lithium polymer battery 3 cell whats it's cut off voltage

[amateur_25]

Hi

I watched daves video on charging lithium ion/polymer batteries. However he was dealing with a single cell.

I have 3 cell battery.

Assuming 2.7v is when the cells are dead, would it then be 8.1v I would deem the battery dead at?

[paulca]

Hi

I watched daves video on charging lithium ion/polymer batteries. However he was dealing with a single cell.

I have 3 cell battery.

Assuming 2.7v is when the cells are dead, would it then be 8.1v I would deem the battery dead at?

Below 3V per cell the voltage will RAPIDLY collapse under load.  Therefore it is usually not recommended to take them below 3.0V, 2.9V absolute minimum.  RC Heli's and models I fly have a soft cut set around 3.3V where they start to limit the current which gives you a warning and lets you land.  They hard cut at 3V and in a plane you glide to a dead stick landing, in a heli, unless you are really good at autp-rotation, you crash.

Manually doing the cut off is not recommended as it will drop incredibly fast under any kinda of load.  You should use a cut off circuit or as a minimum an audible alarm.

I was testing using a 3S 5000mAh pack at the weekend to charge USB devices via a regulator.  The regulator had an LED volt  meter.  It was reading 9.8V feeding about 1A to a USB power bank.  I went to the kitchen to get a cup of coffee and when I came back, literally 3 minutes later the voltage was descending rapidly though 5.8V.  I immediately charged it back up above 9V on very limited current of 1/20th C (250mA).

Also note it is not the pack voltage you should be concerned with but the individual cell voltages.  If the pack is not perfectly balanced and matched, because the voltage collapses so fast you can get into a situation where the pack is producing 9V but that is made up of two cells at 3.5V and one at 2V.  That 2V cell will be under considerable stress!

What is your use case out of interest and I might be able to help.

[amyk]

The cutoff voltage you should use depends on the current --- at low discharge rates, something closer to the manufacturer's recommended minimum is OK, but when discharging quickly (common situation with RC, as explained by poster above), the cutoff should be higher. This has to do with the internal resistance of the cell.

[David Hess]

Assuming 2.7v is when the cells are dead, would it then be 8.1v I would deem the battery dead at?

Yes but ... if one cell has less capacity than the other two, then its voltage will fall first and it may for example reach 1.5 volts with an 8.1 volt cutoff applied to 3 cells.

If it was a high reliability application, then I would monitor the voltage of every cell during discharge so that no cell falls below its discharge cutoff point.