Damaging lithium ion batteries

[nlaham]

So I have learned that all batteries have an internal resistance which will limit the current that it can output, so that you can't discharge a huge battery in a split second because of that internal resistance. But what I am wondering is, at what point is it dangerous to draw too much current on lithium ion batteries.

I took an 3.7V iPhone battery out (1900 mAh) and found the internal resistance to be very low through a couple tests, around 0.2 ohms or something. Later, I attached the leads to a very low ohm resistor and measured the current (like 0.1 ohms, barely anything). I actually maxed out the current on my ammeter, which is 10 amps, so I went over that mark. Hopefully I didn't mess up my meter, but I removed it quickly. The resistor also burnt out, but basically this tells me, I can pull a TON of current from this tiny little battery when it's not hooked to all the circuitry in the phone. Normally, it would be limited to only allow 500mA or something of the like.

I am wondering, is there a good rule of thumb that will tell me what current is unsafe to draw off a lithium ion battery? I would think at some point, if I shorted the battery out for more than a few seconds, the internal resistance in the battery would get very hot and destroy, explode or catch the battery on fire.

I just want to be safe here. I am not into short circuiting batteries, but I want to know how much current I can safely draw off of a lithium ion battery without causing it damage.

 

[mfb]

I would think at some point, if I shorted the battery out for more than a few seconds, the internal resistance in the battery would get very hot and destroy, explode or catch the battery on fire.

That internal resistance is not a separate component, it is an effective description of the battery itself and emerges from its way to store and release energy.

Try to find some data sheet. The maximal current the iPhone requires gives a (weak) lower limit for maximal current.

 

[CWatters]

There is no general rule. For model car/aircraft use some batteries are sold with a "C rating". eg for a battery rated to "20C" the max recommended current is 20 times the AH capacity of the battery.

These cells are rated 60C 800mAH so are good upto 0.8*60 = 48A

http://www.overlander.co.uk/extreme-800-2s-7-4v-40c.html

These are 60C 5AH... Do the sums yourself!...

http://www.overlander.co.uk/batteries-chargers/lipo-battery-4400mah-3s-11-1v-60c-extreme-pro-1010.htmlThey may well be capable of delivering higher than that if abused. Take care now...

 

[nlaham]

That internal resistance is not a separate component, it is an effective description of the battery itself and emerges from its way to store and release energy.

Try to find some data sheet. The maximal current the iPhone requires gives a (weak) lower limit for maximal current.

Thanks guys. I think the iPhone requires at most 1 to 2 amps when it's being used heavily (high processing power). Are you saying that the the battery then will have that as its max as a lower limit? What is the upper limit then? I am hoping I can draw 3-4 amps off of it safely, but there is no "C" rating on it.

 

[mfb]

Are you saying that the the battery then will have that as its max as a lower limit?

If the iPhone can draw 1-2A (that is a lot), I doubt that this value is dangerous for the battery (otherwise it would be a poor design). However, it could tolerate higher currents, too.
Maybe you can use multiple batteries in parallel?

 

[nlaham]

If the iPhone can draw 1-2A (that is a lot), I doubt that this value is dangerous for the battery (otherwise it would be a poor design). However, it could tolerate higher currents, too.
Maybe you can use multiple batteries in parallel?

That is a good idea, I didn't consider that. I might be able to arrange 2 smaller batteries easier that way too.

Thanks for the tips.

 

[mrspeedybob]

I took an 3.7V iPhone battery out (1900 mAh) and found the internal resistance to be very low through a couple tests, around 0.2 ohms or something. Later, I attached the leads to a very low ohm resistor and measured the current (like 0.1 ohms, barely anything). I actually maxed out the current on my ammeter, which is 10 amps, so I went over that mark.

In order for a circuit to draw 10+ amps from a 3.7 volt source the total resistance must be 3.7/10 ohms or less, so your observation of 10+ amps with a load of 0.3 ohms is plausible. I'm just wondering how you measured the cells internal resistance. I may be wrong but from your post it doesn't sound like ohms law is something you were familiar with, but that's the only way I can think of to measure a cells internal resistance.

 

[nlaham]

In order for a circuit to draw 10+ amps from a 3.7 volt source the total resistance must be 3.7/10 ohms or less, so your observation of 10+ amps with a load of 0.3 ohms is plausible. I'm just wondering how you measured the cells internal resistance. I may be wrong but from your post it doesn't sound like ohms law is something you were familiar with, but that's the only way I can think of to measure a cells internal resistance.

I have been familiar with Ohm's law, just not too familiar with batteries and internal resistance. Well, I should say I was more familiar when I was in school, but forgot all of those things and needed a refresher, I'm only a Mechanical Engineer. I also have never really applied these principals to actual applications so I am sort of relearning some concepts.

Anyway, I measured the internal resistance using the second equation here. http://en.wikipedia.org/wiki/Output_impedance#Batteries

I measured no load voltage, then put a load on it, and measured current and loaded voltage. I had to measure the two separately since I only had one multimeter. Since the voltage keeps dropping off as I keep the current running and draining the battery, I can't be sure my loaded voltage and current measurements matched up exactly, but it was pretty close. The best way to do it would be to use two meters, and record my values at the same time. Hence the reason I said ABOUT 0.2 ohms. It could have been 0.15 or 0.25.

I want to test it again, but as of now I believe it's a really low internal resistance, so low that the max current I can draw off the battery would in fact be a dangerous level for a Li-Ion batery. I have been reading that 1C to 3C is a safe discharge rate for most Li-Ion batteries. (C being the capacity which I think is 1900mAh for iPhone 4, so anywhere from 1.9A to 5.7A would be safe for this battery, probably closer to the middle). Thanks to CWatters for this explanation on discharge rates.

I may start using a Lipo battery, since those can discharge at much higher currents, some as high as 60C (or 300 amps according to the battery CWatters pointed out.http://www.overlander.co.uk/batteries-chargers/lipo-battery-4400mah-3s-11-1v-60c-extreme-pro-1010.html

 

[CWatters]

Something else to watch is the discharge voltage - you may need to build a voltage cut off to prevent the cells being over discharged. They can react badly to that.