RE: EVERYTHING YOU WANT TO KNOW ABOUT ELECTRIC FLIGHT
The Low Voltage Cutoff Feature of your ESC
By Ed Anderson
aeajr on the forums
Many electronic speed controls include a feature called the low voltage cutoff
circuit, the LVC. The LVC watches the voltage that is being delivered by the
battery. When it gets below a certain level, it will cut power to the motor to
preserve power for the radio system. This will allow you to keep control of the
plane and land it in a glide.
Power draw by your receiver and servos is a tiny fraction of what the typical
electric motor draws. As the battery drains it will exhibit a voltage drop. You
may feel this in the way the plane flies. The plane may become sluggish or it
may not be able to climb under full power. This is a clear indication that the
pack is getting low.
A battery that can't sustain voltage when the motor is on, can still provide
plenty of power for the flight electronics and may be able to do so for quite a
while, but don't test it. If your motor cuts, enjoy the glide, but set up to
land as soon as possible. I always teach new pilots how to glide their planes
so, if the LVC cuts the motor, they don't panic.
If you practice flying your plane with the motor off, then an LVC cut will be no
big deal. You might even find you enjoy gliding, which can extend your flying
time. I often glide and thermal my electric planes just for fun.
LITHIUM BATTERIES CHANGE THE ROLE OF THE LVC
If you drain NiCd or NiMh packs too low, usually there is little damage. Just
bring them back to charge a little slower than normal. If you drain a lithium
cell below 2.5V resting voltage, typically the cell will be damaged. So, in
this case the LVC is protecting your plane and your battery packs.
Most lithium friendly ESC will cut the motor off if the pack voltage drops below
2.7 to 3.0V per cell under load. At this level there is very little useful
charge left in the pack and the voltage will continue to drop fast.
Note that when you cut the load of the motor the voltage will likely pop back to
3.1, 3.2 or even 3.3 V per cell. If you check your batteries after you land,
you may think that LVC has malfunctioned, but it has not. The battery may be
3.3 V/cell resting but it can't sustain it with the motor running.
One thing you might want to be aware of is that the voltage sag will be less at
lower throttle settings. If the LVC cuts the power at a particularly bad time,
you may be able to get a short burst of motor operation at a reduced throttle
setting. A short run at half or quarter throttle may be all you need to get you
over that fence, past that tree or properly aligned with runway. But don't push
it by trying to extend your flight with lots of short bursts. However if it
will help you avoid a crash, or two short runs, to save the plane, are worth the
risk to the battery pack.
CONCLUSION
The LVC was put there to protect the radio, but if you are using Lithium
batteries the LVC can protect them too. It is best to be sure your ESC/LVC is
lithium friendly. That means either that it can be set manually, or that it
senses how many lithium cells you have and sets automatically. Even if it is
not designed for Lithium cells, if you can set the cut-off at something above
2.75V per lithium cell, then you should be OK.
Understanding how the LVC works will make it your friend.