NiMH (nickel-metal hydride) packs don’t get as much buzz as newer lithium packs do these days, but they still play an important role in model aviation. Among other things, NiMH is still the most common battery type found in RC transmitters and they continue to be a popular choice for receiver packs in gliders because that type of aircraft can actually benefit from the extra weight. Gliders are also frequently flown over fields and hills covered with dry vegetation so NiMH packs are preferred because the cells don’t have a tendency to “vent with flame” when they’re damaged like LiPo cells do.
All three of my RC transmitters are powered by NiMH packs and I’ve been building and flying slope planes with NiMH receiver packs in them since 2004, so I have considerable experience charging and maintaining them. Not all of it was good, though. For example, my first attempt at charging a transmitter with a peak-detect charger didn’t go well and resulted in a heat-warped battery cover and I’m sure I hurt the battery pack, but that wasn’t apparent right away. It also took me less than one season to ruin all the the NiMH packs in my slope planes and I had to cut the EPP planes open to replace them. That’s when I knew I had to learn how to charge NiMH packs without ruining them and the purpose of this article is to share what I learned with you.
The crux of what I learned can be summarized with one short sentence: Warm is OK but hot is not! NiMH cells get warm when you charge them and the key to long cell life is to never let them get hot. I’ll add that at reasonable charge rates, NiMH cells don’t start to heat up until they’re about two-thirds charged. As the cell chemistry approaches energy saturation it’s ability to absorb more energy drops and the excess charging current becomes heat, and the trick is to not let the heat build to damaging levels.
That said, let’s examine the two common methods for charging NiMH packs: Constant current (CC) and automatic peak detection. Actually there’s three methods because automatic peak detection comes in two flavors which are voltage peak (-deltaV) and temperature peak (deltaT), but -deltaV is by far the most popular and the only one I will discuss:
CONSTANT CURRENT CHARGING
Constant current chargers are typically “wall wart” style transformers and as the name suggests they provide a constant flow of regulated current. These chargers are considered “dumb” because they can’t sense when a pack is “full” and they continue to charge until they’re disconnected.
When matching a CC charger to a pack, the most desirable charging rate is 0.1C or 1/10th of the pack’s mAh rating. In other words, to charge a 2000 mAh battery pack you would typically choose a CC charger that is rated at 200 mA output or less (2000 x 0.1 = 200). The reason for this is a 0.1C charge rate usually isn’t enough to cause a NiMH pack to overheat if it continues after the pack is fully charged.
I said I wasn’t going to get scientific, but I need to include some information about the role Ohm’s Law plays in the constant current charging method: Simply put, if A CC charger is capable of delivering at least as much voltage as a fully-charged pack, then Ohm’s Law will handle the voltage regulation as long as the charger can regulate the current. In other words, a charger with a higher rated voltage than you need (within reason) will typically work fine if it can regulate the current, but a charger that can’t provide enough voltage won’t be able to charge the pack. That means (for example) that you can usually use a 6-cell charger to charge a 4-cell pack or an 8 cell charger to charge a 6-cell pack (as long as the current rating is appropriate) but the reverse won’t work. Don’t try that with Lithium cells, though!
The problem with CC charging at 0.1C is it takes as much as 12 hours to charge a depleted pack. CC charging at a higher rate will reduce the time, but then you risk overheating the pack if you don’t disconnect it as soon as it’s charged. This problem is compounded by the fact that we don’t always discharge our packs completely before we charge them and it’s next to impossible to determine how long it will take to recharge them with any degree of accuracy.
CC charging can also lead to overheating if the pack doesn’t have adequate ventilation, and that’s true even at low rates like 0.1C. The heat buildup will be slow, but it can definitely reach damaging levels if given enough time. Been there, done that, and the slope planes with the dead NiMH packs in them are now wall decoration.
PEAK DETECT CHARGING
NiMH cell voltage exhibits an interesting characteristic during a charge cycle: The voltage actually peaks when they’re fully-charged and then it drops off a little. This voltage drop after the peak is known as the negative voltage delta (-deltaV) and it’s measured in millivolts (mV). A peak detect charger monitors the pack voltage while it’s charging and either ends the charge cycle or switches to a trickle mode when it “sees” enough -deltaV after the peak.
The -deltaV setting can be called just about anything in a programmable charger’s menu and there doesn’t seem to be any standard. I’ve seen it called “deltaV”, “Sensitivity”, “Delta Peak Sensivity”, “Delta Drop”, and other variations, but they all do the same thing and the setting is almost always entered in “mV Per Cell” terms. That means you’ll almost always enter the -deltaV for one cell and let the charger do the math.
Typical -deltaV settings for NiMH cells will range from 4mV to as much as 7mV but it’s important to understand the advantages and consequences of incorrect settings. High -deltaV settings charge packs more completely, but the packs get hotter and chargers might miss the -deltaV event altogether. Lower -deltaV settings may not charge packs as completely and they might be prone to false peak detection (premature charge cycle termination), but the lower settings also result in lower pack temperatures at the end of a charge cycle.
Default NiMH -deltaV settings are almost always way too high. 7mV seems to be the norm and manufacturers do that to avoid complaints about false peak detection. Unfortunately, -deltaV settings that high ruin battery packs so the first thing you should do before charging your packs with a new or unfamiliar charger is check the setting and adjust as necessary.
The -deltaV setting is probably the most important setting when it comes to long NiMH pack life, but charge rate is almost as important. Higher charge rates result in faster charge cycles, but they don’t charge packs any better than low rates and there’s plenty of evidence than low rates result in longer pack life. A good rule of thumb is you should never charge NiMH packs faster than you need to and except in extreme situations (read: bad planning) you should stick to rates between 0.3 to 0.5C.
There’s one more important thing to remember about peak-detect charging: The -deltaV becomes a lot less distinct at low charge rates which is why it’s a bad idea to charge NiMH packs with a peak detect charger at rates below 0.3C. In other words, at rates below 0.3C the charger might not be able to detect the peak. Some chargers might even miss the peak at 0.3C so it’s important to pay attention to pack temperature at low rates until you’re familiar with the peak detection characteristics of your charger.
HOW PGR DOES IT
Now that I’ve covered the background information it’s time to explain how I actually charge and maintain my own NiMH packs, and the short version is it depends on whether I’m charging loose packs on the bench or packs in airplanes and transmitters. I really have only one charging scheme, but the “depends” qualifier in the last sentence covers how much I’m willing to deviate from that scheme in a pinch. I’ll add that I’m not a fan of CC charging, but it’s a personal thing and I can’t criticize the method as long as packs aren’t left on chargers once they’re charged. I’ve had outstanding results with automatic peak-detect charging, though, and I haven’t heard any compelling reason to change.
My default charging scheme is a 0.35C charge rate with a 4mV -deltaV setting. I use these parameters for all my NiMH packs unless I have a specific reason for deviation.
For AA and AAA receiver packs embedded in foam I’ll raise the charge rate to 0.4C and reduce the -deltaV to 3mV if I’m in a hurry, but I really try to avoid doing that.
I’ll charge my transmitters at as much as 0.6C with a 4mV -deltaV if I’m in a hurry, but I’ll take the battery cover off to provide more ventilation.
I have charged loose packs on the bench at 0.75C with a 4mV -deltaV, but only because I was irrationally impatient and I manually interrupted the charge cycle when they started getting warm.
I haven’t “broken in” or “conditioned” any new NiMH cells since I started using Eneloops, and that was in 2005. I do one charge-discharge-charge cycle at 0.3C to check for defective cells and then I put the pack to work
Once a year I’ll discharge every pack to 0.9V per cell and do a 14 hour CC charge at 0.1C to bring the cells back into balance. Then I do a discharge-charge cycle at 0.3C to check the capacity and if they test good I don’t worry about them for another year. I’ll also perform this test if I suspect a pack is having issues, but that’s only happened twice with my Eneloop packs and the packs tested good both times.
HOW IT’S WORKING FOR ME
I’ve used Eneloop packs in all my transmitters and gliders since 2005 and all but one pack still tests within a couple percent of the readings I got when they were new. The one pack that failed on me was a victim of physical damage and as far as I’m concerned it doesn’t count against a perfect record. That record supports two opinions and I don’t hesitate to express either one:
- Eneloops are excellent NiMH cells.
- I’m maintaining them correctly.
A QUICK NOTE ON CHARGERS
Chargers come in a huge variety of makes and models and some are a lot better than others, especially when it comes to charging NiMH packs. My favorite for NiMH packs is the Hobbico AccuCycle Elite and I have two of them. The -deltaV setting is adjustable down to 3 mV and it’s not prone to false or missed detection. Another charger which seems to work exceptionally well for charging NiMH packs is my iCharger 4010DUO, but a $350 charger with 40A outputs is definitely overkill for a NiMH charger.
I also have a Hitec X2 and X4 which both seem to work reasonably well for NiMH packs, but typical of many mainstream chargers the -deltaV setting will only go down to 5mV and they heat up my NiMH packs a little more than I prefer. I’ll use them to charge TX packs in-transmitter and RX packs I can remove from aircraft, but I won’t use them to charge packs embedded in foam.
The bottom line is if NiMH charging capability is important to you then there are some extra considerations you need to keep in mind when choosing a charger. If battery pack life is important to you, then the simple fact that a charger has NiMH settings is only one of several things you should research.
Final note: I’m not affiliated with Hobbico in any way, shape, or form and I’m not being compensated for promoting the AccuCycle Elite. It’s just an exceptional charger that I personally use, and something I can unconditionally recommend with a clear conscience.