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!