Below you’ll find email answers I’ve provided to many friends and customers over the years.
Will a small charger hurt my big pack? (The C/10 myth)
A: 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 an 1100 or 1650 or 5,000 pack would take the same amount of time to fill.
A: No, NiCads don’t have that characteristic either. It’s just a common (though not universal) recommendation from manufacturers. Too often people will try to apply extra meaning to recommendations that was never intended.
A: You won’t have a problem as long as you understand the minimum charge times involved. However, you’re rarely charging an "empty" battery. You’re 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 our 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 it off again.
Where these types of myths are started and perpetuated is a fellow buys a new pack. Let’s 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 people’s minds. Most people will not take the time to test such advice and presume their 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.
Why won’t my pack test at its rated capacity?
A: 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, the 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 you’re 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 the Alpha’s specs) set as 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 C×1 the first time through. In your example of an 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 setting 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 discharger’s accuracy.
What goes into a pack is "meaningless" as to a pack’s true capacity. You can only measure the capacity of a cell accurately by discharging it. Rare, but not unheard of, is the clock on a unit running fast or slow, introducing an error into the numbers.
BIGGEST thing to understand here is there are NO chargers/dischargers 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 "500mah" (or .5 amp), then the calculation goes like this: 500mah × discharge time in hours = capacity. So, if a charger runs 4 hours at a 500mah discharge rate then 500×4=2000, or 2000mah. Now, the nugget of information to understand is that 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 the user-set number (500) by hours — when truly the real capacity at this real, measured rate of 460mah would be 460×4 hours = 1840mah capacity. A common problem with older Accucycles (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) × 4 hours = 2000mah, when in reality it tested out at 583×4 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 its 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) as 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 a 1000 cell a 1050, for example.
I know this is a lot to digest and it does require a little work on your part to really get accurate on testing a pack. Hope this helps get you right down to the brass tacks.
Happy Flying! Dave
Maintaining a NiMH TX pack on a trickle charger
A: 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 ascribe 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.
A: 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. You’re 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.
A: 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 on the internet, and 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 Digipulse goes down to trickle after 14 hours or so; as long as you have the rate set to 10% of the cell’s capacity or less you’ll be fine. IT DOES NOT MATTER that the pack may only be half 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 (run down on a discharger, or the switch left on by accident), then you’ll need more than one charge cycle to fill the pack.
You’ll find that if you load-test 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), you’ll still have about 40% of the cell’s 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, stopping when the battery 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 used about 780mah (60% of 1450mah).
Happy Flying!
Dave Thacker
First charge on new packs with a wall charger
A: You are correct; you may use your wall charger if proper time is allowed.
A: The rule for NiMH charging on "dumb" style slow chargers is capacity of pack divided by 10, or less. Your 2700 pack could take up to 10% of its capacity, or a 270mah charge rate.
A: You’ve got a perfect grasp on how to calculate charge times at low rates.
A: Your wall charger will work fine given enough time. Very popular chargers for this task are the Ace DDVC and the Ace Super Digipulse, which allow higher rates to be set 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.
Happy Flying!
Dave
LiPo voltage cutoff — and how to make packs last
A: 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 LiPos at events. YES, I have some 4-year-old packs that are still good. If you’re in the habit of flying to voltage cut-off you’ll be wearing out your LiPos 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.
Blue skies,
Dave
Do I need to cycle new packs before flying?
A: 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.
A: No problem to charge and fly. You only cycle if you wish to prove the 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 a more pessimistic discharge number, lower rates more optimistic.
A: 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 × 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.
Happy Flying!
Dave Thacker
Radical RC
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