Safely managing parallel LiIons?

[MrNaz]

I am making a flashlight with a 2S3P array of LiIon batteries. I've heard that LiIons in parallel require care or else it gets dangerous. Assuming I know very little about the dangers of parallel LiIons, does anyone have comments on this, or can anyone point me to threads where parallel LiIon use was discussed?

Thanks in advance!

 

[yellow]

when You have got them to the same "charging state" and finally wired the pack, there is no danger from paralleling them (except for using a protection driver for over-/undervoltage, overcurrent, ...)
but if You think of individually charging each "stick", You have a short every time they come into contact - Li-Ions dont like shorts, even less than Ni-Cads(Ni-Mhs.

but when that 2S3P will be fixed like that and get used like this, no problem:

get all Your cells, or the BP pack You order and dismount
have them charged (or at least almost charged)
solder wires to + and - of each cell,
solder a resistor thinking: 20 ohms?) to +,
now connect all - (solder)
and connect all resistors,
leave for 1 week.
that way possible differences in charging state get evened out.

(if You want, You could - after some time - connect a low output bulb or resistor to the whole package to discharge the cells a bit before connecting to final pack and protection circuit. But check with a multimeter that voltage does not go under 3 V, under no circumstances!)

connect the "now same" cells as You wish.




PS: 2S3P is the typical BP945 7.2 V 6600 mAh Li-Ion pack. Thats to be bought ready-made. Why building such a pack, just use the one of Your Video-cam.

 

[MrNaz]

when You have got them to the same "charging state" and finally wired the pack, there is no danger from paralleling them (except for using a protection driver for over-/undervoltage, overcurrent, ...)


get all Your cells, or the BP pack You order and dismount
have them charged (or at least almost charged)
solder wires to + and - of each cell,
solder a resistor thinking: 20 ohms?) to +,
now connect all - (solder)
and connect all resistors,
leave for 1 week.
that way possible differences in charging state get evened out.

(if You want, You could - after some time - connect a low output bulb or resistor to the whole package to discharge the cells a bit before connecting to final pack and protection circuit. But check with a multimeter that voltage does not go under 3 V, under no circumstances!)

connect the "now same" cells as You wish.

That's a lot of soldering, and I'm not entirely clear on what you're telling me to do. Are you saying I should put a 20Ohm resistor to the + of *every* battery, and then connect them all? So all + are connected on the other side of resistors, and all - are directly connected?

Also, does it have to be soldered? Can I just use magnets and alligator clips?

Finally, does it have to be a week? What if I was lazy/impatient and only did it for a day or two?

 

[VidPro]

because lithium-ion uses a voltage based charge alogrythm the only issue becomes if ONE of the cells goes bad, then your charging at 1C for BOTH cells, which can mean putting 2C into a Bad cell overheating it.
Then
you want them to be close to the same voltage when pairing them, or the one will charge the other, possibly way to quickly depending on how different they are.
and
i like to put them down like say 3V before i pair, that way any misteaks made when messing with them the short isnt as bad, then charge it after its all together.

my best diatribe on parellel series battery thoughts, is here
http://www.candlepowerforums.com/vb/showthread.php?t=238760
so i wont write that again

also check out some other threads that have valuable info like
http://www.candlepowerforums.com/vb/showthread.php?t=240054
seriesing

http://www.candlepowerforums.com/vb/showthread.php?t=223061&highlight=2p4s
how not to do it

http://www.candlepowerforums.com/vb/showthread.php?t=135777&highlight=2p3s
sum math stuff

http://www.candlepowerforums.com/vb/showthread.php?t=227174
charging

 

[yellow]

That's a lot of soldering, and I'm not entirely clear on what you're telling me to do. Are you saying I should put a 20Ohm resistor to the + of *every* battery, and then connect them all? So all + are connected on the other side of resistors, and all - are directly connected?

Also, does it have to be soldered? Can I just use magnets and alligator clips?

Finally, does it have to be a week? What if I was lazy/impatient and only did it for a day or two?

the idea is to get them all to the same state,
by connecting them all in parallel, that would do it, but that were -basically - a short, till they even out,
therefore the resistor, to fight a too high current.

the days to ensafe that they are even, dunno if much less time is enough
(what difference makes a few days of resting?)

Use "smaller" resistors - for higher current - and less time will also be appropriate.

soldering is no must, if You want clips, why not. As long as there is electrical connection ...


PS: I see an error in my advise --> there is no chance to find duds by doing so.
To be on the real safe side, You would have to test each cell. Now thats really time consuming, therefore one considers, that all cells are equal, when bought at same time.

 

[MrNaz]

PS: I see an error in my advise --> there is no chance to find duds by doing so.
To be on the real safe side, You would have to test each cell. Now thats really time consuming, therefore one considers, that all cells are equal, when bought at same time.

Just out of interest, how do you test them? I've bought all 8 at the same time, so can I safely assume they're all at a similar state of maintenance?

 

[rizky_p]

aren't parallel lithium safe, you can even pair a different capacity of lithiums together? It is lithium in series that need more care and attention.