Testing Small Power Connectors for Hobby RC

By Terry Dunn

Whether you're talking about aircraft, cars, or boats, one of the keys to success with electric-powered RC models is using high-quality power connectors.

Whether you're talking about aircraft, cars, or boats, one of the keys to success with electric-powered RC models is using high-quality power connectors. The electrical connection between the battery and vehicle is critical. A sub-par connector can cause poor performance or even be the root cause of a crash.

Early battery-powered RC models borrowed Molex power connectors from the electronics industry. It didn't take long to figure out that those connectors were far from ideal for RC. They simply weren't designed to handle the high amperage, countless connection cycles, and dirty conditions that RC vehicles endure. Although Molex connectors can still be found in some beginner-oriented kits (often called "Tamiya" or "Kyosho" plugs), numerous RC-specific connectors have emerged over the years. There are a lot of high-quality power connectors for RC drivers and pilots to choose from. That is, unless you like small models.

I used a simple test rig to stress the JST and XT30 plugs and combine their relative performance levels.

When I talk about "small models", it's not a well-defined category. Let's just say that it encompasses cars and boats that are 1/18-scale or smaller and aircraft that weigh less than a pound. These little RC models have comparatively few connector options. Many small models come factory-equipped with a connector commonly called the "JST plug". It's another carryover from the electronics industry. Like Molex connectors, JST plugs are a poor fit in the RC world.

The Lowly JST Plug

Although called the JST plug in RC circles, this little red connector is actually the RCY-series plug made by the JST company (Japanese Solderless Terminal). That's an important distinction to make because other JST-manufactured plugs are also found on RC items, namely the balance connectors on multi-cell lithium-polymer batteries. It's worth noting that those plugs are completely adequate for that application. Just to keep things simple, I'll stick with the tribal terminology and refer to RCY-series connectors as JSTs.

JST plugs are indeed small. A mated pair of plugs measures around 23x6x4mm. Each plug has two contacts (positive and negative). The plastic housings are keyed to prevent them being plugged in backwards. Every JST plug that I've ever seen came with a pigtail of factory-crimped wires, but I assume that it's also possible to buy them in raw form.

The JST datasheet indicates that the maximum current rating is 3 amps when the plug is connected to 22-gauge wire. Several of my JST plugs have 20-gauge wire attached to them. Such a configuration is not reflected on the datasheet. But that's not an issue. The point is moot even if the larger wire allows another amp or two of current. JST plugs are frequently bundled with RC models that demand amp draw far outside of that window. I've had models that pushed more than 15 amps through a JST. That's just asking for trouble.

JST Alternatives

The relative sizes of the JST (top), XT30 and Ultra Plug also reflect their power handling limits.

The plug I use for most of my larger models is the Deans Ultra Plug. It is simple, robust, and can handle a lot of power. Unfortunately, it's just a little too big to be practical in most small models. I used the Ultra Plug's smaller cousin, the Deans Micro Plug, for several years. The Micro Plug is great for small applications in terms of size and performance. I eventually abandoned it, however, because the contacts have a disturbing tendency to pull out of the plastic housing. Not good…dangerous, actually.

Since ditching the Deans Micro Plug, I've been making do with JST plugs. I briefly considered trying E-flite EC2 connectors, a mini-sized version of their popular EC3. The one example that I had worked great. However, at $4 per pair, it would cost a pretty penny to retrofit my fleet of small models and related batteries with EC2s.

One of my recent multi-rotor acquisitions came equipped with XT30 connectors. Like the EC2, the XT30 is a downsized version of a popular connector…in this case, the XT60. For what it's worth, there are larger versions of both plugs available as well (for BIG models)…the EC5 and the XT90. My first glimpse at XT30s was very promising. Although a little larger than JSTs, the XT30s are sufficiently small for all but my tiniest models. More importantly, the large contact pins suggested that they would handle much more current than JSTs.

I needed a few XT30s to build charging leads for my new quad, so I also ordered some spares to experiment with. I was pleased to see that five male/female pairs of XT30s cost less than $4. It's hard to complain about that.

XT30 plugs are easy to solder and readily accept 16-gauge wire.

Once I had the connectors in hand, I soldered the charge leads that I needed. I also soldered a few simple jumpers that would allow me to analyze the connectors' performance. The exposed contacts on the XT30 are easy to solder. The half-pipe area of the contacts tins quickly and helps hold the wire in place. I soldered examples using 16, 18, and 20 gauge wire. I insulated the solder joints with heatshrink tubing.

Testing XT30 Connectors

There are lots of ways to analyze the performance of an electrical connector. For my purposes, I figured that the most direct path was to put the plug in its intended environment and see how it compares to the competition. I made two pairs of jumpers that allowed me to alternately put XT30 and JST plugs in the same model. I did my best to limit the variables by using the same lengths of identical wire on all of the jumpers.

I used these jumpers in my Tower Hobbies Racer Red P-51 model, which is already equipped with Hobbico Star plugs (compatible with and equivalent to Ultra Plugs). I don't consider the Mustang a small model. However, its stock power system pulls about 25 amps. This is the ceiling of what I expect my small models to be capable of. It's way more amperage than I would expect a JST to tolerate, but it provides a handy data point for comparison.

The XT30 is a micro connector that addresses the shortfalls of other power connectors for small RC models.

Using my ElectriFly PowerMatch meter, I took a set of baseline readings using only the Star Plugs. The PowerMatch takes a snapshot that displays the simultaneous amperage, voltage, and power output of the system. I then took several readings with the JST and XT30 jumpers in place. The test was also repeated with a smaller propeller bolted to the Mustang (to lower the amp draw).

The results surprised me. The XT30s performed almost as well as the Star Plugs. Power loss was only 4%. Some of that loss can certainly be attributed to the two extra Star Plugs and 20-gauge wire in the XT30 jumpers. Based on those results, I have no reservations about implementing the XT30s in applications up to 25 or 30 amps. The only remaining question is how well they stand up over time with frequent use. That analysis is ongoing.

An ElectriFly PowerMatch provides a snapshot of data to compare each connector.

The poor little JST plugs actually held up pretty well to my abuse. During data collection, I only ran each configuration long enough to get a stable reading…usually 5 seconds or less. With JSTs in the loop, amperage and voltage values were consistently lower than the XT30 and Star Plug readings. Since power equals amperage multiplied by voltage, watt levels were down as well.

In the most extreme case, the difference in power between the XT30 and JST plugs was 31.9 watts (11.5%). The concern here is not so much the reduction in power caused by the relative inefficiency of the JSTs. This P-51 would certainly fly just fine on the 244.1 watts remaining. The issue is that the watts aren't actually missing. They are manifested as heat at the source of the bottleneck…aka those tortured JST plugs.

My results explain why JST plugs sometimes fail from overheating. It also suggests that XT30s are adequate for all of my small models.

I was curious how quickly 31.9 watts would heat up the JSTs. So I ran the system at full throttle for 30 seconds. The connectors were too hot to hold in my hand for more than a second or two. I've read of instances where JSTs have melted together during use. Now I see how easily that can happen.

A look at the male contacts on the JST (left) and XT30 illustrates why the latter is able to handle more amperage.

Again, none of this is intended to be a knock against the JST RCY connectors. Their use in RC vehicles pushes them well beyond their stated capabilities. My testing nudged them even further. My goal was simply to find a practical alternative to JSTs in small models. Thus far, it seems that the XT30 will be a nice fit.

Terry is a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.