Every now and then I get a question from someone at an RC field, and it’s nearly always the same question: “How many milliamps is that battery?” The first time I heard that, I didn’t know how to respond because the question didn’t make sense to me. So my “What do you mean?” response resulted in another question about how big the battery was or how much capacity it had. That I could answer.
The lithium polymer (LiPo) batteries that we use today in our electric RC aircraft are typically described using several standard electrical terms: “voltage” or “cell count”; “storage capacity”; and “current” or “discharge rate limits.” Take a look at any LiPo label and you’ll see at least these three items. These terms aren’t unique to the batteries we use in RC; they’re terms that are used in all electrical fields, so it’s important to know what they mean and to use them properly. TEXT & PHOTOS BY JOHN KAUK
A battery is composed of cells, which are connected in series and/or parallel to make up the battery. The voltage of any battery is determined by the chemical composition of the material within the battery’s cells. Nickel cadmium (Ni-Cd) batteries have a reference, or nominal, voltage of 1.2 volts per cell. Lead-acid batteries have a nominal voltage of 2.0 volts per cell. A typical LiPo cell has a nominal voltage of 3.7 volts per cell.
A battery’s total voltage is given as a multiple of the cell voltage, so six lead-acid cells make up the 12-volt battery we carry in our cars. A three-cell series-connected (3S) LiPo is labeled “11.1 volts,” and a 6S battery’s label is “22.2 volts.” At a state of full charge, a LiPo battery’s voltage will be near 4.2 volts per cell, and the cutoff, or minimum allowable, voltage is 3.0 volts per cell.
Two 5000mAh battery labels show different C-rates. The Pulse battery shows a single 45C rate, while the Turnigy shows a range from 25C to 50C. While it’s not stated explicitly, I’d treat the lower as the continuous rating and the higher as a 30-second rating. Note that neither label specifies a charge rate.
The labels for these batteries both show energy capacity in watt-hours in addition to the storage capacity in amp-hours. The E-flite label specifies a charging voltage, while the ElectriFly specs the charge current.
A battery’s storage capacity (C) is described as the amount of charge that it can deliver over a period of time while staying above the cutoff voltage, and is basically determined by the size of the battery. In general, bigger LiPo batteries have more capacity, as do bigger Ni-Cds. Capacity is measured in amp-hours (Ah) or milliamp-hours (mAh), and those are defined by the number of hours that a battery can provide a given discharge current. This means that a battery with a capacity of 1Ah is capable of providing a current of one amp for one hour before it gets to its cutoff voltage. It can also provide 500mA of current for two hours, or two amps for half an hour. Josh Barker of MaxAmps confirmed for me that the industry standard for labeled capacity is a one-hour discharge rate.
Storage capacity varies; it isn’t a constant. Increasing discharge current will decrease a battery’s capacity as will temperature extremes. It’s also worth noting that we rarely use a battery’s full capacity anyway as doing so might cause damage to it and shorten its lifespan. I time my flights so that I land when the battery is near storage voltage: 3.8 volts per cell. That leaves about 45 percent of the capacity unused, but it allows a safety margin for failed landing attempts and it’s easy on the batteries. It’s also easy on me because I don’t have to charge or discharge to storage levels once I’m done flying.
USES OF “C”
In all batteries, capacity is used to define several other rates, such as charge and discharge rates, and this is where things can get a little confusing.
Charging a battery incorrectly can damage it, so manufacturers specify a safe maximum charge rate in multiples of C. With the LiPos we use, a 1C charge rate is almost always safe and easy on the batteries. Some manufacturers specify higher charge rates. For instance, Pulse Batteries and MaxAmps specify a 5C charge rate, so I’d be comfortable using that rate from time to time. For routine charging, I stick to the gentler 1C rate because I think that it helps the batteries last longer.
The term “C-rate” is used to define the discharge current for a battery. As with charge rates, this number is specified as a multiple of C, such as 20C. Sometimes the label will show a range, like 25–50C, and sometimes it will show continuous and pulse, or 30-second rates. A continuous C-rate is the maximum discharge current that the battery can provide for the full discharge, from full charge down to the cutoff voltage, without damaging the battery. The 30-second C-rate is the discharge current that the battery can supply for short-term pulses up to 30 seconds without damaging the battery. For a 5000mAh 25-45C battery, that means a continuous current of 125 amps and a pulse current of 225 amps.
This old Astro Flight Whattmeter has served the author well over the years. Knowing current, voltage, and power allows a modeler to confirm that a power system is within its battery’s specs to avoid damage. Watt meters are available at many RC vendors.
How these maximum discharge currents are determined is a bit of a mystery to me. I’ve talked with people at various companies about it, and there isn’t a consistent answer. In most cases, the limits are defined by the cell manufacturer to prohibit excessively high currents that would damage the battery. Things like cell chemistry, cell construction, intercell connections, internal resistance, and wire size all have an impact on the final maximum current rating for a battery.
This data log from the Castle Creations Edge HV 120 in the author’s Top Flight Corsair shows voltage and current graphs for its first flight. A maximum current of about 58 amps and maximum power less than 3000W mean that the power system is well within its limits on this flight.
I try to set my models up with moderate current demands, for reasons I’ve discussed before. An advantage of doing this is that I don’t have to worry about fanciful C-rates causing problems for me. If I keep my maximum current to 75 amps or less, a battery rated at 25C is sufficient for larger planes. They’re less expensive and last a long time because I don’t stress them much.
If you’re interested in more general information about batteries, there are plenty of reliable sources on the Internet. One that I’ve found helpful from time to time is batteryuniversity.com, and MIT’s Electric Vehicle Team has a nice guide to battery definitions as well (web.mit.edu/evt/summary_battery_specifications.pdf). A more in-depth discussion of LiPo lore that relies heavily on Internet forum sources is “Learning About LiPo Batteries” by Ken Myers, available at theampeer.org.
BuddyRC AB Clips
One of the most annoying things about charging batteries is getting the JST-XH balance lead plugged into and out of a balance board. The plug bodies are small and fairly thin, and when they’re stuck in a socket, they can sometimes be hard to grip well enough to pull out easily. Resorting to a firm pull on the wires risks pulling them out of the plastic plug and causing a short circuit in the balance leads. Trust me, I’ve done it and it’s no fun.
BuddyRC’s AB Clips solve that problem, and they do it cost effectively. The one-piece molded polypropylene clip snaps tightly around the balance plug and its wires, forming a larger piece that’s much easier to grip. That makes it simpler to connect and disconnect the balance leads with no risk of damage. I’ve got them on all of my batteries now, and I haven’t pulled a wire out of a plug in a long time.
Available to fit 2S through 6S balance plugs, the AB Clips come in packages of five for a regular price of $1.95. buddyrc.com
BY JOHN KAUK