LI-Ion at high amp draws

[abinok]

OK... so 2C is about the limit of what most folks say you can run Li-ion at.
So... what would the effect of running at higher draws be?
fire and explosion? or reduced life?

Im thinking speciffically of a pair of 14500s in series to drive one of a few lamps for seconds at a time. Amp draws from selected lamps would range from 1 to 4.5 amps.
700mah would put the 2 c range at about 1.4amps...
Thoughts on driving for seconds at 4C, 6C and 8C?

 

[cfromc]

I've driven a 2400mAh at 4amps for several minutes and the batteries got very hot. I think the life of the batteries is shortened when they are frequently discharged at 2C or more.

 

[Fallingwater]

Voltage will sag very quickly indeed and the cells will overheat (and possibly catch fire if you REALLY push them).

If you want to run high draw devices on small cells, get some lithium polymer packs meant for RC use. They can routinely withstand draws of several C.

 

[mudman cj]

The problem with high discharge rates (>2C) for Li-ion cells is overheating. If the cell reaches an internal temperature of 140F then a runaway chemical reaction may start than cannot be stopped just by removing the load. This thermal runaway causes the explosion by producing hydrogen gas. So, running the cells at over 2C is dangerous given enough time for heat to build up, and running at 4C, 6C, or 8C will cause the heat to accumulate much faster because heat production equals the current squared times the resistance (internal cell resistance in this case).

For high current applications you can use Nimh cells designed for high current draws available at the likes of cheapbatterypacks.com (I know, not enough voltage - right?), or other lithium chemistries such as those used in A123 systems or Emoli cells (or lithium polymer as Fallingwater suggested). Some folks get these cells from power tool battery packs. Of course, these cells are typically 18650 size. And one other thing, those 700mAh cells really only have at most 600-650mAh actual capacity, so 1.4A is already over 2C. It would be best if you could keep the current at or below 1.2A for safety because you never know when that light could get turned on and run longer than you planned...

 

[mailint]

Time ago while I was walking with 8x notebook lithium cells in my hand each long like an AA and 2 times the diameter I accidentally short-circuited a cell and it became incandescent in colour and emitted a scaring flam that reached about 4cmx1cm in less than 5 seconds. I immediately throwed it to the floor and it came back normal in about 1 second.
This is how I learned to have respect for lithium cells

EDIT: they were Li-ion cells, not lithium cells

 

[DM51]

Li-Ions can withstand short bursts of high current quite well. I wouldn't like to put any figures on this by saying they will be OK at xC for ySeconds, but if the bursts are truly short, you should not have problems. Maybe SilverFox has more info on this.

 

[mailint]

Li-Ions can withstand short bursts of high current quite well. I wouldn't like to put any figures on this by saying they will be OK at xC for ySeconds, but if the bursts are truly short, you should not have problems. Maybe SilverFox has more info on this.

Oh, you're right! they were Li-ion cells, not lithium cells

 

[LEDite]

Abinok;

Most "unprotected" Lithium-Ion cells have a protection device which opens the circuit if you draw much more than 2.5C from the cell for about 30 seconds or so.

It is designed keep the cell from venting due to heat build-up.

There is also a PTC device which increases in resistance as current increases.

Larry Cobb

 

[mailint]

Abinok;

Most "unprotected" Lithium-Ion cells have a protection device which opens the circuit if you draw much more than 2.5C from the cell for a minute or so.

It is designed keep the cell from venting due to heat build-up.

There is also a PTC device which increases in resistance as current increases.

Larry Cobb

I found a video of Li-on cells that get on fire for overcharging: http://youtube.com/watch?v=vS2hGoJVmlA