Here's the answers to our most frequently asked questions regarding care, feeding, technology types and system applications*

 NOTE: We keep this data current! Page last updated 05/01/2008

 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 agressive aerobatic enviornments 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 paramaters and what type (Nicad, NiMH, Lithium, Gell/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 Freaking 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?

Temeprature is the key.. always, always, always! IF THE PACK AIN'T WARM AT THE END OF THE CHARGE ROUTINE, IT AIN'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 AIN'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 gonna get 3 gallons out of it.  Engage your brain.. check temperature at the end of the charge routine!

Q: Temeprature?? 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 recomended; 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 temeprature!  

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 proceedure you use.. do it the same way each time, because if you change ANY parameter in a cycle test proceedure 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!

Q: I have a 3300 NiMH pack I got from another vendor, and since there were no instructions, I never ‘formatted’ it. Have I damaged it? Will it hold a full charge now?

A: ‘Forming’ on a new NiMH pack gently ‘teaches’ the little buckets of chemical soup (the battery industry calls them ‘cells’) that it’s in fact a ‘battery pack’. It brings all the cells up to full charge gently, and if any cells in the pack get to full charge before their brothers and sisters in the pack, those cells are much less likely to receive thermal damage. (‘Forming’ is also called ‘equalizing’ or ‘cell balancing’) The formation process ALSO garners capacity data on the new pack to use for comparison when evaluating the general condition of the pack as it ages. If your situation matches the criteria in the question above, by all means, run a ‘formation’ (slow charge) routine which will re-equalize and balance the cells in the pack and then cycling the freshly charged pack will get you your answers… if it reaches to within 10% of the mfg’s capacity numbers and passes the charge retention test, I’d say your pack is serviceable.

Q: Enh? Charge Retention? What’s up with that?

A: All stored energy systems (batteries) have a ‘self-discharge’ characteristic. This means that a ‘fully charged pack’ is in fact only ‘fully charged’ at the point it comes off the charger in a warm state. Cycling out the pack at that point gives you the nominal ‘capacity’ number for the fully charged pack. If you then re-charge the pack and disconnect it from all devices and let it sit for 2 days and THEN discharge it, it will cycle out with a capacity number up to about 25% below the nominal capacity. More than 25% capacity loss over 48 hours and its possible there’s a weak cell in the pack. Not all battery technologies have the same self-discharge characteristics. As an example, high impedance NiMH Tx packs self discharge at a more rapid rate than Nicads. This is why we always check the voltage display on out Tx’s when we turn them on and we check on- board packs with a load tester before the FIRST flight.. Why?? Well, you may have charged that pack last night… but is it safe to fly now?? Only a load check will reveal if the pack is indeed at a high enough charge state to safely fly it.

Q: Is 'self discharge' an issue, if I charge on Friday but don't make it to the field till Sunday do I have a problem?

Nope. Think about it for a minute. Do you fly without first checking your pack at the field with a load tester? If you do, THAT'S a problem. (scroll down for ESV info) I'd hope that anybody smart enough to buy good packs was also smart enough to have good field habits and would ALWAYS check the pack with a loaded ESV appropriate for the kind of pack and plane he's flying... before each and every flight; including the first one. We ALL stop flying when the loaded voltage displayed is lower than 1.2v per cell. AND, we all know the pack is 'officially' discharged as far as a capicity test is concerned at a much lower voltage than that (usually .8v per cell). That means we're never actually flying out the full capacity of the pack.. we stop and recharge or go home BEFORE we get to a dangerously low voltage level. So, using your brain this time; the answer is?

Q: Do I need to ‘load test’ my Tx pack before flight?

A: Just turning on the Tx ‘loads’ the pack with about a 250ma load, so an external load tester is not needed for Tx packs since the voltage displayed on your Tx is the loaded voltage of the pack. Below 9.5v, I’d charge. 9.6v and above.. your pack has more than enough of a charge to fly. Bear in mind that the cell mfg’s consider a cell is discharged when it reaches .8v per cell… in a Tx pack that’s 6.4v!!! Nobody in his right mind would fly a Tx pack down to that level, and most of you have Tx’s with low-voltage alarms built right in… they usually start hooting at you when the Tx pack gets under 8.5 volts… so relax, and stop beating the crap out of your Tx packs with a fast charger when you see less than 10v on the Tx voltage display.

Q: My 2700 NiMH Tx pack seems to self discharge pretty fast. After 3-4 weeks in the Tx the voltage of the pack is below a flyable level. Is there an easy way to keep the pack charged and ready to go?

A: Yep.. high capacity, high impedance NiMH cells self discharge at a more rapid rate than Nicads. Since I'm not a fan of 'trickle' charging (it's like leaving your car running at the curb 24/7) I use Red's method of adding a simple mechanical 'light' timer to my high output wall wart and set it for 2 hours of charge out of 24. This keeps the pack 'topped' and ready to use without wearing out the pack via 'trickle charging'. More details on this simple 'maintainer' is available on Red's web site:

http://www.rcbatteryclinic.com/timer.html

Q: My (insert your pet glorified battery toaster make and model number here) indicates my 3300ma pack only has 2200ma capacity at the end of the charge. Is something wrong with my battery?

A: I have no clue with just that small piece of data to go on.. But; I’d suspect right off the bat that if that pack was warm at the end of the charge routine that the pack had some charge in it when the charger started in and that the MAH display on the charger is telling you that it put in 2200ma before it decided to stop. You DID remember to check the pack temp at the end of the charge, didn’t you??

 Q: At the end of my 2700 Tx pack’s charge routine my charger said the capacity was 2965 ma? Is this possible?

A: See the answer to the question above this one… again, the Charge MA display is NOT the packs capacity. To determine the amount of energy the pack has stored, you have to Cycle the pack.. That’s Capacity. The charger display just lets you know how much energy got pushed at the pack… not how much it stored.

Q: My charger only ran for 12 minutes and shut off. What’s up? Is the pack charged?

A: So, here goes; one more time, the basics: Temperature is the key to determining if the pack is fully charged. A pack that is not warm at the end of the charge routine is NOT fully charged. A pack that is not fully charged cannot return full capacity. ‘Smart’ chargers are anything but ‘smart’ in determining ‘what’s up’ since they cannot reason and evaluate ALL the info the charger exibits. YOUR brain is the crucial element here.. so your next move is to check the pack temp. If the pack is warm at the end of a slow charge routine it indicates the cells can no longer absorb the energy being shoved at it by the charger and the cells are giving off the surplus energy as heat. Please note that should the charge rate be too high, the cells will warm up very quickly… since again; the cells cannot absorb all the energy being pushed at them at the too-high rate, hence the energy NOT reaching the cells is coming off as heat. This is why we publish both the max slow charge rate and max fast charge rate we recommend for that type of pack right on the pack label. If your battery supplier does not provide that basic info, then try using 10% of rated capacity for a slow charge rate and a maximum 1C rate for fast charging… AND CHECK THE TEMP OFTEN DURING THE CHARGE ROUTINE.

Q: On your Sanyo 2700 Tx packs, the label says ‘Slow charge 150ma, Fast charge 1.0 amps max. Why so much less than the 10% and 1C rates?

A: High Impendence, High Capacity AA NiMH cells are very fragile with regards to thermal damage. On our end we’ve learned to take significant steps to protect them from thermal damage even in the assembly process by utilizing lower weld temps. These ultra high capacity (and very high impedance) cells are indeed unique and delicate.. and that’s why you’ll never see us offer the Sanyo 2700 AA NiMH cell assembled as a receiver pack since they just can’t handle high servo current loads, high temps and high vibration associated with using them on board an aircraft. Since your Tx pack runs at well under a half amp constant current load, and it isn’t ‘up there’ being beat to death by high vibration, wild and erratic servo current loading and suffering with 130 degrees of heat under that big ‘ol canopy, the Sanyo 2700’s can do a fine job keeping your Tx going all day on a fresh charge.

With regards to charging the 2700 AA’s, even just a few minutes delay in shutdown at the full 1C (2.7 amp) charge rate can result in a fried pack. There’s just no room for error at the ‘industry standard’ 1C NiMH fast rates with these cells. Even at the industry standard 10% slow charge routines (270ma) a few extra hours of charge can force then into venting. Using the charge rates recommended on our pack labels will significantly reduce the chance of doing damage to the cells and they will definitely perform to spec longer if you stay with our recommended charge rates AND STOP THE CHARGE WHEN THE PACK WARMS UP.

Q: My NiMH pack is not warm, and the charger still keeps cutting off. Now what?

A: Most ‘smart’ chargers employ some form of ‘peak detection’ circuit… this or cruddy charger cabling (leads too long or too light guage, worn connectors and plugs, etc) is usually the culprit. Replace your charger cords and on-board switches every few seasons and use decent quality field gear. If the pre-mature cutoff persists, (the pack is not warm) try charging the pack DIRECTLY instead of through the Tx charge port or the Rx pack charge port/switch. If cycling confirms that indeed, the charger has shut off early you can attempt to get around the chargers peak detection circuit by using a higher Mv cutoff point or by using a different charge sub-routine, like using the Nicad charge routine instead of NiMh routine (higher Mv cutoff). Again, as always, don’t check your brain at the door. Check the temp often and stop charging when the pack warms up… since your charger ‘detection’ circuitry is being by-passed in this work around, you become the control system. I suggest you check out Red’s Battery Clinic and eyball the charger reviews, he details how to turn most ‘smart’ chargers into constant current slow chargers for forming and timed slow charging routines without a confused peak detection circuit stepping in and shutting the charger down early. Bear in mind, again and as always, temp is the key.. when the pack warms up YOU have to stop the charge.

Q: My Triton keeps cutting off with about 1000ma put into my NiMH 2700 pack. The pack is not warm, but the charger keeps shutting down. What's wrong with the pack?

A: Wrong with the pack?? It's pretty doubtful this is a 'battery' problem. In this particular situation it's your Triton's Auto Presets. The charger, being 'smart;' sticks to it's programing parameters.. and the default 'auto' NiMH program for that particular charger has a value of 1000ma set in it's default maximum charge parameters for NiMH. See your charger manual (page 14, bottom of the page, item #8) for what's 'wrong' and re-program that function. BTW.. the 'smarter' the charger, the more you need to review and become familar with it's programing structure, capabilities and 'decision making' paramaters. In other words RTFM (read the freaking manual) and make sure you're 'smarter' than it is so you can correctly interpet what it's trying to tell you.

Q: I’ve had a couple of packs fail after just a few seasons. How long do packs usually last?

A: Cell type and technology, dimensioning, duty application stresses, charge rates, charger types and calendar age all impact directly pack performance and lifespan. As a rule of thumb, 2-3 seasons of normal use can be reasonably expected in most R/C applications, but bear in mind that some high capacity systems can fare worse than others under fast charge and high current demands. We recommend you check your packs with a cycler once or twice per season to keep tabs on it’s relative health and the use of an ESV every flight to avoid any unwanted battery surprises. For longest service life, use a slow charge routine for normal recharging.. charge till the pack is warm, no more. Use the peak charger only when necessary at the field to extend flying times. Don't peak charge NiMH Tx packs before the voltage display on the Tx drops below 9.6v.. constant 'peaking' of NiMH Tx packs ruins them pretty quickly.

Q: What’s an ‘ESV’ and why do I need one??

A: An ‘Expanded Scale Voltmeter’ as it relates to the hobby is a test device that combines a voltmeter with a load. When activated, it applies a pre-determined fixed load to the pack and displays what the voltage of the pack is while that load is applied. A meter without a load is just a voltmeter, and unloaded voltage is NOT the info needed to make a ‘fly’ or ‘don’t fly’ decision. We recommend the load be at least 250ma for 10 seconds on any Nicad or NiMH pack used in .60 sized and smaller aircraft and a 500ma load applied for 10 seconds for larger aircraft. We recommend a safe minimum ‘flyable’ voltage while the load is applied to be no less than 1.2v per cell, or 4.81v for a 4 cell pack and 6.01v for a 5 cell pack.

Q: Ok, seems easy enough, but how often should I check my pack with an ESV??

A: Prior to every flight, including the first one. Most standard switches provided by the radio mfg’s do have a ‘charge lead’ plug and wire, but unless you add a ‘charge port’ for that lead in an accessible place on the outside of the plane you’ll likely not want to pull the plane apart to get at the plug. Either upgrade to a chargeport inclusive HD switch or add a $3.00 ‘charge port receptacle’ to your OEM switch’s charge lead to gain immediate access to the pack. Get in the habit of checking the pack before every flight. If you do, you’ll likely never lose a plane to an under performing or weak pack.

Q: How do I find out how many flights I can safely fly with my pack?

A:  The answer is different for just about every application. Factors like how you fly, what servo's you have, what the temperature is and what the type pack and it's relative overall condition is will all impact the number of flights you can safely fly with a full charge. A battery cycler can establish what the capacity of the pack is, and your ESV checks will reveal when it’s no longer safe to fly. To determine the amount of energy required to fly your plane just fully charge the pack and fly two or three of your routine flights (while checking between flights with an ESV to stay safe) and record the amount of time flown. You can then return the pack directly to the cycler and record the capacity remaining. Subtract that from the normal capacity of the pack to get the amount of capacity your flights used. Next, just divide the amount of capacity used by the number of minutes flown and you have the average amount of capacity consumed per minute of operation.

Q: Ok, now that I know how long I can fly my pack, why keep checking with my loaded ESV?

A: The value of doing a loaded ESV test before every flight becomes even more apparent when the number of safe flights count begins to materially change. Lets say my 'Divit Digger .90' is getting to the safe minimum loaded voltage after 3 flights when just a few months previously it averaged 5 flights. The big question now becomes ‘What's changed.. and why?'. Time to re-charge and cycle the battery to see if the pack is aging and needs replacement or if the charger's the culprit. If the pack and charger check out ok, what on board the aircraft is pulling all that juice? Most often I find a servo problem, either stalling at full transit (throttle setup needs re-adjusting, fairly common) or a condition developed with servo gears or control system linkages. Point is, because you had a data point to start with you were able to detect a material operational change and head off a problem before it swallowed an airplane.

Q: Are Nickel Metal Hydride packs better than Nicads?

A: The answer depends on the application. NiMH in some cell types can have higher internal impedance, this means the voltage drop under significant current loads can be noticeable; particularly in electric flight motor pack applications. In most smaller sized aircraft (up to .60 sized using 'AA' sized cells) normal servo current loads seldom impact system security. Even in aerobatic environments most control systems don’t threaten a modern 4 cell Sanyo NiMH 'A' or 'Sub-C' sized packs voltage stability while under normal aerobatic flight loads. Nonetheless, I prefer to be very safe instead of very sorry! Years of experience with both Nicad and NiMH discharge curves under demanding or spiking, erratic loads leads me to strongly recommend using a 5 cell 6v pack or a parallel pack system in overly aggressive flight circumstances and particularly in 3-D, IMAC and Giant Scale type applications no matter which cell technology or servos you choose to fly with.

As far as I'm aware, I'm the only assembler that puts the cell impedance value on every receiver pack we build... as well as the reccomended slow charge and fast charge rates. You'll also find the cell impedance ratings included with our pack specs here on the website and on our price sheets. 

Q: The ‘Old Salts’ all say I should stay away from NiMH, that they aren’t dependable. There has to be a reason why. What’s wrong with NiMH?

A: Actually, the question really should be “What WAS wrong with NiMH?”  Initially, NiMH claims commonly made were ‘More capacity and less weight than Nicad’. That was true then.. but the dependability suffered because the cell insulator materials were relatively fragile and the cell plates were pretty thin in order to keep the cells light. It turned out that ‘lighter’ wasn’t better, and when the cells were used in fast-charged commercial tooling applications significant shortfalls in duty cycle lifetimes and rapid capacity deterioration compared to Nicads became apparent. Luckily for us modelers, yet again, following the same development cycle as Nicads two decades ago, the demands of the portable power tool market has lead to the development of far more robust NiMH cells. The current genre of fast charge NiMH cells now sport thicker plates, better insulators and all welded internal assembly, features that reduced impedance and dramatically improved durability and duty cycle lifetimes in high load and fast charge applications. Today’s fast charge safe NiMH cells are no longer lighter than Nicads in the same physical dimensions but they do continue to offer significantly higher capacities when compared to Nicads of the same dimensions.

Q: How about Impedence? Can NiMH cells handle the loads from digital servos in big planes??

A: High impedance isn't native to NiMH cells.. there are more than a few Nicad cells with impedance just as high as a comparable NiMH cell. Most folks react to the 'capacity' number as being the big part of their decision in selecting a pack for their big aggressive birds. This can lead to big trouble on board the aircraft. This is why you'll find we won't sell some AA NiMH cell types for receiver & servo applications that we do provide for Tx applications. Our new 4/5 'A' 1950FAUP NiMH Sanyo Rx pack has the same impedance as a 1700 Sub C Nicad, so impedance by itself in NiMH doesn't mean you can't use them, it just means you need to be sure the cell type is appropriate for the job you expect it to do. Smaller aircraft do just fine on AA sized NiMH packs, big birds should use 'A' sized or larger Nicad or NiMH cells (and in some cases parallel setups or 5 cell packs rather than 4 cell packs) to stay ahead of the voltage drops under load associated with higher impedance cells.