Protected LION, what's the max current?

Anyone know what the max current is on protected 18650 cells before they go into overcurrent cutoff?

Do they actually shut off the current or just throttle it?

Wondering because I have a flashlight where the batteries read about 3.65 volts with no load, but when the flashlight is turned on it's drawing 2 Amps at the tailcap and the light only stays on for maybe 3 seconds then turns off.

This happens on High, it works fine on Medium or Low.

I tried a pair of freshly charged cells, it only draws 1.5 Amps at the tailcap with the fresh ones and the light doesn't turn off on high any more.

Any ideas?

3.65V is dead, charge it up and it will work fine. A 2 amp load from 3.65V would only take a few seconds to cause the low-voltage limit to trip the circuit open on the PCB. You're probably not tripping the max current, more than likely you are tripping that low-voltage cutoff, and with each further attempt to get more juice out of this dead cell, you are over-discharging it a little bit.

different brands of protected 18650 have different thresholds for maximum current, seem to vary between ~2.5-6 amps based on brand so it depends on what you have, none of them throttle, they all just shut down when you exceed the limit for a specified amount of time (a few miliseconds). Check your battery specifications for more information.

Hope that helps,
Eric

3.65v is dead? Since when? I thought around 3v was dead.

What is the normal voltage range of these li-ion 18650's?

I thought it was the same as lithium ion polymer cells, where 3v or so per cell was dead.

Remember it depends on which protection PCB is being used. AW's limit current at 5.2-5.5 Amps as an example. You have to get the spec from individual manufacturers to be more precise.

When Eric said 3.65V is dead, he meant out of juice. Damage occurs to most Lithium Cobalt cells below 3.0V and progresses with duration and degree below that.

Hello Hellbore,

There is also the possibility that we are talking about different conditions when measuring the voltages.

If your question involves the cells ability to perform while under load, and you are measuring the voltage under load to be 3.65 volts, you will get a different set of answers than if you noticed that your battery cut out and after removing the load notice that the rebound voltage was 3.65 volts.

In the first case, it would appear that there was some sort of current limit built into the protection circuit.

In the second case, your cell is discharged and you should charge it back up and try your test again.

Tom

Hellbore said:

What is the normal voltage range of these li-ion 18650's?

Click to expand...

3.50-4.20 Volts, OC for Li-Co cells. Realistically, 3.60 Volts is pretty much dead.

Your light apparently has a regulated driver. With your batteries at 3.65 Volts OC, the circuit was drawing 2 Amps from almost dead cells. When you put in freshly charged cells, the draw was 1.5 Amps. The lower the batteries voltage, the more current a regulated circuit will draw.

On the lower levels, the current draw did not drop the voltage enough to shut things down. Just the same, I doubt it would have run much longer.

Hope this helps.

Dave

The discharge of lithium cells is not a nice smooth line all the way down to 0 (as so many people seem to assume would be the case which causes such great confusion), it holds up pretty steady in the 4.1 - 3.5V through 95% of the discharge of most lighter duty loads (like portable mp3 players and cell phones an stuff)... then right at the end the voltage plummets. So while the difference between 3.6 and 4.2V is literally like 90+% of the capacity of the cell (as measured open circuit), the difference between 3.0 and 3.6V open circuit is practically nothing, only takes a very short time to make this difference in a discharge as this is the point where where the cell is nearly completely dead and will fall on it's face if discharged much more... Stopping at ~3.5-3.7V keeps the cell a lot healthier than discharging to 3.0V, while discharging to 3.0V only gains a very small amount of runtime over discharging to 3.5-3.7V.

Eric

I recently purchased some protection boards from a seller on eBay and they had two different flavors. One had a higher maximum discharge rate and the other, the ones I bought, had a lower maximum discharge rate. The physical difference in the two boards is in the form factor: the lower discharge rate boards are round boards designed to be placed at the end of the cells while the higher discharge rate boards are long rectangular boards intended to be placed alongside the cells.

Now I just need to figure out how to solder the things to the batteries!

PCC said:

Now I just need to figure out how to solder the things to the batteries!

Click to expand...

Soldering directly onto a lithium cell is considered a big no-no.

Soldering to the tabs that were already welded to my 18650s. These batteries were taken out of a brand new laptop battery that was never used and the voltage on the cells dropped below the protection cut-off limit so that the pack would not charge. The batteries still read 3.6V so I think they're fine.