Tech Tricks: Homemade Balance Charging Cradles.

[KiwiMark]

Also, since you are charging loose cells, there is no need (or advantage) to balance charge them in series - just charge them in parallel. Here's how I charge 1-8 cells at the same time.

There is one obvious disadvantage to charging in parallel - you have absolutely no way of knowing how much capacity is going in each cell. If they are all the same model/age cell that are drained together you could GUESS that they will each take about the same as each other, but if one cell is failing you wont be able to tell when charging.
Balance charging enables you to get data on each cell - you can even graph it to see how each cell compares to the rest.

I'm not saying that parallel charging wont work just fine, just pointing out a drawback.

 

[benckie]

thats not correct and only should be used if all batteries have the same voltage befor charging

 

[SilverFox]

Hello Benckie,

thats not correct and only should be used if all batteries have the same voltage befor charging

What is not correct?

Tom

 

[Mr Happy]

DasFriek, from what I understand, there is no such thing as "break-in" cycles for LIxx chemistry. It will actually hurt because battery loose capacity for every cycle. No way around it.

Not necessarily. Some lithium ion cells show improved internal resistance after a few cycles from new. There can therefore be an advantage to "breaking in" lithium ion cells to get the best performance out of them.

 

[45/70]

Not necessarily. Some lithium ion cells show improved internal resistance after a few cycles from new. There can therefore be an advantage to "breaking in" lithium ion cells to get the best performance out of them.

Hi Mr H. I've seen this suggested from time to time, however I've never seen where any manufacturer recommends a "break-in" for Li-Ion cells. Maybe this applies to the newer high cap cells with the nickel safety layer?

In my experience with LiCo, LiFe, and LiMn cells, Ive never seen any significant improvement after cycling. I've always thought this suggestion was just a leftover from the NiCd/NiMh days, or possibly due to the tester's (myself included) cells being warmed up during the testing, or perhaps even the runtime in a light increasing due to the Vf of a new LED light lowering, due to the breaking in of the LED itself.

In any case, I seriously doubt that fully discharging and recharging, such as is the practice for breaking in NiCd/NiMh cells, is necessary. Normal use, eg. discharging to 50% and recharging, should suffice to "break in" Li-Ion cells, if it is even necessary.

If you have any links referring to this "breaking in" of Li-Ion cells, I'd appreciate it. As I said, I'm not familiar with the newer cells, such as the Panasonic "NNP" cells. There may very well be a different regimen for these.

Dave

 

[KiwiMark]

In my experience with LiCo, LiFe, and LiMn cells, Ive never seen any significant improvement after cycling.

I have received new Li-ion cells, charged them, drained them (noting capacity received from them) and then charged them again - this is something I often do so that I know what the capacity of the cell is. I have tried a 2nd discharge occasionally - every time I have tried that I get about the same capacity as from the first discharge, no significant improvement. This would seem to match your experience which makes me think that you are correct.

I have heard of letting a laptop or phone or other 'smart' device run flat and then fully charging it so that the battery management software can get some better data to calculate remaining run-time from - but I doubt that this gives any improvement in actual run-time achieved.

 

[45/70]

Hi Mark. Well that makes two of us anyway. As I said though, some of the newer Li-Ion cells may actually be different.

I meant to respond earlier to your previous post.

There is one obvious disadvantage to charging in parallel - you have absolutely no way of knowing how much capacity is going in each cell.

While I agree with you that a balance charge is probably better, when parallel charging you can usually tell if there is a "bad" cell by checking the cell's voltages after letting them sit for a while after charging. If a cell's voltage has dropped significantly, compared to the other cells that were charged in parallel with it, this is a sure sign of an under performing cell.

Dave

 

[Mr Happy]

In response to Mark and Dave:

I have read that while the capacity might not improve, the voltage under load/internal resistance might improve. Also I don't mean break-in in the C9000 sense, just normal usage with a few charge/discharge cycles.

Here is one reference I came across recently, relating to this:

http://www.youtube.com/watch?v=2rUoKBrUPnA#t=04m00s

Of course this might be bullshit on the part of the cell vendor, but it's not the only place I've come across the idea of breaking in lithium ion cells. I don't know the outcome of the particular story in the video.

 

[benckie]

Hello Benckie,



What is not correct?

Tom

i stole this from another site from another person as i cant be stuffed

Im a fan of series charging for this reason, because of the increased convenience, and no need to worry about differences in charge levels. Even the slightest voltage difference means large charge capacity differences, and connecting these cells together is effectively shorting the cells until the capacity differential is conducted, limited by the internal resistance of the battery.

As a Theoretical example, this gives an idea of the kind of current involved. If there is a voltage difference of about 0.5v, given a total internal resistance of 0.05 ohms, then the instantaneous current flow will be (V/R) = 0.5/0.1 = 5 Amps. So the instant you connect cells in parallel with a voltage difference of 0.5V, you will be discharging one cell at 5A, and charging the other at 5A for a short period.

A difference in voltage between a fully charged cell and an empty cell (3.0v - 4.2V) = 1.2V. Divide this by 0.1 Ohms and you could get an instantaneous current of 12A.
Better cells (like my 2000mA capacity RC lipo packs) with a measured internal resistance of 0.005 - 0.01 Ohms would share a current of 20A for a difference of 0.1V! (240A for a difference of 1.2V)

there is a reason why all the hobby li-xx rc battery packs are normaly in series and balance charging is used, just my thoughts do what works for you.

 

[Lightfoot98]

As to what okwchin says,

I've checked the inrush current of cells that were 0.5V different, and only had about 230-250ma current.
It quickly tapers down to lesss than 50ma in a few minutes.

Some a little more, some a little less.

I'm comfortable with paralleling cells with up to 0.4v difference going onto the charger for parallel charging.

 

[Lightfoot98]

There is one obvious disadvantage to charging in parallel - you have absolutely no way of knowing how much capacity is going in each cell....

The cell will only accept what it needs.

My example with the lights...

Plug a 40watt, a 60watt, a 200watt, and a 10 watt lightbulb into the same electrical outlet, it will only take what is needed.

The voltage will rise according to the highest capacity cell, and the other cells are just along for the ride.

A 1000mah cell and a 3000mah cell charged in parallel will be just fine. The larger capacity cell will take more, the lower capacity cell will take less.
The charger is only supplying the current at a specified voltage.
Both cells will complete the charge at the same time at the same ending voltage.

 

[rmteo]

...there is a reason why all the hobby li-xx rc battery packs are normaly in series and balance charging is used, just my thoughts do what works for you.

The reason why hobby packs are charged in series (with balancing) is because the packs cannot be disassembled for parallel charging unlike loose cells used with flashlights. Take a look at a typical 3S2P pack. ONLY the 3S is section is series balanced charged (note the 4-lead balance connector). See here http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7655

 

[45/70]

As to what okwchin says,

I've checked the inrush current of cells that were 0.5V different, and only had about 230-250ma current.
It quickly tapers down to lesss than 50ma in a few minutes.

Some a little more, some a little less.

I'm comfortable with paralleling cells with up to 0.4v difference going onto the charger for parallel charging.

We're getting OT here, as this thread is about series charging with balancing. Just the same, I thought I'd link to a thread by SilverFox where he explains some testing he did parallel charging Li-Ion cells. The conclusion from his testing was, that as long as the cell voltage of the cells being parallel charged is within 0.5 Volts, there is no problem.

I myself prefer that the maximum voltage difference between the lowest cell voltage and the highest be no more than 0.1 Volt. This way the higher voltage cell, or cells, are not being discharged any more than if the cells were charged in series with the typical "on board" balancing feature, that most hobby chargers offer.

Also, note that taking the maximum 0.5 Volt figure, if you think about it, there are not too many situations where you might be charging cells with this much difference in voltage. You would pretty much have to be charging fully discharged cells along with fully charged cells to run into trouble parallel charging.

Dave

 

[SilverFox]

Hello Benckie,

The theory is sound as far as it goes...

The problem is that it is looking at an instant of time and can not be substantiated by actual measurements. As soon as the two cells are connected there is a surge of current, but Li-Ion cells are fully capable of handling surges without damaging the cells. Surge performance is different than constant current performance. When cells are connected in parallel the voltage of the higher voltage cell drops and the voltage of the lower voltage cell increases.

A cell that is discharged to 3.0 volts should rebound to 3.6 to 3.7 volts once the load is removed. This means that if you happen to parallel a fully charged cell with a fully discharged cell the voltage difference is only 0.6 volts initially. If you are discharging the cells before charging them in parallel, the voltage difference ends up being quite small.

When charging multiple cells it is important to keep the cells in balance. You can do this by charging in series and using balancing taps or by parallel charging.

Tom

 

[rmteo]

Not saying that anyone should try it but I have connected a fully-charged and a discharged 18650 in parallel (one several occasions and subsequently parallel charged them with a hobby charger) and there is absolutely no drama whatsoever.

In fact, you would be hard pressed to tell which cell is which when they are connected. You can tell them apart only by disconnecting them and measuring with a DVM. The discharged one will be at a lower voltage.

 

[45/70]

That seems to go along with what Tom found in the post I linked to above, rmteo.

The reason I prefer to have the cell voltages much closer is that the discharging of the higher voltage cell, or cells that takes place is cumulative. For example, If you pair two cells together that are 10% different in capacity, after 10 charges this works out to the same as if you had actually used the cell one discharge cycle, but you didn't get any use out of it!

The same applies when charging cells in series with balancing, as well. Unfortunately, as far as I know, all balancing chargers, as well as separate balancing units, discharge the higher voltage cells, rather than charge the lower voltage cells, to balance the "pack". So, whichever way you charge cells, it's best if the voltages are close, as if it isn't, you're just adding that much more wear to the cells when charging. It's a minor amount, I know, but......🙂

Dave

 

[rmteo]

On a personal level, I have no cause to charge more than one cell at a time as I do not have/use multi-cell lights, so loss of capacity is a non-issue for me. However, for someone who has something like a TK35 it probably won't matter much either as each pair of cells are going to be close (in voltage) prior to charging.

I think that parallel charging is the simplest, safest and most fool-proof method (assuming you have a hobby charger - which is also needed for balanced series charging). The reason I did those tests was to ensure that charging multiple same type cells in parallel can be safe even if you (accidentally) have charged and discharged cells in the group.

One other caveat - this was done using unprotected (16340 and 168650) cells (I do not have/use protected cells) and may not be the same with protected cells.

 

[KiwiMark]

The cell will only accept what it needs.

Yes, parallel charging should work fine. But what I was saying is that you don't KNOW how much capacity is going into each cell. When I charge a single cell the charge finishes with a mAh reading for how much capacity the charger put into the cell - I could take a note of that and see how the cells capacity holding changes over time. Some hobby chargers are able to provide a graph through a computer for each cell while balance charging. When parallel charging several cells you just don't have that information available - the charger has no idea of how much of the power it is pumping out is going to each cell. As far as the hobby charger is concerned it is just charging one big cell.

So it all comes down to whether you just want to get your batteries charged or whether you want the info on each cell.

 

[benckie]

we got back to the start and what most have been saying if the cells are close in voltage parrallel charge i think i said that atlest once, test each cell and go through them all to find the ones that have close voltage and charge them in parrallel, or just put them all together and series ballance charge, i have 3 multible cell torches that run in series and after hard running the cells are always a little bit to far out for my liking for parrallel and my expernice from lipo,s makes me like series.

but people do what ever floats your boat

 

[Lightfoot98]

but people do what ever floats your boat[/B]

Don't worry, we do.