By Wayne A. Diercks – 5/4/2015

Intro 

I spent many hours of test flights and battery voltage testing in composing
my article on Battery Test Results for the SYMA X5C-1. Before and after
each flight I took voltage readings using my six-dollar, digital voltage tester
from Harbor Freight. In fact, mine was actually free using a coupon with
another purchase. It has been one of the best six dollars I never spent.

I always found it cumbersome to test battery voltage using the supplied
three-foot (1 meter) test leads on the tiny connector test points of the male
battery connector plug. I longed for something better that would also allow testing
while under load. I felt that if I could simply plug the male battery connector into a
female connector these voltage tests would be much more enjoyable and handsfree.
Well, it has been said “necessity is the mother of invention” and even though I am a slow
learner, I finally invented one.

Harness Procurement

I purchased three cable harnesses from Banggood (a popular Chinese RC model
dealer) for well under two dollars each for this and other projects in the works. They
are actually advertised to be used for charging up to five batteries at the same time
when connected to your model’s included, regulated, USB charger cord. However, I would not recommend it for charging multiple batteries, as each battery should have its own distinct regulator for charging safety and maximum battery life.

For that purpose I highly recommend using a 4-port (or more) multi-charger,
each port independently regulated, which costs just a few dollars. It also
consistently charges each battery to the correct voltage – safely. Often this
charger is bundled in a package deal of four or more batteries.

This cable harness consisting of white “Losi Micro 2.0 2-Pin Connector
Plugs” is hard to find and you must search on their sites using the words,
“charging cable for 3.7v battery” at Banggood, Amazon or eBay. You
must spell this exactly and have no space between “3.7” and “v”. Trust me.

Construction

I simply spliced the positive and negative leads of my digital voltage tester (actually a
multipurpose volt, ohm, milliamp tester) into the middle of the cable harness. I stripped
the insulation of each lead of the tester a short distance from the banana plugs that insert into the tester. I also stripped the middle of the heat-shrink connection of each lead of the cable harness and I wound the stripped wire from the corresponding color lead of the tester around this area. I then soldered the joints for a quality, permanent connection and
wrapped electrical tape over each joint to prevent shorting. I also took the opportunity to
wrap electrical tape around most of the exposed testing tips at the end of the tester leads to minimize the possibility of accidental shorting.

You actually need only one female connector out of this cable harness
assembly but it does no harm to leave the other leads and connectors. The
red colored connector does not even apply to the X5C quadcopter. Besides,
there are other uses of this assembly using the remaining connectors.

Battery Tester

Once built, your new battery tester can become functional by
simply plugging a battery into one of the female connectors and insuring the black banana
plug is inserted to the “COM” jack and the red banana plug is inserted in the jack
immediately above it. Set the dial to point to 20 volts on the DCV portion of the voltage
selection and turn on the meter. Accurate voltage measurements to the nearest hundredth of a volt will be clearly displayed, which is of prime concern where hundredths of a volt matters.

Battery Maintainer

One other important use of this assembly is draining down one or more fully charged
batteries to the recommended 3.8 volts for long-term storage of over a week or an entire
winter. To do this, simply insert the white male connector into the X5C quadcopter while
connected to the tester with one or more nearly identically charged batteries – all connected in parallel. Then, simply turn the quadcopter on,  bind and idle the motors as you monitor the voltage drop – taking care not to lift off by weighting down the quad and avoiding tangling any wires.

Once again, I must stress the importance of using only batteries of nearly
identical discharge state whenever connecting multiple batteries to the
harness as the higher voltage battery will far too quickly drain to the lower
voltage battery causing excessive heat, probable battery damage and
possibly a flash fire. If unsure about the current charge state of your
batteries, before beginning you can easily test the battery voltage of each
with the tester – or simply drain them one at a time. If you are still
uncomfortable with this, you can simply snip off four of the five female
connectors at the harness connection point insuring always-safe single
battery operation. LiPo batteries are nothing to play with.

Other Diagnostic Uses

You could also use this assembly to monitor a battery under load by
observing the voltage drop as your quadcopter is being powered by a
particular battery, as in the previous configuration. You might want to
compare its rate of discharge as compared to a similar battery in determining
the battery’s health.

Conversely, you can test the charging rate of a particular battery or charging
port by monitoring it under load. To accomplish this feat, simply select any
one of the female connectors and plug your battery into it. Then plug
the only male connector into one of the charger ports and monitor the
charging rate. This may help in diagnosing a suspected bad battery or
even a flaky charger port.

Other RC Models

I am sure that you can build a similar battery tester assembly for other
quadcopters and RC models with one cell (1S) LiPo batteries using the
appropriate wiring harness. However, a study of balance chargers should be
undertaken before applying to 2S or 3S applications for safety.