Li packs (all chemistries) for many years now and there has never been any concern with having to rotate cells. In fact, just about every Li pack there is has fixed-position cells that cannot be rotated. And there's no issue there.

As the link you posted noted, cell imbalance can cause all sorts of issues but that's independent of cell location in a series string. But, if we remove cells to charge them individually then cell balancing isn't a problem either as the cells get balanced every time we charge them.

 

[Thrasher]

Damage from cell reversal-

Subjecting a discharged cell to a current in the direction which tends to discharge it further, rather than charge it, is called reverse charging. Generally, pushing current through a discharged cell in this way causes undesirable and irreversible chemical reactions to occur, resulting in permanent damage to the cell. Reverse charging can occur under a number of circumstances, the two most common being:

When a battery or cell is connected to a charging circuit the wrong way around.
When a battery made of several cells connected in series is deeply discharged.
In the latter case, the problem occurs due to the different cells in a battery having slightly different capacities. When one cell reaches discharge level ahead of the rest, the remaining cells will force the current through the discharged cell. This is known as "cell reversal". Many battery-operated devices have a low-voltage cutoff that prevents deep discharges from occurring that might cause cell reversal.

Cell reversal can occur to a weakly charged cell even before it is fully discharged. If the battery drain current is high enough, the cell's internal resistance can create a resistive voltage drop that is greater than the cell's forward emf. This results in the reversal of the cell's polarity while the current is flowing. The higher the required discharge rate of a battery, the better matched the cells should be, both in the type of cell and state of charge, in order to reduce the chances of cell reversal.




Most of what you heard refers to the older days when stacking cells in a mechanical. Because there were no electronics to tell you the batteries are getting too low.

A reverse charged cell could go into meltdown or explode when recharged.


This is also the reason you see many pull or recharge hi drain series batteries at around 50%


Since the forward most battery carries the most stress it will also receive the greatest amount of wear, changing position just guarantees the batteries wear out more evenly lessening the chances of reverse charging because the forward battery is less likely to run down before the others if they are kept equal

 

[Martnargh]

I'm new to vaping lingo but I thought stacking cells meant that they are in series. Not true?

Series is when theyre both connected in same circuit. I think connection differs when theyrr both positive in the same direction (sigelei 150) and when the positive is opposite (hex ohm v2) direction but i get same results either way.
When theyre stacked i believe top batt takes the brunt of the charge but i could be wrong, ive never stacked batteries.

 

[Mooch]

Damage from cell reversal-

<snip>

Most of what you heard refers to the older days when stacking cells in a mechanical.
A reverse charges cell could go into meltdown or explode when recharged.

This is also the reason you see many pull or recharge hi drain series batteries at around 50%

Since the forward most battery carries the most stress it will also receive the greatest amount of wear, changing position just guarantees the batteries wear out more evenly lessening the chances of reverse charging because the forward battery is less likely to run down before the others if they are kept equal

But cell reversal, for any particular cell, will occur no matter where it is in a series string of cells. Rotating cells won't have any effect on that. I agree that it's a very not good thing if it happens though. A strong reason to always use some sort of balancing method (either a balancing charger, BMS chip/board, or individually charging the cells).

Why does the forward most cell undergo the most stress? It carries the same amount of current as the other cells.

 

[Martnargh]

I have no knowledge of stacking cells and the effects that take place. Just thingd ive heard. Never had a mod that allows or requires stacked batteries.

 

[Thrasher]

~~~~~~~~~~~~~~~~~~

Edited for wrong content( my bad)

The idea behind rotating is so any unbalance no matter how small means one cell is not always pushing, Rotating gives them all a chance to wear evenly

 

[Mooch]

Because the anode or positive pole is now producing in excess of the 4.2 volts it was designed for, not a bad thing, but at the end of the chain that forward cell is now pushing up to 8.4 v out of its tip( in a 2 cell circuit) since the forward cell is pruducibg 2x the voltage things like heat, chemical stress etc are also higher.

But each cell in a series string only produces 4.2V (or less). The total string voltage is the sum of the individual cell voltages but each cell is only creating, and experiencing, the same voltage it would if it were operating alone. It's never dealing with more than its own voltage no matter how many cells are in series.

The reason we read 8.4V at the most positive point of a 2-cell string is because the top cell has its cathode at 4.2V (which is the anode of the bottom cell) so its anode then has to be at 8.4V because of the 4.2V the top cell is creating. The top cell still only "sees" 4.2V. It doesn't start at zero volts at one end of the cell and go to 8.4V at the other end. It starts at 4.2V and goes to 8.4V. It experiences the same 4.2V that the bottom cell does (assuming an "ideal" cell).

The voltage-related stresses the two cells see are the same.

 

[Thrasher]

Its getting late and your correct the theory is the front battery has to handle the brunt of the current load not the voltage. I read something out of context there.

, I don't study battery theory and I don't stack cells. i dug up some old posts

So I'll state it again. rotating was to make sure one wasn't always pushing.

And befor you say anything lol yes it is the same current but if the front cell is doing the work that means its wearing out faster then the back cell.

If you want to debate this stuff head over to Wikipedia, battery university candlepower forums and reddit, don't shoot the messenger lol

 

[Mooch]

Oops, sorry Thrasher, you edited your post as I was typing mine out.

What do you mean by one cell always pushing and the other always pulling? Both are producing their own voltage no matter what their position is. How does this pushing/pulling cause different rates wearing down of the two cells?

 

[Mooch]

Its getting late and your correct the theory is the front battery has to handle the brunt of the current load not the voltage. I read something out of context there.

but I don't write this crap, I don't study battery theory and I don't stack cells. A question was asked and i dug up some old posts

So I'll state it again. rotating was to make sure one wasn't always pulling and the other isn't always pushing.

And befor you say anything lol yes it is the same current but if the front cell is doing the work that means its wearing out faster then the back cell.

If you want to debate this stuff head over to Wikipedia, battery university and reddit

Sorry if I am frustrating you. Not trying to debate, just understand what you mean when you say certain things.

 

[Thrasher]

I hear ya man and I'm glad when someone calls out my mistakes, I'm not perfect and I try not to act like I know everything. So I don't have a problem being called out if I post the wrong info. Glad you saw it actually as I feel stupid if I post wrong stuff and no one says anything lol

All I know is this is a long standing widely believed theory.( about current not voltage)

And if ya dig around on CPF you can see pics of what happens when stacked cells go south. Dude almost lost his hand and destroyed half the kitchen

So in that respect I personally would adhere to the steps the real experts recommend.

As for pushing, didn't you read any of the post on reverse charging?

If one cell is worn or can't hold the same voltage as the other cell from being worn out more the other cell is going to try and push it anyways

 

[Mooch]

I hear ya man and I'm glad when someone calls out my mistakes, I'm not perfect and I try not to act like I know everything. So I don't have a problem being called out if I post the wrong info. Glad you saw it actually as I feel stupid if I post wrong stuff and no one says anything lol

All I know is this is a long standing widely believed theory.( about current not voltage)

And if ya dig around on CPF you can see pics of what happens when stacked cells go south. Dude almost lost his hand and destroyed half the kitchen

So in that respect I personally would adhere to the steps the real experts recommend.

I completely agree, and thanks. I appreciate the time you took in answering my posts!
I have had a couple of LiPo conflagrations and several cell "jellyroll" events here over the years (testing cells for clients). Very, very serious business!