7.4v Li-ion battery pack

[simples]

Hi

Im suffering a bit of Li-ion battery pack confusion, and was hoping someone might help me get my head around this. Apologies in advance if youve answered this a 100 times before.

Right, here goes....

I have built a rather crude little lamp that uses a 7.4v 2 cell li-ion pack (18650s). This has a little PCB sitting across the top of both cells that protects it against everthing but mad cows disease. Im charging it with a 7.4v 1A smart charger that I got from the same place (all battery) a year or two ago. Would I be correct in the assumption that without the little board that these would otherwise be unprotected cells?

Therfore, if I wanted to build my own pack, should I buy the little pcb board and 2 unprotected cells? Ive noticed that individually protected
cells now seem to be available which makes me wonder if I can buy 2 of these and wire together in series negating the need for the PCB on top?
Im sure I can discharge them in this way because that must be what happens if you shove 2 such cells in a flashlight, but am wondering if it would all go horribly wrong charging them as a 7.4v series pack?

I guess the question is basically, is there a significant difference between the 7.4 volt 2 battery pack pcb, and the circuit in individually protected 18650 cells.

thanks




Im probably missing something obvious here, so please go gentle on me.

thanks

 

[simples]

To muddy the waters further, the smart charger Im using starts charging an empty battery pack at 7.4v and 1.2 amps. The current reduces fairly constantly as the battery pack charges, and is down to a trickle just before the pack reaches 8.4v and cut off.

Not sure why, but I was expecting it to have a more fancy charging profile.

Does this sound reasonable?

 

[SilverFox]

Hello Simples,

Welcome to CPF.

Your pack most likely is made using unprotected cells. The protection circuit is designed for 2 cells.

You can use individual protected cells as long as you don't hook up the protection circuit for the 2 cell pack. All of the protection circuits add resistance that can effect the battery packs performance.

The proper Li-Ion charging algorithm involves charging at a constant current until the battery pack reaches 4.2 volts/cell, then it holds the voltage constant at 4.2 volts/cell while the current drops off. When the current drops off to a predetermined amount, the charger should shut off completely.

The fancy thing about this algorithm is holding the voltage at 4.2 volts/cell to insure long cycle life from the cells.

Tom

 

[simples]

thanks Silverfox

Ive relooked at the charger and it appears to work exactly as you described.

re. batteries, if Im reading this right then I can either build a spare pack with 2 unprotected cells and a suitable 2 cell 7.4v protection board, or with 2 protected cells simply wired in series without an external protection board. Either approach would be OK.

Also, Ive adjusted the programming in my lamp so that it cuts out at a voltage slightly higher than the protection circuit cutout voltage, in order to avoid relying directly on the battery protection. Any opinions on whether this is either sensible or unnecessary?

thanks again

 

[SilverFox]

Hello Simples,

I think you are on the right track...

Tom

 

[simples]

Cheers Tom.
Am finding these high power LEDs and li-ion batteries quite fun, and the CP Forum seems excellent arena. Hopefully Ill learn a bit more and have something more useful to add in future.