Below you’ll find email answers I’ve provided to many friends and customers over the years.
"Dave, I read the battery info on your website but did not find the answer to the Following question (it may have been there but I didn't see it). It is my Understanding that if I charge my NiCad Tranny pack at significantly less than 1/10C internal shorts will develop. NOW, if I regularly use my stock 60mA charger on my 1100mAH NiCad tranny pack (about 1/20C), I am going to loose some of my 1100mAH capacity. My limited experience tells me this is true."
Not true, it's a myth that's been perpetuated for years. You’re filling a gas tank. In this case, your tank is larger so more time is required to fill it when it's empty since your flow of fuel is at the same "rate" as it was on the old pack. However, exactly the same amount of time is required to fill the same size hole. So, if a trip to the field used up 300mah in your old stock TX pack, it would be filled in 7 hours at 60mah. The same 300mah hole in 1100 or 1650 or 5,000 pack would take the same amount of time to fill.
"Does NiMH have this same characteristic of wanting to be charged at 1/10C minimum?"
No, NiCads don't have that characteristic either. It's just a common (though not universal) recommendations from manufacturers. Too often people will try to apply extra meaning to recommendations that was never intended.
"I am going to order a tranny pack from you and am trying to decide what to get. I don't mind long charge times. I am trying to avoid a decreased capacity situation.
Thanks for your help, Mike"
You won't have a problem as long as you understand the minimum charge times involved. However, you’re rarely charging an "empty" battery. Your only replacing what you last used. The size of the "hole" in the pack is what determines the fill time. A more extreme example is out 1650 TX packs. 90% (or more) of the people who own these are charging them on a traditional 50 or 60mah wall wart charger. Once you get the pack full, overnight will keep it full as long as you haven't deep discharged it. So, 42 hours to charge initially, but after a 4 or 6 hour trip to the flying field, it's not empty and 50mah overnight is plenty good enough to "top" top it off again.
Where these types of myths are started and perpetuated is a fellow buys a new pack. Lets say it's an 1800 Sub C RX pack for a new larger model he just finished. He charges the thing on his stock 50mah wall charger for 14 hours. Overnight always filled the stock factory pack so why wouldn’t it fill this one he thinks. Later he cycles the pack after some use and discovers it only has 600mah or so in it. He charges again overnight, cycles down again. Again it has 600mah in it. He tells his buddies you can't charge large packs on a small charger. He's totally 100% wrong. He just did not understand the concept that a bigger gas tank takes longer to fill. Problem is, the falsehood gets repeated so many times it becomes "fact" in peoples minds. Most people will not take the time to test such advice and presume there good buddy old “Albert” would never guide them wrong.
A great question even if the answer is a bit long, lots of good meat here.
"Dave, I recently purchased 15 single cells from you. I made two 720 MAH 6 volt NiMH packs and one 830 MAH 6 volt pack. I can't get the 830 pack to take over 770 MAH. Both 720 packs will take 760 and 780 MAH. I charged on C/10 for all packs (Litco Alpha 4 charger). After I cycle them using a C/10 charge and they peaked I gave them another 3-5 hours at C/10. I gave the 830 pack another 8 hours on C/10 after it peaked and read having discharged 770 MAH. It still did not take more than 770 MAH when I cycled it again. Do I have a bad cell in the pack? I am worried about using the 830 pack in my plane. I read all you info on your site about how to charge the packs and followed the guidelines you have there.
Help please, Dick"
I never judge a NiCad pack until the 3rd or 4th cycle. I never Judge a NiMH pack until the 4th or 5th cycle. That's about how many charge cycles it takes for packs to "Wake Up" and break in fully.
What most people fail to do is measure the testing device.
First most common way an error is introduced into testing a pack is discharging above C/5 (Capacity/5). The correct rate for these cells to match factory test is C/5 or 160mah for the 800 cells. If your discharging faster (over 160mah) you get lower numbers, slower discharge (under 160) you will get higher capacity numbers. The rating is "relative" to a 5-hour discharge. If you can't set exactly 160 mAh (not sure of Alpha spec.'s) set near as you can. 150 or 200 is reasonable. I can't give you a factor for how to adjust your numbers for over discharge at 200mah. But I'd expect 10 to 40mah.
If a customer were to send me a pack he thought might be bad, I will often discharge it at Cx1 the first time through. In your example of 800mah pack that would be .8amp (800mah) discharge rate. I realize my first test won't be dead on accurate but I can get a pretty quick idea if the pack is good or not. For example, if I get a reading of 700 (or in the neighborhood) then I know right off it's a good pack, if I get a reading of 400, clearly it's not going to be a good pack or needs to be rebalanced or broken in fully or has some sort of problem. Of course, before returning such a pack, I always do the 5-hour discharge in the interest of providing a "real" accurate number.
Second most common problem is slight calibration error. If you were discharging at 160mah and the charger was really discharging at just 6 mAh higher than you had it set for, it would under-rate the pack by 30mah. You need a volt-ohm meter with an AMP or MAH settings on it. You break one of the wires running to the pack (we sell a handy test Y harness that helps here) and let the pack discharge through the meter.
This gives you an independent check on the dischargers accuracy.
What goes into a pack is "meaningless" as to a packs true capacity. You can only measure the capacity for a cell accurately by discharging it.
Rare but not unheard of is a clock on the a unit running fast or slow introducing an error into the numbers.
BIGGEST thing to understand here is there are NO chargers/discharger's that actually measure their own discharge current (discharge rate). That sounds amazing but here is how it works. If the discharger is set by the user to "500 mah" (or .5 amp) then the calculation goes like this. 500mah X "Discharge time in hours" = Capacity. So, if a charger runs 4 hours at 500mah discharge rate then 500x4=2000 or 2000mah. Now, the nugget of information here to understand is this number it uses in the math formula is "set", not measured. So if the discharge rate is really 460mah, the unit has no way to know this. It just multiplies by the "user set" number (500) x hours. When truly the real capacity at this "real measured" rate of 460 mah would be 460x4hours=1840mah capacity. A common problem with older Accucyle's (it can happen to any brand) is they are often discharging above the "user set" rate. Giving "Pessimistic" numbers. If the real rate were 583 for example, the pack would discharge in 4 hours and read out at 500 (user set) x 4 hours= 2000mah when in reality it tested out at 583x4 hours= 2332mah. If you were testing a CP2400 NiCad, you'd have an inaccurately "low" result for the pack which might cause you to condemn it before it's time.
Really, 50mah either way on a pack like this (720’s and 800) is a good pack. Battery packs are not scientifically precision containers like a test tube is. They are rough approximations of the overall batch "AT" the time the cell was first introduced. They tend to creep upwards in capacity over time (years) and more and more batches of cells are made and the factory generally improves its technology. Once every few years when cells are consistently passing higher tests they will at times increase the rating of the cell. Usually it's the very same cell, they just got good enough at making them over time that they can now call an 1000 cell a 1050 for example.
I know this is a lot to digest and it does require a little work in your part to really get accurate on testing a pack. Hope this helps get you right down to the brass tacks.
Happy Flying! Dave
"Dave, I just finished reading your Q&A's regarding Nicad vs Nimh packs. I have been using a 1450mah nimh tx pack in my Hitec Eclipse 7 transmitter for the last approx 1.5 years. I use the Ace Super Digipulse to maintain all of my battery packs. After a flying session at the field, I will recharge my tx nimh 1450mah pack at the 1500."
Mmmmm, I'm not sure of the quality or brand of this cell, at that capacity, it's not a FDK (Sanyo). So, if it has a short life (could last a long time, who really knows) you can't necessarily subscribe the problem to charging practice, it would be a brand issue. I find underperformance initially or over time a common occurrence with non FDK (Sanyo) (or non SR) brand packs.
"...setting on the Digipulse and leave it charge for the full 16 hours and then it will go to trickle where it will stay until I again go to the flying field which could be once a week or once every 2-3 weeks. Yesterday, I saw a thread in the battery forum of RCU that said that I will destroy my NiMH tx pack using the charging/maintain charge methods that I have been using."
Yes, it sounds like you've got the rate programmed to 150mah, which is 5mah over the limit. I find you don't get full life when you do that. Your a little over the limit and a lot of people would just shrug their shoulders about it. However, I've found that cells won't hold up consistently when you do this. NiCad can take abuse longer but they will be killed in time just the same. Best to set the rate at C/10 (capacity/10), which is 145mah (or less) in your case.
"Now I'm confused. Your comments/advice will be appreciated. If I decide to switch back to a NiCad tx battery for my Eclipse 7. What do you recommend? I currently am storing 3 aircraft on this transmitter. Thank you. Jerry, Fairfield Glade, TN"
There is no need to switch back to NiCad on your TX. I’ve no idea what you’ve been reading. Bear in mind there are all types out there in the internet and they everybody seems equally credible when they write things about a subject you might be a little weak on. Don’t let them cause you to call into question technology (NiMH) that is in use and the most popular (80% of our sales or more) in the RC Community.
The Digipluse goes down to trickle after 14 hours or so, as long as you have the rate set to 10% of the cells capacity or less you'll be fine. IT DOES NOT MATER that the pack may only be 1/2 empty and after 7 hours it's re-filled and you are continuing to charge for another 7 hours. Though this may be slight wear on the pack, it's not going to be a problem that builds up faster than the "Age" of the pack. The calendar should be the first thing to get the pack. After 3 years you are nuts to fly an RC pack of any flavor.
Presuming you are "not" running them flat, there is no harm in setting rates lower than Capacity/10. For example, at 100mah you’re still going to fill the "hole" in the battery after 14 hours. The issue with this practice is if the battery is flat (ran on discharger or switch left on by accident) then you'll need more than one charge cycle to fill the pack.
You'll find that if your load testing a given pack at around 500mah (1 amp for 2400 and above) and you stop use at 1.2V per cell (4.8V for 4 cell, 6V for 5 cell, 9.6V for 8 cell) that you'll still have about 40% of the cells capacity remaining. This is a rough estimate but you'll find it reasonably on target if you do some discharge testing after a flying session where you stop when the batter reaches this level). Bearing this in mind. In your 1450 example, when it starts to fail a loaded ESV (Expanded Scale Voltmeter) test you've only use about 780 mah (60% of 1450mah.
"I got the following packs from you:
Item - 44 RRC4H2700F Radical RC 2700mah 4 Cell 4.8V NIMH RX pack 1
Item - 51 RRC8H1650S 1650mah 9.6V NIMH Square TX Pack 1
I have read that I should charge these the first time without using my SuperNova 250 charger can I use the charger that came with my radio even though it is made for NiCad's?"
A. You are correct; you may use your wall charger if proper time is allowed.
"It is a Futaba charger with the following charge rates:
9.6vdc at 50mm & 4.8vdc at 50mA"
The rule for NiMH charging on "dumb" style slow chargers is Capacity of pack divided by 10 or less. You 2700 pack could take up to 10% of its capacity or 270mah charge rate.
"My guess is this will work but I did not see anything on the site that spells this out. If so, are the following the correct charging times?
Capacity (2700) x 1.4 = 3780. 3780 / charger output (50 mah) = charge time in hours of 75.6.
Capacity (1650) x 1.4 = 3780. 2310 / charger output (50 mah) = charge time in hours of 46.2"
You've got a perfect grasp on how to calculate charge times at low rates.
"If this charger will not work for the first charge what is the best charger to purchase to service these packs?"
Your wall charger will work fine given enough time.
"Any and all help would be greatly appreciated. S. Miller, Lake Mary, Florida"
Very popular chargers for this task are the Ace DDVC and the Ace Super Digipulse which allow higher rates to be set and at many small intervals. Using these more advanced tools you can do the break in charge on a wide variety of packs overnight (14 - 16 hours). Not a tool you must have with the above packs but a very nice addition to your workshop when the time comes to treat yourself to something new.
"Hi Dave, I have a question on your Blue Bird 15A ESC. I'm planning to use a 2-cell Li-Poly with it...does the Blue Bird have a programmable volt cutoff so that I can program the cutoff for 6 volts (3 volts per cell)? If it's not programmable, do you think it would be safe to use with a Li-Poly, or should I just go with the Castle Pixie 20P?
Mike, Great Lakes"
Mike, The Blue Bird is not programmable. I use mostly Castle personally. However, I have NEVER bothered to set voltage cut-off for any lipoly pack and I've been flying them for 4 years, one of the very first pilots in the country showing and flying Lipo's at events. YES, I have some 4 year old packs that are still good. If your in the habit of flying to voltage cut-off you'll be wearing out your lipo's REGARDLESS. Best to fly until you first notice the pack getting soft (this is at about 60% use) and stop, switch packs and put the first on charge.
This is how I've always dealt with lipos. I realize I'm in the minority on this opinion. However, it seems the "impatient pilots" who change their packs just as soon as the flight performance drops off a bit are the ones who have the packs that last a long time. The ones who fly to BEC cut-off routinely are the ones who seem to report packs fading away prematurely.
It is my view that any lipo flight that ends in a BEC cutoff is a pilot error flight. It's like a bad landing. An occasional occurrence isn't going to kill you but if you do it often you're going to wear out your packs.
Lipos have brought us a whole new world of e-flight. One of the things that has to change is this "fly to terminal empty" philosophy. Some may disagree with me on this but I feel they will find in time that their packs don't last quite as long. Cheaper in the long run to own a few extra packs and treat them (on average) more gently.
"Hi Dave. I read through your web site reference on battery selection. Of interest was the section new battery packs (obviously because I just got a pair of 1650mah TX's from you).
I am new to the RC sport. New meaning that I have not yet gone through any battery failure that has caused me any problem and have only recently bought new batteries. I bought 1100mah RX, two of them, from Airtronics and the two TX 1650's from you.
Now your document stated that I should initially charge at C/10 overnight. Does that assume anything?"
Yes, to clarify "C" is the common abbreviation for "Capacity". So, the capacity of your pack divided "/" by 10 is the ideal overnight rate. A C/10 rate for 14 hours will fill an empty battery.
"Should the batteries be 'cycled' first or can I just put them on the TX side of my AccuCharge using the 150 ma charge current setting for 8 cells and just let it go overnight?"
No problem to charge and fly. You only cycle if you wish to prove pack is good. Be aware that NiMH take 3 to 5 cycles to come up to snuff. Some people will do one cycle to verify 80% or better capacity (all you should expect first time through) just to feel secure. Charge rates for accurate tests should be set to Capacity /5. Higher rates mean more pessimistic discharge number, lower rates more optimistic.
"The AccuCycle, when using the cycle mode (you probably already know this), the discharge cycle is dependent on the discharge rate, in this case, for TX it is 250 or 500 ma with the 500 ma setting for batteries over 1000 mah. Then after the voltage reaches the cutoff point, I believe is 1.02 volts, it starts to charge at one of three settings, 25, 50, and 125 ma. It says that it will charge for up to 15 hours if necessary. (1650 / 125 = 13.2 hours). Am I on the right track?
You've got it almost right. No battery or charger is perfectly 100% efficient. You must put in about 140% to be certain of filling the pack. So, 1650 x 1.4=2310mah needed input. 2310/ charger output = charge time in hours. So, at 125mah it would take 18.48 hours to fill an empty 1650 pack.