NoBS Batteries
No Bull.. Just Great Batteries!
No Bull.. Just Great Batteries!
Q:
What's the big deal about 'Impedance'? What is it and what does it do?
A: Most folks react to the
'capacity' rating alone as being the biggest part of their decision in selecting
a pack for their big aggressive birds. This can lead to big trouble on board the
aircraft. Just as cells are evaluated for capacity, in high load applications
you need to take into consideration what the cells voltage performance will be
when loads are applied. All battery technologies react to servo loads with a
voltage drop while the load is applied. It's really very simple.. the higher the
cells impedance rating, the greater the voltage drop will be while servo loads
are applied. Rule of Thumb for cell selection: Impedance Ratings are like a Golf
Score. The lower, the better. Aircraft in aggressive aerobatic environments
employing digital or high torque servos should carry a cell impedance score of
10 mOhms or less. As an example, one of the most popular lightweight packs used
in IMAC aircraft is the Sanyo HR 2700AUX.. plenty of capacity, but the impedance
rate on the cell is 20 mOhms. Pretty high.. but if it's used in parallel with
another 2700 pack the impedance rate is cut in half to 10 mOhms (and the
available capacity is doubled). This is why you usually see this pack employed
in parallel in IMAC birds and used without issues as a single pack in a giant
Cub.
Q:
Can I use my (insert the name/model of your charger/cycler here) to charge my
new packs?
A: I get this one
via email an awful lot... the answer is always the same: "Every charger mfg
ships their chargers with a manual... in that manual will be the operating
parameters and what type (NiCad, NiMH, Lithium, Gel/Lead Acid, etc) the charger
is designed to charge.
Folks.. I build
battery packs.. I don't do brain rentals and I'm surely not the planet's charger
manual librarian... and you absolutely have got to do your own 'homework'
regarding how to use your charger. If you don't have a manual for your charger
and you made it this far, you can certainly figure out how to use a web
browser/search engine to locate either a RCU or magazine review or the charger
mfg's website for technical info on their product.
So, read the
manual to learn how to use it, and how to set the charger up for the type of
battery you intend to charge. Then check the battery pack label. On our pack
labels you'll find the cell technology type and the recommended charge rates for
both fast and slow/formation charging.
Q:
What's up with 'Formation Charging'? Can't I just charge and fly a new pack?
A: The term
'Formation Charging' describes the initial charge/discharge 'forming' process
that fully activates the 'chemical engine' and balances the cells in a new pack.
NiMH packs have a particular need for repetitive c/10 charge/cycle conditioning,
a 'break-in' procedure needed to get them to full rated capacity. Both NiMH and
NiCad packs benefit from the process and the intent is to make sure that any new
pack has been verified to be fully operational and that a 'start-up' capacity
number is established for the pack before it's put into service. To 'Form' a new
NiMH pack do three 10% 16-24 hour charges followed by a 300 to 500 ma discharge
routine between each charge. In other words: Do a slow charge at the slow charge
rate (or as close to it as you can reasonably get with your equipment) as shown
on the pack label. Charge till the pack is warm, followed by a controlled
discharge with a cycler. Do it 3 times. The recommended Formation Charge/ Slow
Charge rate for our packs is printed right on the label of the pack as well as
on the data card the pack was shipped with. New NiCad packs should get at least
one slow charge followed by a 300 to 500ma discharge. Record the capacity
numbers reported by your equipment for comparison cycling as the pack ages.
Never check your brain at the door! As one respected modeler put it, "There's
nothing more suspect than a new battery pack." Before you fly anybody's pack be
certain it's fully operational and safe to fly.
Q: I
have a programmable charger. What discharge rate and voltage cutoff value should
I use for the Formation Process?
A: Most
programmable charger/cyclers allow programming the discharge current and the low
voltage cutoff level for the discharge. If using a 250-300ma (.3) discharge rate
set the cutoff voltage value at 1.0v per cell. If using a 500ma (.5) discharge
rate set it at .8v per cell. For formation charge/discharge routines on Rx and
Tx packs we don't recommend using a discharge rate higher than 500ma (.5) and
DON'T discharge under any circumstances below .8v per cell !! Packs with cell
impedance ratings below 10 mOhms CAN de discharged at 1.0 amps AFTER going
through the formation process.. BUT NOT BEFORE.
Q:
What’s ‘cycling’ and why do I need to do it?
A:
Battery
‘cycling’ in it’s simple form is the two step process of charging and then
discharging the battery under controlled conditions. Normally the purpose of
the procedure is to ‘condition’ or ‘format’ a new pack or to verify the capacity
or check the condition of a pack that’s been in service. ‘Routine’ cycling, or
discharging before recharging a pack after every use is not necessary for either
NiMH or NiCad packs, and discharging to ‘zero’ voltage under load is in fact
harmful. A ‘cycler’ or controlled discharger will limit the discharge voltage
level to prevent damage to the pack, whereas ‘leaving the switch on’ in your
radio system to discharge a pack is very poor practice and may result in damage
your radio system components or the battery.
Most ‘hobby’
cyclers or dischargers operate at a predetermined load and cutoff level that is
different than the cell mfg’s rating system. This leads to discrepancies that
can be a high as 10-15% of rated capacity between the mfg’s rating and the
rating presented by the cycler or discharger you are using. If your cycler or
discharger system’s numbers are lower than the comfort zone described above then
you need to verify the calibration of the cycler and check the charge rate and
charge time to insure the pack did indeed get a full charge before the discharge
function began. If the numbers are still uncomfortably low after re-running the
test with your new pack, we suggest you contact the assembler or retailer for
evaluation assistance in determining what the cause is before using the pack in
a critical application. Any pack cycling more than 25% below
the mfg's rating or the original numbers established for the pack with your
equipment when it was new should be removed from critical application service.
Q:
How do I know if my pack is fully charged?
Temperature is
the key.. always, always, always! IF THE PACK ISN'T WARM AT THE END OF THE
CHARGE ROUTINE, IT ISN'T CHARGED! Note I said 'warm'. Not HOT! Hot is NEVER GOOD
at the end of the charge routine.. but warm is OK. If using a temp probe, set it
for 10 degrees above AMBIENT. If it's a peak controlled charger and the charger
shuts down before the pack is slightly warm to the touch.. IT ISN'T FULLY
CHARGED. If it's a timer controlled charger and at the end of the charge period
the pack is not warm.. it's NOT fully charged yet! By far and away the biggest
reason for low capacity numbers is undercharging.. if you have a 5 gallon pail
with only 3 gallons in it, your only going to get 3 gallons out of it. Engage
your brain.. check temperature at the end of the charge routine!
Q:
Temperature?? I thought voltage was how you could tell the pack was charged??
A: More packs
have been fried by guys looking for a 'number' instead of temperature on a
charger than any other cause of premature battery failure. The 'finish' voltage
of a pack will vary under an astounding number of variables.. but temperature
rise ALWAYS signals that the cells are charged in a slow charge routine. Why?
Because when the cells can no longer absorb the energy being shoved at them by
the charger, they begin to give the unstored energy off as heat.
Taking the
Temperature story to the next level.. fast charging. Here we can generate even
more heat. Some high-impedance cells like AA NiMH's can get warm when being
charged at 'normal' 1C (fast) or even 10% (slow) rates. They can get warm BEFORE
they get to full charge. Why? Because high-impedance cells don't absorb energy
as efficiently at high current levels.. just like they don't give up energy
efficiently at higher discharge rates. So.. before you crank up the current on
your charger on your new NiMH Tx
pack.. check the pack label on our packs.. and set the charge rate recommended;
which is often LESS than the 'industry standard' for NiMH cells.
Final word on
temperature: Aside from being a key indicator for correct charge time, rate and
final charge condition bear in mind that heat is the bitter enemy of NiMH Cells.
Drive the pack into thermal overload (hot) on the charger and it's likely you'll
ruin it. Pay attention to the temperature!
Q:
How long should I wait between charging and discharging for capacity checks?
A: I actually had
a guy that sounded like he knew what he was doing run me through hoops for days
regarding low cycle numbers on his new packs. Finally he dropped the little
missive, "I've been waiting 24 hours after the charge to start the cycle...."
ARRRGH! Where do folks come up with this stuff? Here's the deal.. ALL battery
technologies have a self-discharge characteristic. NiMH self discharges at a
higher rate than NiCad. Lithium has the lowest self-discharge rate.... but the
point is, waiting a day (or an hour) to start a discharge after a charge will
trend the capacity return downwards. Nominally, depending on cell type, age,
impedance and temperature; a 10-15% capacity drop over 24 hours is not unusual.
So.. whatever goofy procedure you use.. do it the same way each time, because if
you change ANY parameter in a cycle test procedure you will effect a result in
the returned capacity number. Time span between charge termination and discharge
start impacts test results. Long leads impacts test results. Cycling through
switch harnesses impacts test results. Cycle testing in cold or hot conditions
impacts test results. Here, we cycle immediately after the charge completes. If
your cycle a day later, your numbers won't even be close to mine.. or the cell
MFG's. Same goes for long leads, cold days, running your cycler through system
switches.. ENGAGE THE BRAIN... YOURS, NOT THE CHARGER / CYCLERS!
