Need help with charging 9 Li-Ion 18650 cells

[protocol217]

I need help with charging 9 Li-Ion 18650 cells connected as on the picture:

i want to charge the all at the same time, and i was hopping i can do this with this dc-dc converter:
**broken link removed**

If yes can somebody tell me what output current and voltage should i set with pots.
all 9 Li-Ion 18650 cells are from a laptop battery, so they are 3.7v and 4.2v when fully charged, for the capacity i don't know.

 

[MrAl]

Hi,

What kind of cells are they, protected or non protected?
That's the first question to answer before any decision is made.

The protected cells are safer but it's not too good of an idea to charge them in series unless each cell has a charger balance circuit. That keeps the voltage from going over the max voltage which can cause heating and fire.

Also, these kinds of cells require a very strict charging technique. Limited current and very well regulated voltage.

If at all possible it is much better to charge cells individually. If groups are kept in parallel (no series connections) then you can get away with charging parallel banks.

 

[spec]

Hi proto,

Afraid to say that the simple step down converter will not do the job of charging your batteries very well. What is needed is a LIIon charger, either bought complete or built yourself using chips available from a number of manufacturers. In general, for maximum life and capacity, batteries of most types should be charged at capacity/10 or less. This is normally inconvenient so capacity or capacity/2 charging is commonly used, often with a battery temperature sensor.

The circuit you have drawn looks like an 11V laptop battery pack.

Just a word about charging batteries connected together: it is very unsatisfactory for a couple of reasons which I will not go into now. It is much beeter to have individual charging circuits for each battery. Of course, this is often not practical for some applications.

If you post more information about the intended application perhaps we can work out the best approach for charging your batteries.

PS: MrAl our posts crossed

 

[protocol217]

All of these cells are unprotected, yes they are from laptop battery, i want to use them for my cordless drill, right now i have old 12v NiCd in my c.drill and i want to relapse them with Li-Ion and get a little more capacity.
So if the single cell is 1200 mah and 4.2V when fully charged, for 9 of them connected as i described i should charge them with 12.6V and 360 ma am i right ?

 

[alec_t]

In the interests of safety I suggest you start by checking this.

 

[camerart]

Hi P,

As the others say "Be careful"

I have a similar set-up and have connected between each paralleled cell with a fuse, to try to keep them balanced as well as balanced by the charger.

Best to get a proper charger, especially with a temperature sensor. This will have settings for LIMIT which will be set for LI-ON cells, also TIME, so as not to keep charging for ever. Charge them in a 'safe place' i,e, not in the house if possible. And when they eventually catch fire, don't throw water on them

C.

 

[Externet]

The way I do it...
Attached a 18650 quad holder to the bottom of the 12V drill. Cells are left in parallel pairs as pulled from the laptop pack, so each holder slot gets a pair for a total of 8 cells ~14V
Recharged on a double charger, still in paralleled pairs. If you had two double chargers, all can be recharged at once.

Chargers are cheap ----> **broken link removed**

And holders ----> **broken link removed**

 

[audioguru]

It sounds like fun.
Connect the ebay power supply to the Lithium battery cells outside on concrete, not asphalt. Then make and post a video of the explosion and fire. Do not put water on the fire because it makes the lithium burn even hotter.

I have two 18650 cells from a laptop in series in a portable vacuum cleaner I found that had a dead 6-cells Ni-Cad battery. I charge the cells to 8.40V with an LM317 voltage regulator with the current limited by the AC-DC adapter rating. Since the cells are from the same battery then they are matched fairly well and I do not need a balanced charger for them, yet. I unplug the charging the next morning.

 

[MrAl]

Hi,

First, it is a MYTH that you can not use water on a Li-ion battery to put out the fire. That precaution is for Lithium metal batteries. I keep a chemical fire extinguisher nearby anyway though, you probably should too.

As far as charging 9 cells arranged in 3 parallel groups of 3 cells each in series, for unprotected cells this is a definite DONT DO. Almost any other method would be better. Charge in parallel groups of 3 for example. At least that way you have less risk. Charging unprotected cells in series is very bad because there are so many things that can go wrong. You might get away with it for a year, then all of a sudden it explodes into flames. That's because as cells age their characteristics change and there is no way that the three cells in one parallel group will track the changes in the other two groups. That means eventually three of them overcharge, maybe worse.

I would not do it even with protected cells, but with unprotected cells i'll tear the pack apart before i did them in series.
Yes, you would need 4.2 times 3 which is 12.6 volts, but you cant simply apply that to the pack anyway because there also has to be current limiting. Current limiting protects each cell and for groups of 3 it protects each cell only if the current set point is set for the max of of ecell only.

Take it from everyone here, you should not do it in series and you should also read more about this first, MUCH more. These cells are not your average type of cell, they are more serious.

 

[protocol217]

Ok i get it, overcharge or higher voltage per cell = fire or worse. So lets say that's 100% not safe.
How about charging in parallel groups of 3 as MrAl suggested, i would need to limit the voltage to 4.2V and the current to 660 ma (on the laptop battery was standing 11.1v and 6600 mah and inside were 9 cells so 1 cell should be 2200 mah) am i correct?

and how much risk is this method now, on the scale from 1 to 10?

 

[audioguru]

If your charger does not detect that the lithium battery is fully charged and does not turn off then the trickle charging will cause the lithium ions to turn into very dangerous metallic lithium (says The Battery University). Something happens (a nice hot fire?) if a do-it-yourself battery charger tries to charge a lithium battery that has discharged to a low voltage. A battery charger IC designed to charge a lithium battery should be used as a charger. Your do-it-yourself charger has a risk of 11 out of 10.

 

[MrAl]

Hi there audioguru and protocol,

first audioguru:
I dont know if i would go that far, i think the risk is much less because we usually take the batteries off charge after a while anyway. That's actually the way i do mine. But i also monitor the current so i can tell when it gets low.

protocol:
That's a lot better. There are still some points to be considered carefully, and one as as audioguru pointed out, but i'll list here:
1. Charge voltage tolerance. The tolerance on this is that you can not go over 4.200 volts. You can always stop before this, but cant go over this. Due to voltage meter tolerances, it would be best to stop at 4.150v instead. I use a 50000 count volt meter to measure mine but most meters only have three digits at 4.2 volts, so your reading might be 4.20v, so keeping it down to 4.15v is a good idea. The absolute max is 4.250 volts but we dont want to go too close to that. Getting this kind of accuracy at room temperature isnt too hard, using a device like an LM317, but for a large ambient temperature range a better reference would be required.
2. Current limit. The current limit has to be set based on the manufacturer's data sheet. If each cell has a limit of 660ma then 660ma should be a good setting for three cells in parallel, because that means even if two cells are weaker and dont draw much current the third cell still only gets 660ma. So that should be a safe setting. Charge time for 2000mAHr cells maybe around 4 hours each, which comes out to about 12 hours for three in parallel (time approximate).
3. Low current cut out. Once the current gets down to about 1/20 fo the normal charge current, the charger should be shut off or cells removed.
4. Safety timer. The safety timer allows only a certain amount of time for a charge, no matter what else happens. In the case of 660ma and 2000mAHr cells, three in parallel, the timer would be set for around 12 hours. Some experimentation would be required to get this time right though. What kind of timer you use can vary, as long as you dont forget to check the cells.
5. Never leave the cells alone when charging. You should keep an eye on the cells as they charge, and never leave them alone.
6. Because there are so many cells in the pack, it would be a good idea to have a good fire extinguisher nearby ready to go. If you do three sets of three in parallel, you can keep the sets apart from each other too if possible.

If you follow the above you should not have any problems. The risk factor must be pretty low, and goes up as the current is increased because if one battery has to take all the charge then that one battery cell could get a current that is too high for it. A common mistake would be to try to charge three in parallel at three times the normal charge current reasoning that there are three cells and therefore three times the current is ok. But if two cells get weak then one cell gets three times the normal current...which is not good.

Other members might care to share their experiences too with this.

One last thing though, never feed them after midnight <chuckle>

 

[protocol217]

Thanks very much for this info MeAl.
Yesterday i opened one power bank for charging cell phones, inside was 3 Li-Ion cells connected in parallel, so i took multimeter and tested the charging current and voltage, the charging current was 500 ma, and when the charger stops the battery voltage is 4.15V - 4.16v

So i look on eBay what can i use for charging and i have find this charging module: **broken link removed**

From the description i see i can adjust the charging current with resistor change, so i would set it to 580 ma output for 3 cells (1 cell is 2200 mah), and it has to led-s for showing the charging and when it is finished charging.
Next: i would like to use it as follows:

is it good to charge them like this while all tree groups are connected and making together 12v ??

 

[MrAl]

Hi,

That looks like it might work. There is a catch however, and that is the three inputs (apparently from USB) must come from isolated power supplies themselves. That means you cant plug all three into the computer for example, and you cant plug all three into any single power supply with a single output voltage. The three inputs must be from separate, isolated power supplies.
You could use those little cell phone wall wart chargers probably, that put out 5v because they are isolated from the line voltage.

The problem with using non isolated supplies is that all the grounds connect together, and with the cells already connected that means some will then be shorted out. If you disconnect the two series connections however then you could use non isolated supplies, then put them back together when ready for use.

I did not check the chip they are using for charging so maybe you could also check that out first too.

 

[protocol217]

Onboard chip is TP4056
So with three separate cell phone USB chargers i can charge them all connected like on the picture.
i can connect the three cell phone chargers to the same 220v power outlet right?
And on the scale of 1 to 10 what is this method for a risk?

 

[MrAl]

Hi,

That sounds ok as long as the cell phone chargers take 220vac input of course.

I'd say very low risk, although it is hard to put a number on it. Probably less than 1. Always keep an eye on them though in case anything unforeseen goes wrong.
Just make sure the chargers are all isolated from the line voltage. If not, the risk is 10 because something shorts out.

BTW, cute drawing

 

[protocol217]

Thanks, one last thing, in battery holder i have room for 9 cells, my cordless drill is 12v, what would be a better option:
1. to make a pack of 3 groups of three cells making together 12,6V when fully charged with 3 chargers at 580 ma current or
2. to make a pack of 4 groups of two cells making together 16.8V when fully charged with 4 chargers at 400 ma current

 

[MrAl]

Hi,

How many cells does it actually have inside?

 

[audioguru]

The 12V Ni-Cad cells produced about 14.5V when fully charged. Four Lithium cells in series produce 16.8V when fully charged which is only 16% higher and will be the same small increase as in my little vacuum cleaner.

 

[kinarfi]

I don't remember where I got this design, but I use it to charge 2 in series and then take that 8 volts down to 5.1 to charge my phone, I also have a 4 cell charger using 4 of the single cell designs. Haven't had any problems.