Watt's Up Applications
25
A battery pack whose individual cells are all balanced delivers the most energy since all
cells are exhausted at the same minimum voltage. If any cell is "out of balance" it may
reach the minimum safe voltage before the others and continued pack discharge will
damage the cell.
If the cells in a pack all equally contribute to the overall pack voltage they are considered
"in balance". So we can check each cells voltage at various states of pack charge and see if
their voltages are the same. If they are, the pack is balanced. If not some individual cell
reconditioning or replacement is necessary.
Cell balancing measurements need at least 0.02 volt resolution so that we can tell the
difference between a 1.22 and 1.20 volt cell. More resolution is better because it allows us
to recognize the lower voltage cell which will get exhausted first. The "Watt's Up" 0.01
Volt resolution is great for cell balancing where resolution is more important than
accuracy. This is because we are mostly comparing our batteries to each other seeking
equality rather than wanting to know what particular voltage they're at.
There are two ways to measure cell voltages.
1. Measure each cell individually
2. Measure the pack’s voltage and a few cells and subtract the cell voltages from the pack
total to get the remaining cell voltage. Let's call this the "Sum and Difference Technique"
- SDT for short.
The SDT is a bad approach for some very technical reasons. The short explanation is that it
requires more accurate and linear voltage measurement equipment than most people have
in order to prevent "quantization errors" due to digital measurement effects. A more
complete explanation is beyond the scope of this user’s manual. Without that high
performance equipment the value you calculate from the pack voltage minus a cell or two
may be inaccurate to a degree that interferes with your cell balancing.
We, therefore, recommend you measure each cell independently. Be careful not to short out
battery cells doing this.
Remember that to measure below 4 volts, the Watt's Up needs to have a battery or other
power source attached to the auxiliary connector.
Protect the Watt's Up's LOAD side leads so they can't short.
After powering up, simply apply the red lead to the plus side of the battery cell being tested
and the black lead to the minus side. Read the battery voltage and write it down. Repeat for
all cells in the pack. Your goal is to have all the cell voltages be as equal as possible when
the pack is charged and discharged. If you want to load the cells when taking
measurements, you can attach the load to the LOAD leads of the Watt's Up while testing.
This will also show you the load current being drawn.
10.6
Receiver and Servo Testing
A loss of receiver power can be catastrophic. This can be difficult to diagnose and
troubleshoot. The most likely cause, other than bad connections, is drawing more current
than the receiver battery or battery eliminator circuit can provide resulting in an
unacceptably low receiver voltage. An undetected “stalled” servo is an example cause for
this.
You can use the Watt's Up to determine the peak current and minimum voltage used by
your receiver and servo loads and verify they are within acceptable limits over the full
servo operation range.
You should use the Auxiliary power connection for doing this sort of testing because
receiver operation voltage can be near the low end of the Watt's Up voltage range without
it. Connecting the Auxiliary power after the SOURCE may be better for eliminating
measurement of power dips immediately following SOURCE power connections. Try both
before and after.
"Watt's Up" & "Doc Wattson" Watt Meter and Power Analyzer User's Manual
RC Electronics, Inc.
