Are there 2 types of Li-ion batteries? I see everyone talking about 4.2 volts, but all the cell phone batteries say 3.7v. Can I use the 4.2v charger to charge the 3.7v cell phone batteries?

I see everyone talking about 4.2 volts, but all the cell phone batteries say 3.7v. Can I use the 4.2v charger to charge the 3.7v cell phone batteries?

From : Don Stackhouse

They're the same thing.

A fully charged Li-ion or Li-poly cell (same chemistry, different construction) is at about 4.2 volts per cell. This quickly drops down quickly over about the first 5-10% of the discharge cycle (typically the first couple of minutes running time) till it levels off at about 3.7 volts per cell. It stays near that level for most of the discharge, till near the end it starts to drop again, untill you reach the cutoff voltage of about 2.7 to 3.0 volts per cell (the exact cutoff recommended depends on which manufacturer you ask).

In the E-tec cells we typically use in our Roadkill Series models, E-tec recommends a cutoff voltage of about 2.7 volts per cell, unloaded. The actual setting for the low voltage cutoff in the ESC is typically about 2.35 volts per cell, because as soon as you shut down the throttle, the voltage will rebound to about 2.7 volts per cell.

Of course it's very difficult to actually run the pack that low in flight, especially with a 2-cell setup such as what folks typically use on our Piper Cub or Curtiss-Wright Junior. The power absorbed by a prop is proportional to the cube of the RPM, and the RPM is linearly proportional to the supplied voltage. As the battery voltage decreases near the end of charge, the prop loses RPM, which therefore means it isn't loading the motor as much, and is therefore not drawing as many watts. This means that when the battery voltage is down to 3.0 volts/cell, the watts absorbed by the prop are only 36% of what they were at 4.2 volts/cell, and 53% of what they were at 3.7 v

This means that by the time you get down to about 3.1 or 3.0 volts/cell on a 2-cell setup, the prop can't screw itself through the air fast enough to pull enough watts to sustain flight anymore. The plane struggles to sustain altitude, even at full throttle, and then settles into a slow descent. It's telling you it's time to land.

With a 3-cell pack and one of our MPS-2 twin motor systems, the same thing happens at about 2.7 volts/cell (unloaded), right at the desired cutoff. The cutoff in the speed controller becomes just a backup, with the plane's natural flying characteristics becoming the primary means of keeping you from over-discharging the batteries.

Don Stackhouse
DJ Aerotech