Li-Ion Discharge Cutoff Voltage

[Mike Warren]

I've seen contradictory reports about how much a Li-Ion battery can be safely discharged, but 3V per cell seems to come up most often.

I tested a Panasonic video camera, and it shuts off at 3.15V. Are they just being conservative?

The problem I have is that I'm shutting off a circuit at 6.3V (2 cell) that's run from a generic Sony video camera battery and only getting half it's rated capacity.

Although I don't expect the full capacity from an after-market battery, these batteries come from WES Components, not eBay, so I wouldn't expect them completely fake, so I'm guessing that I'm cutting off too soon.

[mazurov]

What is your current draw? "Rated capacity" can easily be given for like C/20 discharge current or even less - you can learn this from battery's datasheet.

[Mike Warren]

I'm drawing 380mA, which is probably very close to what the original cameras would draw from it.

[mazurov]

You may want to measure internal resistance and also discharge couple times at 50-100ma to see if you can get anywhere close to stated capacity. In any case, discharging below 3V at your current won't give you any noticeable increase in run time - google "li-ion discharge curve" for details.

[Mike Warren]

I'm only discharging to 3.3V, but I'll set up some tests on my electronic load and see what I get.

[ohmineer]

Li-Ion discharge safe cut-off voltage is usually between 2.75 V and 3.0 V.

Cell datasheets usually state that you can draw the full available capacity discharging at a rate of 0.2C, which in your case is 400 mA. Therefore, if your camera is demanding 380 mA, battery should give its nominal capacity (2000 mA).

Sometimes, aftermarket batteries does not include cells of the capacity they show at the label. I've seen this kind of fraud hundreds of times.
My second guess would be that the battery has a high internal resistance but if the pack is new this should not be the case unless it is a "second life" battery.

[Mike Warren]

I found out what the problem was. The battery meter was giving an incorrect reading. I got close to 2Ah from it on the load test before I ran out of time and had to go home.

So all is good. Thanks for the replies.