My inbox is often filled with press releases about the latest RC hobby products. I try to focus my attention on the truly unique and innovative stuff. That's why the ARRMA Granite Voltage ($140) caught my eye. On the surface, it appears to be a run-of-the-mill 1/10-scale monster truck. But this is no ordinary backyard basher. The Granite Voltage is equipped to use a power source that I've never before seen in an RC truck, cylindrical Lithium-Ion (Li-Ion) batteries. I'm a battery nerd, so I had to take a closer look.
The 18650-sized (18mm diameter, 65mm long) Li-Ion batteries used by the Granite Voltage are not new. Derivatives of these cells have been around for more than 20 years. They've always been a popular choice for small electronic devices because of their high energy density, convenient size, and robust housing. However, these cells never really captured the interest of RC hobbyists. The issue was that Li-Ion cells couldn't discharge rapidly enough to meet the high-current demands of most RC vehicles. Simply put, Li-Ion batteries were great at storing energy, but not so great at expending that energy quickly. Most hobbyists adopted a different flavor of Lithium-Ion technology with more favorable discharge characteristics, the soft, flat Lithium-Polymer (LiPo) cells that now dominate RC applications.
Recent developments with cylindrical Li-Ion batteries have finally made them contenders for hobbyists. The latest generation of 18650 cells have high discharge capabilities, giving backyard drivers another battery option. I also found it interesting that Tesla and the vaping community have also embraced the allure of modern 18650 Li-Ion batteries.
About the Truck
The Granite Voltage is one of several trucks in ARRMA's lineup with the Granite name. Only the Voltage variant is Li-Ion compatible. So "Granite" in this context of this article refers specifically to the Granite Voltage. This is a 2-wheel drive monster truck that uses a brushed motor. The Fazon Voltage is the same basic vehicle as the Granite Voltage, only with different body, smaller wheel/tires, and slightly different gearing.
This Granite is near the bottom end of the price point for a hobby-quality RC vehicle. Your $140 gets you a factory-built and painted truck with waterproof electronics, a 2.4GHz radio system, two 1500mAh 18650 cells, and a charger that can be powered by USB or AC power. I was surprised to see that the Granite even includes a full set of ball bearings rather than bushings. The only required items are four AA-size alkaline batteries to the pistol-grip transmitter.
My only other ARRMA vehicle is the 1/8-scale, brushless-powered NERO 6S, a much different truck in terms of size, power, and cost. The budget-minded Granite does not appear to yield anything to the NERO in terms of quality. All of the components are well-molded and fit together with no slop. While I've only had a few days of testing thus far, no weak points have emerged.
Using 18650 Cells
The Granite's plastic chassis was obviously designed with Li-Ion batteries in mind. A compartment in the middle of the chassis holds a battery cradle that accommodates up to six 18650 cells. Each cell has a nominal voltage of 3.7v, and 4.2v when fully-charged. Two fresh cells in series provide 8.4 volts to get the Granite moving.
Up to two more pairs of serial 18650 cells can be installed in the cradle. These additional pairs of cells are connected in parallel with the first duo. So the voltage never goes above 8.4v, but each additional pair increases the collective capacity of the arrangement by 1500mAh. Additional capacity equates to more run time.
Not only is the use of 18650 batteries unique, the practice of installing individual cells is also a departure from the norm with hobby-quality vehicles. Every other RC car battery that I've ever used had the necessary cells soldered or welded together in an inseparable array…aka a battery pack. The Granite's method of using individual cells means that the necessary electrical contacts between each cell must be integrated into the chassis. ARRMA accomplished this by implementing two strips of contacts that press against the ends of the cells when the battery compartment door is closed.
Before I actually had the Granite in hand, I had a couple of concerns about its use of spring-pressure electrical contacts. First of all, I worried that there could be intermittent connectivity as the truck bounced its way over rough terrain. Since the same batteries also power the radio system, intermittent power would also mean intermittent control.
My second concern was that the spring-contacts would not be able to effectively carry the electrical current demanded by the motor. In my mind, each contact was a potential electrical bottleneck that could bring the Granite to a crawl.
My experience with the Granite thus far suggests that neither of my concerns was justified. The contacts are held against the cells with considerable pressure. Also, the door and latching mechanisms for the battery compartment are very beefy. It's hard to imagine a scenario that would knock things loose. I've not had once instance of power loss, and I don't expect any. I don't know if there is much power loss through the spring contacts, but there is certainly enough power remaining to make those fat rear tires break loose and spin like crazy. I'll be keeping an eye on things to see if the contacts deteriorate with use and age.
The Granite can also use traditional NiCad, NiMH, and LiPo battery packs. These packs fit into the center section of the battery cradle and plug directly into the Electronic Speed Control (ESC). There is a notch in the door for routing the battery wires. ARRMA has a video that illustrates how each of the batteries is installed. It's a pretty clever system. I felt that the door notch and parts of the cradle had edges sharp enough to potentially cut through the wire insulation over time. I rounded off those areas with a small file.
I had a little trouble getting the cradle in and out of the battery compartment when using NiMH and LiPo batteries (not necessary with 18650 cells). I tied a short piece of stout string to the middle part of the cradle. This gives me a convenient handle for pulling the cradle and does not interfere with the door.
There is a jumper on the ESC that you must position according to the type of battery you're using. This sets the proper cutoff voltage and ensures that you do not over-discharge your batteries. The jumper position labels were difficult to read, so I colored the embossed lettering with a Sharpie marker.
Speaking of the ESC, I removed the bullet connectors between the motor and ESC. The ESC wires are long enough to reach the solder pads on the motor directly. This mod certainly is not mandatory, but it omits two unnecessary connections and little bit of superfluous wire.
Driving the Granite Voltage
I have driven the Granite with numerous different battery configurations. The stock setup uses two 18650 cells. While these batteries are capable of relatively high discharge rates, they are still limited to a maximum of 20 amps. I didn't know how that limit would translate performance-wise. It's actually pretty good. The Granite has impressive acceleration and top speed. It is certainly on par with my other brushed-motor bashers that use NiMH and LiPo batteries.
I'm seeing run times of about 7 to 8 minutes when driving on a mixture of hard-packed dirt and pavement. The Granite will slog through grass just fine, but run times suffer. The stock tires work well on all of these surfaces.
When using multiple pairs of 18650s, it is important to make sure that they are all at the same charge state. Combining cells in parallel with significantly different voltages could damage them.
As I mentioned earlier, adding a second pair of 18650s will effectively double your run time. It also has the ancillary benefit of doubling the available discharge capacity. This gives the Granite slightly better acceleration when running two or more pairs of 18650s. Honestly, I rarely feel the urge to drive more than 7-ish minutes at a time, and the performance boost is minor. So, I doubt that I'll often use more than a single pair of cells at a time.
I've also driven the Granite with a 6-cell Duratrax Onyx 3000mAh 7.2-volt NiMH battery and a 2-cell Duratrax Onyx 5000mAh 7.4-volt LiPo battery. The LiPo is a tight squeeze in the battery compartment, but it does indeed fit.
Even though there are several ounces of weight difference between each battery option (LiPo=10.1oz, NiMH=12.3oz, 2x18650=2.9oz), performance is similar across the board. I subjectively rate the performance with NiMH and LiPo batteries to be nearly equivalent to using two pairs of 18650 cells…with the LiPo having a slight edge. I'm not sure I could tell the difference if I didn't know which battery was installed.
Why Choose Li-Ion
I'll assume you now share my opinion that the Granite Voltage has proven the viability of using 18650 cells in RC vehicles. But that conclusion does not address the question of why you should choose Li-Ion. For backyard bashers, there really is not an overwhelming reason to choose any battery type over another. I think it all boils down to what you're comfortable with.
NiCad, NiMH, LiPo and Li-Ion all work similarly well in moderate-power applications like the Granite. Each cell type offers unique pros and cons. Li-Ion provides the high energy density and weight advantages of a lithium-based chemistry, but without the electrical and physical fragility of LiPo. I know several people who shy away from LiPo batteries because of fire concerns. The same folks may be willing to consider Li-Ion. NiCad and NiMH are relatively abuse-tolerant, but they are considerably heavier than their lithium-based equivalents.
I think the biggest downside for Li-Ion is that the charge rates are relatively slow. For the cells included with the Granite, the suggested charge rate is .75 amps (2 hours charge time) and the maximum is 1.5 amps (1 hour charge time). The provided charger refills two cells at a time in about 2.5 hours when using AC power.
There is not a huge disparity in price between the battery types. Six-cell NiMH packs generally cost $20 to $40, depending on the cell capacity. The 2-cell 5000mAh LiPo I used costs $43. Currently, ARRMA's 18650s are sold in pairs with a charger for $30, or in a 6-pack without a charger for $25. You can also get a 6-cell charger separately for $40. I am told that additional 18650 cell options will be available soon.
My hunch is that, much like LiPo cells, high-discharge Li-Ion cells will continue to come down in price while also having ever higher capacity and charge/discharge rates. This will make them an even more attractive option for many. Consequently, the Granite Voltage may be the first step in a new Li-Ion battery trend for RC vehicles.
Terry is a freelance writer living in Buffalo, NY. 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.