I have some sony laptop cells (3.7v 2000mah) I want to make a 4 cell pack at 3.7v 8ah. Do i need a PCB for each cell, or will one alone work. For charging can i use my ultrafire WF-139? I am going to make a custom 18650 dummy cell with leads that will let me plug my 4 cell pack into it....
5 unprotected 18650 in parallel, help?
- Thread starter willymcd
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cool, thanks! will this PCB be adequate for the job? for 3.7v they all say one cell only, or is that only apply to batteries in series?
that depends on how much current draw you need, but if you don't need very much power, it should be ok
the cut-off voltages for overcharge/overdischarge are a bit high/low though
mdocod
Flashaholic
The pack and charging method should work fine, however, there is another problem that has been overlooked, allow me to elaborate...
The MTE 2-mode direct drives the P7 on high (as I understand it, correct if I'm wrong here), and it is more by happenstance that a single 18650 happens to drive one well, without overdriving, ...because the single cell experiences an ideal amount of voltage sag to come to a sort-of ideal "happy medium" with the P7 for reasonable drive levels. My concern is that by piling 4 cells in parallel, the load split between the cells will allow the cells to hold a higher voltage, this could be the difference between pushing the P7 at a "normal" ~2-3A range, and maybe more like 4-5A+, which would probably end up tripping your PCBs high current limit, or, if you had a PCB with much higher current limit, would just end up overheating the P7 rather quickly.
I'm under the impression that the reason for putting 4 cells in parallel on this would be to extend useful runtime, but I'm concerned that in doing so, you'll actually end up creating a thermal problem that limits useful runtime substantially, and potentially damages the LED prematurely.
This is not to say that this project can not be done! You might just need to add a regulator into the circuit! Search for some of the driver solutions people are using to push P7s around here for some ideas.
Eric
The MTE 2-mode direct drives the P7 on high (as I understand it, correct if I'm wrong here), and it is more by happenstance that a single 18650 happens to drive one well, without overdriving, ...because the single cell experiences an ideal amount of voltage sag to come to a sort-of ideal "happy medium" with the P7 for reasonable drive levels. My concern is that by piling 4 cells in parallel, the load split between the cells will allow the cells to hold a higher voltage, this could be the difference between pushing the P7 at a "normal" ~2-3A range, and maybe more like 4-5A+, which would probably end up tripping your PCBs high current limit, or, if you had a PCB with much higher current limit, would just end up overheating the P7 rather quickly.
I'm under the impression that the reason for putting 4 cells in parallel on this would be to extend useful runtime, but I'm concerned that in doing so, you'll actually end up creating a thermal problem that limits useful runtime substantially, and potentially damages the LED prematurely.
This is not to say that this project can not be done! You might just need to add a regulator into the circuit! Search for some of the driver solutions people are using to push P7s around here for some ideas.
Eric
DM51
Flashaholic
Not a good charging solution - it will take ~20 hours to charge...
yeah your right! the wf-139 only charges at 450mA
So i will probably want to get a 1.5A or 3A, smart fast charger like the ones on batteryspace?
mdocod
Flashaholic
3.6-4.2V is the RANGE of possible Vfs that a P7 could be. It is not the range that you can operate any single die at. Some will REQUIRE 4.2V to achieve 2.8A forward current, while others will only require 3.6V, they probably average somewhere around 3.9V. Taking P7 with a Vf of [email protected], and pushing 4.1V into it, could very likely push it up around 4 amps or more. If it were a 3.6V Vf LED, and you hit it with 4.0V, it would probably jump to around 6 amps.
LEDs have a very narrow voltage "range" that they can be operated within, and there are inconsistencies from batch to batch in the manufacturing process, this is why they usually require current regulation in the design. The exception to the current regulation "rule," is if you have a power source that happens to come to an equilibrium with the load and Vf of the LED chosen, sometimes with the assistance of a low value resistor. Kind of like taking 3 AAA cells and direct driving a regular LED, like a Luxeon III or Cree Xlamp or Seoul P4, the cells end up sagging under the load of the LED and coming to an ideal equilibrium with the LED. It can be assumed that most alkaline and consumer oriented rechargeable AAA cells will suffer a fair amount of sag under the ~500mA-1A load of a single die LED, enough that you never have to worry about overdriving, but taking that same 3xAAA light, and running it on a single larger Li-Ion cell, or 3AA cells instead, can have the undesired side effect of substantial overdrive since the larger more capable cells hold a higher than expected voltage under the load of the LED, capable of pushing it well into overdrive/overheating territory. Switching to a 3AA or 1xli-ion design would require a modification to the resistance in the path to the LED, either in the form of a simple resistor or regulator. My feeling is that the same considerations need to be taken into account when you add more cells in series to a direct driven P7 setup....
Personally, I think it would warrant at least some testing to be sure, start with 1 fully charged 18650 and test the current on high, then try 2, then 3, and see what's happening. There's a very good chance that you'll be under 2.8A on 1 cell, but this depends on the Vf of the P7 in your particular unit. There's always the possibility that you could have an unusually high Vf LED that actually NEEDS 4 cells to actually HIT 2.8A. I would personally assume that they would choose LEDs for the 1 cell design that at least semi-complement the behavior of a single cell under that load.
Eric
DM51
Flashaholic
Depending on how many other applications you will have for a charger, it might be worth considering a hobby charger. Your 8Ah pack would ideally be charged at somewhere between 5-8A. Any less than that would obviously take a increasingly long time.
With a hobby charger, you would have the capability to do high-current charges such as that, and depending on the one you choose, you could also use it for single/multiple cells of different sizes and chemistries, in series/parallel configurations...
You would also have the ability to do proper charge/discharge cycles. You might actually end up saving money otherwise spent on buying several single-purpose chargers.
