Noob Questions: Unprotected Li-ion in Parallel...

[Gryloc]

Hello all. I have done a few searches and feel semi-confident on attaching li-ion cells in parallel now. I understand that the cells must have voltages very close from one another before connecting them in parallel in their intended application (within 0.5V from each other -thanks SilverFox) to keep in-rushing current to a minimal, but it is just best to charge them fully before hooking them in parallel. I understand that the cells will automatically balance each other as they are charged. Even after they are disconnected, they will continue to balance each other.

I feel better after some searching on the CPF, but I still have some further questions to sooth my mind. I am thinking about making a battery pack with either 2 C-sized li-ion cells (available at DX), or 2 D-sized li-ion cells (available at KD), in parallel. I have protection circuitry (3.7V, 8A), fresh cell wrap for both sizes, thin kapton tape (for insulation), and some copper tape for connecting the cells. Should I use one protection circuit per cell, or can I use a single protection circuit (that can handle the current) for both in parallel?

Can a "dying" cell that seems to lose it's charge drag down the other cells (due to the continuous balancing) to a point where the battery pack would be a safety concern?

Finally, a bit off topic though, would two C or D cells (li-ion) face much voltage sag when there is a current draw of about 3A compared to 3 series connected C- or D-sized NiMH cells? I want to find the best combination of cells to power a 3A load without too much sag (which would reduce current dramatically with a DD LED load). By the way, to maintain this 3A load, the cells must be operating at ~3.5V (the Vf of my Seoul P7 at 3A), so maybe it is good to have some voltage sag. Is this correct? In a Maglite body, there will be a bit of resistance, which may drop the voltage down to the required level at high currents. I can always add a resistor in there, too. Am I in over my head, and should I just use regulators with 2 series connected li-ions?

Thank you for the help!!!

-Tony

 

[Mr Happy]

I think your last paragraph is the most important one, and is perhaps the most on topic part of the whole question.

High power, high output LEDs are current driven devices, and should be powered by current-regulated driver circuits. Connecting such LEDs directly to batteries without regulation will lead to highly uncontrolled currents and unpredictable results including early failure of the LED.

I'd say the right way to go is to use a buck regulator circuit with an input voltage somewhat higher than the maximum Vf of the LED (e.g. use two Li-ions in series to give about 7 V to the driver).

To drive the LED at 2.8 A will require a special regulator, and I'm not able to help with a recommendation for that. I do believe you will need to find something suitable though. Maybe others will have suggestions...

 

[SilverFox]

Hello Tony,

To address the battery side of your question...

You can either charge them in parallel, or hook them up in parallel after they are charged.

One protection circuit should handle parallel cells fine.

A dieing cell will drag the other cell down with it, but you will notice it because of reduced runtime. If one cell shorts out, there may be some safety issues for the few moments while the other cell dumps, but even then, with quality cells, you should not get into a rapid venting with flame incident.

Tom