I am planning to make a bluetooth speaker and use a 4 x 18650 battery holder like this:
I do not plan to charge the batteries onboard the battery holder. So Im going to charge them in my good charger then insert the 4 cells into the holder after they are all fully charged. My question is will the cells discharge evenly by themselves or will I need to get something to make sure of this. I will be using mostly Samsung and LG cells from laptop pulls. Thanks for your help.
Series connected like that it is possible for the cells to wander a little and one cell possibly lower voltage than all the rest. If you use a set, purchased together and always used together that can help alleviate the potential problem.
There are boards available; BMS, battery management system.
Example: http://www.banggood.com/Battery-BMS-Protection-PCB-Board-For-3-4-Pack-18650-Li-ion-Lithium-Battery-Cell-p-1004302.html
Those are made to balance and protect while charging and discharging.
That cell holder will likely only accept flat top cells, BTW. At least it has been my experience that most holders for 18650 are length limited.
I don’t think those boards will actually balance. However, for this simple setup where you don’t discharge the cells to fast and will recharge then separately anyways you won’t run into problems very soon as long as you pick a decently matched set to start with. When i doubt just set your packs lower voltage a bit on the conservative side.
So maybe i’ll try it cautiously at first then check all 4 cells after an hour or 2 of use. How much voltage deviation between the cells is dangerous?
What’s the working voltage for your setup?
The working voltage is 8-25V DC so 16.8V max for 4 18650’s should be good enough I hope.
First, you should use 4 cells of the same type and same capacity. If you use 4 from the same laptop pull most likely they will be very similar in capacity. If you have a charger that will do it, check the capacity of all 4 to make sure they are similar.
Cell imbalance in a series pack only becomes a problem when one of the cells discharges before the others, then its voltage will go out of the safe range and reverse charging can also happen easily. Similarly, imbalance can also cause one cell to become overcharged if the pack is charged in series, but it sounds like you will charge each cell individually.
A pretty easy way to ensure none of the cells overdischarges is to use a voltage monitor like this one.
It monitors all 4 cell voltages and will beep when any one of them reaches the user settable low voltage limit. Just stop using the pack at this point and charge the cells.
The BMS products linked above would also perform this function, but I don’t have experience with using them.
What's up fellows?
Well, here's my smartass solution:
You know, these ubiquitous TP4056 modules with protection ICs. Well, we install one onto each cell in series. If we interconnect the module outputs in series, with the batteries at the battery terminals, we would also benefit from the overdischarge/overcurrent protections.
Now, you may be already aware that, since these modules are not isolated, feeding them from a single PSU would end up in a short-circuit disaster. However, this can have a simple, smart solution, which is to use 2 independent (isolated) 5V supplies, something like this, some other USB/smartphone “chargers” or other standard 5V switching units. The key, then, is to connect the odd numbered modules & attached cells to one PSU, and the even numbered modules/batteries to the other. After each full charge is achieved on the cells, our pack re-balances. :THUMBS-UP: (edited after dreaming about “Short Circuit 4” :FACEPALM: , see below)
Sounds good to me at least. Hope you like it.
Cheers ^:)
0K fellas, not gonna dip into the movie industry yet… 
Well, a better thought simple solution to feed the in-series TP4056s: an adjustable supply with a bigass diode in series at its output.
For this application, this AC 100V-240V DC 5V 5A Adjust Universal Regulated Switching Power Supply 25W New inexpensive unit could do; as PSU output diode, a 10A05 to 10A10 would suit, or the more efficient 10SQ030-10SQ100 schotty series (cheap also: 5pcs 10SQ045 10A 45V Schottky Rectifiers NEW). We would adjust the PSU output slightly upwards (≈5'3-5'5V) to compensate the diode voltage drop. How about this?
Answer: Short Circuit 5 
Cheers my dears ^:)
All right! :STEVE: A whole set of isolated DC supplies. Salve for the inconvenience, not really expensive nowadays… O:)
AC-DC 5V/12V/20V 2A/2.5A/0.7A Switching Power Supply Module for Replace/Repair
Cheeeap! :-)
Cheers ^:)
None of the boards linked here actually balance at all, but if you are charging them separately in a charger you can actually get away with these default multi-cell protection boards. If one of your cells is out of whack and dips to below safe voltage set by the board the whole battery pack will just quit on you (good thing). If you ever wish to add in-device charging you _will _need balancing. Here’s and example of a protection board that will actually balance;
This example is only for a 2cell setup tho, and even then the cells need to be paired properly as this board will only balance at 10mA.
The simplest method for this is to simply charge them all individually or in parallel.
In a single discharge cycle the chances of the cells getting imbalanced to the point of an issue is virtually nill. When you recharge them it is all fixed.
If you start out with 4 matched cells you should not have any issues at all, simply make sure you have a lot voltage protection setup, even the basic lipo cell tester would work fine and let you check voltage at will per cell as well. Just make sure you have the tester on a switch as it will drain the batteries if always connected.
I have converted a few drills and other items to 18650’s and they are all using Laptop pulls (drills are not big fans of them BTW but can’t justify new cells on an old drill). Even with cheap laptop pulls never being taken out of series I have been surprised by how little imbalance has show up over time.
The most important thing I do is recharge them in series pretty slow, like .5A or so.
In parallel is way easier.
Should be easy here by using a DP switch/volume pot instead of the usual SP ones. Best would be auto shut-down on lowV as it’s easy to forget something like this is on, no need to wreck a cell or worse.
Phil
I’m not sure if this is what you’re after, but I am running a similar set up to power 12v led strips from a battery pack. I’m using a 3sp1 protected battery holder.
https://www.fasttech.com/products/0/10002230/1161200-3s1p-18650-11-1v-battery-holder-case-li-ion-pcm
The holder also comes in 4s1p https://www.fasttech.com/products/1425/10002230/1161100-4s1p-14-8v-18650-holder-with-battery-build-in-pcm
Like you I currently run the led strip lights with laptop pulls and charge them on my opus BTC-3100.
I don’t have the technical acumen to tell you if the pack balances discharge. From my experience I don’t think it balances discharge as one time I had a cell which drained faster than its compatriots in the pack. However, it showed me that the protection works as the pack turned off when the one cell got too low.
There are 2 downsides to using these packs.
1st the things are stupid expensive. $10 for a protected battery pack without batteries? Yikes! The protection circuit adds $8 to a $2 item.
2nd whenever the protection circuit is tripped or you remove the batteries, the protection circuit needs to be ‘reset’. To do this you must charge the pack itself (through the pack leads) with the correct voltage for a few seconds. So for me after I charge the cells individually, I remove the pack then use an adapter so that I can use a 12v charger to ‘reset’ the battery pack protection. You’d have to do the same except with a 4s pack except you’d need to hit the pack with ~15v to reset the pack.
Overall I’m pretty happy so far with performance. The packs are expensive and charging is annoying but the peace of mind that the protection circuit ‘should’ keep the cells from overdischarging and burning the house down if someone forgets to turn off the closet light is probably worth it.
