3x18650 LiCo - Series charge, or parallel?

[45/70]

I have a bunch of 18650 LiCo's (protected and unprotected) that I'm matching up for use in sets of three. While their performance characteristics are going to be close, they of course won't be identical.

Since these cells are individual cells, not a pack, which is the best way to charge them with a hobby charger? Three in parallel, or three in series with balancing? I'm assuming parallel would be best? I have converted charging bays that can be setup either way.

Thanks,

Dave

 

[TranceAddict]

if you hobby charger manage to output sufficient ampere in current, charge them in parallel, if not set them up in series with balancing leads connected.

 

[45/70]

Thanks, Trance. :thumbsup:

That's pretty much what I figured. I don't have a hobby charger as of yet, but I'm considering a 5A unit. This would work out to 1.67A/cell in parallel, or very close to 0.7C, which is considered by many to be the ideal LiCo charge rate.

One thing I wonder, is if a charger rated @ 5 Amps (eg. a Thunder AC6 or the equivalent), will actually be able to supply 5A on a regular basis. The voltage will be ~ 4.20 Volts, rather low, but I'm not sure that's really any easier on the charger's circuitry.

Dave

 

[TranceAddict]

if the maker of the charger is reliable, the rated output is usually conservative. if it is a cheap one, you get the risk of running it into smoke. cool the power electronic part with fan will surely help.


you will find this useful
http://www.progressiverc.com/Charger.html


poor man solution:
get a old laptop no longer in use, take out the battery pack and use it as charging cradle, use the laptop as charger, zero cost with high reliability provided you don't short them.

 

[longleg]

A follow-up, if I may:

If you charge 3 (or more) individually protected cells in series with a balancing charger, what happens when the first cell reaches cutoff (4.2V) and its protection circuit kicks in? I.e. no charging current goes through this cell anymore.

Will this stop the whole charging process for all the cells, or will the balancing circuit make sure that the remaining cells also reach 4.2V?

Thanks!

 

[hybridspecies]

Thanks, Trance. :thumbsup:

That's pretty much what I figured. I don't have a hobby charger as of yet, but I'm considering a 5A unit. This would work out to 1.67A/cell in parallel, or very close to 0.7C, which is considered by many to be the ideal LiCo charge rate.

One thing I wonder, is if a charger rated @ 5 Amps (eg. a Thunder AC6 or the equivalent), will actually be able to supply 5A on a regular basis. The voltage will be ~ 4.20 Volts, rather low, but I'm not sure that's really any easier on the charger's circuitry.

Dave

Sorry for the intrusion, but I just read another thread on Li-ion parallel charging by Silverfox
http://www.candlepowerforums.com/vb/showthread.php?t=161299
in which he says "When parallel charging, set you charger cell count to 1 cell and adjust the charging current to a 1C rate for the lowest capacity cell." Is this correct, or should the charge rate be set a 1C of the combined battery pack energy capacity? If only the lowest capacity cell, then a high amperage charger really isn't necessary right? Thanks.

 

[45/70]

longleg, I believe that a balancing charger pretty much keeps the cells within +/- 0.01 Volts throughout the charge. All the cells will terminate charge at the same time.

As far as protection circuits, most will not trip until something like 4.35 Volts, so they don't really figure in, as a good charger won't go over 4.20 Volts (or 4.10 Volts depending on the setting).

Trance, thanks for the advice, and the link.

EDIT: Sorry hybridspecies, missed your post somehow.

The idea is to not charge any cell at more than 1C. So, you have to go by the C rate for the lowest capacity cell.

Dave

 

[hybridspecies]

EDIT: Sorry hybridspecies, missed your post somehow.

The idea is to not charge any cell at more than 1C. So, you have to go by the C rate for the lowest capacity cell.

Dave

Thanks Dave,

So what would be the benefit of using a 5A hobby charger over an Ultrafire WF-139 to charge five 18650s in parralel if the current output of the hobby charger still needs to be limited to that of a single cell? I've been wanting to build a parallel setup, but was wondering if I really needed to spend the money for a hobby charger or not. I appreciate the advice.

 

[45/70]

So what would be the benefit of using a 5A hobby charger over an Ultrafire WF-139 to charge five 18650s in parralel if the current output of the hobby charger still needs to be limited to that of a single cell?

Well, there are several reasons why, in this example, a 5 Amp hobby charger would be beneficial.

First, you could charge 5, 2500mAh cells in parallel @ 5 Amps. This would work out to 1A per cell (0.4C), which is a 67% higher rate than the WF-139 (or a Pila). Charging all 5 cells would take only ~ 2.5 hrs, as opposed to the ~12+ hrs to charge 5 cells in a two bay WF-139.

Second, and maybe the most important, is that a hobby charger uses the proper CC/CV algorithm when charging Li-Ion cells. One of the benefits to using the proper algorithm is when the cells are charged, the charger cuts off, without trickle charging. As I understand it, the WF-139 while having undergone some improvements with the latest version, still doesn't utilize the proper algorithm (a Pila does, however). This would lead to a higher safety factor, in addition to prolonging the cells lifespan.

Third, and along with the second reason, why I'm looking into this, by providing a balanced charge with a hobby charger, the cells will be charged to as close a relative state of charge as possible. This is important to me, as I'm using three 18650's in a series application. The cells when discharged, should all be discharged to about the same level when the light's circuit starts the "low voltage' warning, or shuts down. Again, the benefit here is not only safety (particularly with unprotected cells), but prolongs cell life. With a WF-139, or a YOHO-122, as I presently use, this is not possible. You have to charge two cells at a time, then charge the remaining cell (or, charge the three cells singly). This, I'm sure, doesn't provide a close match of the cells.

I might add that I'll be using the hobby charger for a lot more than the sets of three 18650's. It's better algorithm alone, makes it more suitable for charging any Li-Ion cells. Of course it can be used for charging NiMH packs as well, which I will do for my M@g 623 et al.

Dave

 

[hybridspecies]

I didn't realize that the WF-139 still had cut-off issues - that is definitely a reason to go with a better charger as you point out.

Regarding your first point though, I'm still misunderstanding something. If we're limited to 1C of a single 2500mAh cell then shouldn't the total current output of the charger be limited to 2.5A regardless of the number of cells when parallel charing? I hear what you're saying about 5A/5 cells = 1A each, but that assumes no charging variance among individual cells. I just wonder if it's possible to have different amounts of current going through individual cells due to internal resistance changes, or protection kick-in of completely charged cells, that could end up exceeding 2.5A for the remaining cells. I don't know if this is possible, but I thought perhaps that was the reason for the caution not to exceed 1C of any individual cell.

Thanks again for the help. I was out soldering battery bays this morning to get a head start on the multicell charger setup - just need a hobby charger now. Take care.

 

[45/70]

When you charge cells in parallel, the charger sees the cells as one cell, or I guess "battery" would be a better term, with the combined mAh of each cell added up. So, in the 5x2500mAh example, the charger sees one cell with a 12500mAh capacity. So, 5A rate with a 12500mAh battery would be 0.4C. As Tom pointed out, all cells should be within 0.5 Volt of each other to do this.

I think what Tom is referring to is when you charge different capacity cells in parallel, then you have to set the charge rate at 1C for the lowest capacity cell.

Being new to all this myself, there is a possibility that I have something wrong here. I think I'll shoot Tom a PM.

Dave

 

[Anders]

Hello 45/70.

Why don't you look at this thread.

LuxLuthors thread about balance charging With Magnets.

Anders

 

[45/70]

Why don't you look at this thread.

Hi Anders,

I'm already pretty much setup for charging bays. I have two 4 bay NiCad chargers that I gutted. I'm thinking of maybe setting one up for parallel and the other for series with balancing leads, although I'm not really sure I even need, or would ever use a series setup.

I like the cleaner footprint of the charging bays as opposed to the magnet setup. I question the resistance they add as well. It probably doesn't make much difference for charging, but I plan on using them for discharging as well with my CBA II.

Thanks,

Dave

 

[SilverFox]

Hello Dave,

I will repeate what I explained in the PM so we can better understand parallel charging.

If you put two 2000 mAh cells in parallel, a 1C charge rate for this battery pack would be 4000 mA.

Now, if you parallel a 2000 mAh cell and a 1000 mAh cell, the 1C rate for the pack would be 3000 mA, but this would exceed the 1C rate for the 1000 mAh cell. In this case if you charged at 3000 mA, each cell would see a charge rate of 1500 mA, which is more than 1C for the 1000 mAh cell.

Dropping the charge rate to 2000 mA would take care of this problem.

Tom

 

[45/70]

Thanks, Tom. :thumbsup:

I thought I had it right, but then a hint of "cornfusion" set in, something we have a lot of here in the Midwest.

So, when charging Li-Ion cells in parallel with different capacities, the capacity of the lowest cell determines the maximum rate the "battery" can be charged at.

The best way to figure out the maximum rate the charger can be set at, can be determined by multiplying the lowest cell's capacity by the number of cells being charged.

An example:

You want to charge three cells, a 1000mAh, a 2000mAh, and a 3000mAh. This, in effect, is a 6000mAh "battery", or at least that's how the charger sees it. A 1C charge rate would be 6000mA. However, since we're charging different capacity cells, we need to multiply the lowest capacity cell's mAh by three (since we're charging three cells), 1000x3=3000. So, the maximum rate of charge for this setup would be 3000mA.

Since the charger's 3000mA current is divided between the three cells, 3000/3=1000mA, which is 1C for the lowest capacity cell being charged.

Sorry about getting a bit OT here, but hey, it's my thread!

Dave

 

[Aircraft800]

@ longleg in post #5

If I am charging 3 AW Protected "C" cells in a series with balancing leads, and I charge at 1C. If the cells were out of balance, one cell will trip, and my charger will terminate the charge saying pack fault. I do not have this problem if I charge at 1/2 C rate.

It may be an issue with my charger, but If the cells are close in balance, or I charge at a slower rate, I don't have the same problem.

 

[45/70]

If the cells were out of balance, one cell will trip, and my charger will terminate the charge saying pack fault.

Aircraft, as I said previously, I'm a noob to hobby chargers. I don't even have one yet, but a "beginner model" is on the way.

It would seem to me that when the cells are charged in series, that balancing would keep any one cell from receiving more than 4.20/4.10 Volts and thus tripping a protection circuit, due to over voltage. I'm having trouble understanding what is happening in your situation.

If one of the cells was really low in voltage in relation to the other two and they were being charged in parallel, I could understand a high Amp current inrush from the other two cells tripping a protection circuit in one or more cells. I don't see how anything like this could happen when the cells are charged in series though. Is it possible that the other two cells are trying to reverse charge the lower cell and that is tripping the circuit of the low cell?

Then, there's the fact that this doesn't happen if you reduce the charge current to 0.5C. This definitely makes it sound like a current related problem. I'm puzzled. :thinking:

The learning experience continues.

Dave

 

[SilverFox]

Hello Aircraft800,

Remember that the balancing circuit has limits as to how much imbalance it can handle. You should be able to find the specification for the maximum current the balance circuit can handle.

At higher charge rates, you can overpower the balancing circuit and end up with what you are describing.

The balancing circuit is designed to deal with small imbalances. If you have cells at grossly different states of charge, it would be much better to hook them up in parallel before charging in an effort to bring them much closer into balance.

I should also point out that not all balancing circuits work the same way. Some bleed off current from the highest voltage cell. These are cheaper to manufacturer and are most common. Others, re-direct the current from the high voltage cell to a lower voltage cell. This type is a little more involved, and tends to be a little more expensive, but it does get around the problems you have observed.

Tom