The curves move downward slightly as load increases because of lead and battery
internal resistance, but the difference is small if the constant discharge current is less than about .5C. This would
be 300 milliamperes for a 600 mAh battery and 600 milliamperes for a 1200 mAh battery.
general practice in R/C modeling is to operate batteries at an actual average discharge rate of less
than .5C in order to obtain a reasonable battery life.
How To Use An ESV
There are two approaches to using an ESV to estimate the state of charge of a fully-charged battery
that has been discharged for a period of time. It is not necessary that the current drain
prior to the voltage measurement be constant. Both require plugging the ESV into the battery charging
receptacle as soon as practicable after landing.
The first involves making the fly-don't-fly decision by observing the color of the scale or LED's at the part of the scale where the reading falls and making the decision indicated. Typically, the scales of ESV's are divided into color zones. The scale on my Ace Voltmaster II ESV for a 4-cell battery is green above 4.8 volts ("OK to Fly"), yellow between 4.75 and 4.8 volts ("Use Caution"), and red below 4.75 volts ("Charge Battery"). When an ESV has an LED readout,
the LED's are usually colored to indicate the zones. Referring to Figure 3, 4.8
volts indicates 72.9 percent capacity used and 4.75 volts indicates that 80.7 percent
has been used. Other manufacturers may recommend different decision voltages, depending
on their risk management philosophy.
The second is a little more trouble but allows you to determine the level of remaining charge at which
you will decide not to fly. It goes this way:
(1) Plot a curve like Figure
3 after multiplying the voltage values shown on Figure 2 by the number
of cells in the battery
(2 ) Measure the battery
voltage with your loading ESV.
(3) Decide at what percent of charge used you
will stop flying, 80% or any other value you choose. The automobile gas gauge analogy
applies here. It is prudent to drive or fly until the gauge shows approximately 1/4-Full,
and then fill your tank or charge your battery.
(4) Look on the graph made in step
(1) and read off the percent of full charge used.If you got a reading of 4.75 volts, it
would mean that 80.7 percent of full charge has been used.
Some ESV's have the option of a load in the vicinity of 500 mA. If all the discharge curves were exactly
alike, we would be fully justified in measuring voltage of any battery at a single constant load
current, but because of the effect of internal resistance, we can reduce estimating error by
using the higher load when the actual discharge rate approaches .5C.
Using an ESV for estimating percent of charge used is not very accurate for two reasons: First,
most ESV's cost in the range of $12 to $50 and are not precision instruments. Second, you can see
by looking at Figure 3 that, because of the flatness of the curve, a small error in the voltage
reading would make a large error in percent capacity used, except near the beginning and ending of
the curve where the slope is greater.
Batteries are like automobile gas tanks. When our battery or gas gauge reads half-full or more,
we're not too concerned about accuracy. We begin to be concerned when the tank or battery is near
empty. It is fortunate that estimating battery capacity used with an ESV is more accurate near the
end of its life.
of error in usingan ESV to estimate the remaining capacity of a battery installed in a
plane with an ESV is lead resistance, and when you measure voltage. Battery voltage will increase
slightly with time after you stop flying. You should make the measurement as soon as you
can after landing.
You can minimize errors involved in making the measurement
by using your ESV to plot a voltage vs. time discharge curve like Figure 1 for your
own particular battery and wiring, and using it to derive a discharge curve like
Figure 3 for use in estimating the percent of charge used. To do this, charge the
battery, plug the ESV into the charging jack, and measure voltage every 10 to 20
minutes until voltage drops to 1.1 volts per cell. Divide each time reading by the life of the battery
in minutes and plot a curve like Figure 3. Doing this is more important if your ESV is one of
the cheaper ones ($12-$15). The load they provide is resistive so the discharge current is not exactly
constant-current. They are also less likely to have accurate meters.
Making A Decision To Fly
In practice, the way an ESV should be used is to measure the voltage when you take the battery off
charge to make sure that charging took place and to check for failed cells; measure it again at the
field before your first flight to make sure the switch was not left on; and measure periodically thereafter
until the voltage drops to a value at which you have previously decided to stop, and at this
point stop flying. Stopping at the 80% point makes use of about 4/5 of the total charge of the
battery and generally proves to be sufficiently conservative to keep the average flier out of trouble.
Estimate Remaining Flying Time
This leads to the question,
how can you estimate remaining safe flying time after you have flown for a period
of time and battery voltage has dropped to near 1.2 volts per cell? Clearly, an ESV will not
tell you how many minutes of flight you have remaining, just as an automobile gas gauge does not
tell you how many more miles you can drive before you have to fill the tank.
those who are of an experimental mind, remaining flying time can be estimated by conducting an
experiment with your plane in which a cycler is used to determine how much charge remained after
a flying session. The detailed procedure, not original with me, is as follows:
Start by determining the average current drawn by your radio in your plane. To do this, charge the
battery and use a cycler to measure its full charge capacity (CFC) in mAh.
(3) Go to the field the
same day the battery is taken off charge. Fly the plane for 45 minutes or more total,
flying as you would normally fly. Keep track of the total time in minutes that the plane is in the
(4) Take the plane home and use a cycler to measure the remaining battery
capacity in mAh.
These data will allow you to calculate the charge in mAh used during your flying session. Since you know
how many minutes you flew, you can calculate the mAh used per minute. Knowing the full-charge
capacity of your battery in mAh, you can estimate the total flying time for a full charge, dividing
this capacity by the mAh/per minute used by your plane. When you go out to fly, keep track
of how many minutes you have flown and how many minutes you have left, remembering to leave
a margin for safety.
A year or so ago my son used the above process to determine the average current drawn by his 60-size
STIK sport plane with five servos (two for ailerons) using an O.S. 60 engine and a Futaba
radio with a 600 mAh airborne battery. The average current drain was found to be 268 mA.
With this plane/battery combination and using 80% of the capacity of a 600 mAh battery, one should
expect about 1.8 hours of flying time from a fully-charged battery.
Is all this worth doing by the average flier? My opinion is no. It's probably not worth the risk to try to squeeze the last few minutes of flying from a battery. To maintain a high comfort level, one should go to the next larger battery or be prepared to recharge at the field from your car battery. he difference in weight between a Sanyo 1000 mAh and a 600 mAh 4-cell battery is only about 2.28 oz.
up, you might be getting by without using an ESV by charging your battery before every flying
session, and flying for a total time that represents less than half of the capacity of your
battery. It is like driving a car with a broken fuel gauge. Such a practice limits your flying time and is a gamble that will someday bite you. You can avoid the risk by buying or building and using an ESV.
Application To NiMH Batteries
While I was in the process of writing this article, I began to wonder how useful an ESV designed for
use with NiCd batteries would be in estimating the percentage of charge used from Nickel-Metal
Hydride (NiMH) batteries that some are beginning to use because of their high gravimetric
energy density. For a given weight they have a capacity of approximately twice that of a
On contacting Sanyo, I learned that NiMH cells have voltage
and discharge curves very similar to those of NiCd batteries. The small difference
between cell voltages at the same percentage of discharge, makes it unwise to go
by the color zones on your ESV. What you should do to use existing ESV's safely
with NiMH batteries is to run a voltage vs. time discharge curve like Figure 1 using your
ESV as described earlier in this article, construct a voltage vs. percent of full charge used
curve like Figure 3, and make your charge/fly decision based on reaching a voltage representing
the percent discharge point you have chosen to stop.
Voltage and capacity
are not the only differences between NiCd and NiMH batteries. Before deciding to
use NiMH batteries, be sure to look into their special requirements and limitations.
Robert S.Hoff, firstname.lastname@example.org
by permission of Robert Hoff