I have 9 18650 cells salvaged from a laptop battery. They have all been tested to work and since they were always used together, I put them in parallel. I'd like to make a power pack, so I got some lithium charging circuits with all the protection bells and whistles, but with just 1A max output current. Now, I've been charging them with the circuit board from another old USB power pack, and it works, it's just super slow. Someone mentioned that the 1A max current simply would not work for 9 parallel cells, but I don't fully understand how it couldn't. As long as I'm charging the batteries with more current than I'm drawing from them, they should charge, right? I'm not super worried about the speed, right now it takes pretty much a full day to charge the pack. My question is, will the pack charge with a 1A total charging current (albeit slowly), or am I missing something?
Electronic – Appropriate charging current for parallel 18650 lithium cells
Edited 2017 - changed recommended long life storage voltage and added comments on fast charging using some recent systems. RM.
What YOU do as regards several of these questions depends largely on what YOU are trying to achieve or test.
Discharge to cutoff is fully discharged (to whatever remaining % that voltage represents). That's the easy one :-)
Percent dropoff of current in tail sets final % of max possible charged reached. There was a superb table given here within last week or so. Can supply later if you don't find it.
Real Men™ plateau at 4.2V and tail down to 10% or even 5% of the constant current rate. This gets the battery full and knocks the stuffing out of it.
Others terminate the current tail at say 25% of cc value.
Optimum lifetime for ongoing usage is at about the end of the constant current phase. That makes it very easy to locate - charge at specified current until desired max voltage is reached, then charge at constant voltage as desired. Here "desired" is to stop immediately. This is the point at which batteries tend to give significantly longer whole of life mAh of storage without grossly reducing mAh capacity per cycle. This is liable to be the point where older "fast chargers" tell you they have finished. Actual % total claimed varies but probably 70% - 80% range.
Newer USB input fast chargers use the term differently. In the case of USB the maximum available charge current at 5V is 5A so that the battery MAY be able to be charged at ~= 6A for the CC part of the cycle using an efficient buck converter to drop voltage and raise current.
[For a buck converter: Vout x Iout = Vin x Iin x efficiency_of_conversion]
Some systems such as QuaqlComms Quick Charge system allow the use of higher charger voltages (9, 12, 20) with specifically designed equipment, so battery charging can be faster for a given voltage provided that the battery specification allows this.
Maximum charge rates for LiIon and LiPo batteries are usually C/1 = 1A per Ah of battery capacity.
At 5V, 5A a USB charger can charge a 6000 mAh 1 cell LiPO battery at max rate - so eg a 10,000 mAh single cell battery used in some larger tablets can not be charged at the allowed 10A ! rate.
For long life storage where actual stored capacity is unimportant, LiIon and LiPo cells should be stored at about 3.7V.
Using cells without protection adds to the rich tapestry of life. As long as you don't mind the occasional scorch mark on the tapestry that's fine. Note that part of the protection is a one time high capacity fuse under the cap for when things get out of control. Undervoltage discharge destroys. Charging from below a certain voltage at full rate can get fun, I'm told. Charging at reduced rate can bring cell up, I'm told. Below another second level they say don't even think about it. I've had very poor success in trying to get LiIon to misbehave. I have a box of unprotected cells that are very uncooperative about venting with lame etc. Strange. Sony and Apple and even HP seem to be much better at it :-).
You theoretically don't need anything, but there are drawbacks:
The charging will be 10 times slower compared to a single cell charged from a TP4056
A damaged cell will cause the others in the pack to drain and you'll never be able to charge or maintain the pack at full capacity.
A damaged cell will also cause even more slower charging.
So if you make sure all cells are good, you should be fine for a while.
Someone mentioned that the 1A max current simply would not work for 9 parallel cells
Nonsense !!! it will work but it will take a very long time to charge. One cell of 2000 mAh (=2 Ah) (a typical 18650 is 2 - 2.5 Ah) takes 2 hours to charge so 9 cells take 9 x 2 = 18 hours to charge. Close to a full day indeed, not all the energy ends up in the cells and is lost so in practice 1 day sounds right.
As long as I'm charging the batteries with more current than I'm drawing from them, they should charge, right?
Yes, that is correct.
Next time someone gives an opinion about things always ask why and ignore their advice if they cannot give a good explanation.