Lithium Polymer (LiPo) batteries are presently the standard power source for most hobby-grade RC vehicles. These batteries have seen steady improvement since they were first introduced to the hobby around the turn of this century. Most of that progress has been related to increasing their fast discharge and charge capabilities. During the last year or so, we have seen the emergence of LiPo cells that evolved in a slightly different direction. High-Voltage LiPo batteries (LiHV) look and perform much like contemporary LiPo cells, but have a tad more voltage.
More voltage fed your electric motor equals more RPMs. That will give your RC toy a zippy performance boost, right? Well, maybe. There is more to consider than just voltage. I recently tested a LiHV battery to determine whether these types of cells are an overall advantage for me.
What is LiHV?
Standard LiPo batteries have 4.2 volts per cell when fully-charged. LiHV cells go up to 4.35 volts. That extra .15 volts may not seem like much, but it could make a significant difference in some applications. Most RC vehicles use two or more cells in series. So the extra voltage of a LiHV battery is multiplied by the number of cells.
A regular LiPo is considered discharged when it drops to 3.2 volts per cell under load. This usually equates to about 3.7 volts at rest. LiHV cells have the same bottom end values. So, LiHV cells operate over a slightly broader voltage range than standard LiPo variants.
There are numerous LiHV options available on the market. From what I can tell, most of these batteries are very similar in size, weight, price, and discharge capabilities to their normal-voltage contemporaries. Yes, there are some differences. But I think most of those deltas would be insignificant to the average hobbyist.
Hyperion provided two similar batteries for testing, one LiPo and one LiHV. The LiPo is their G5 3-cell (12.6 volts) 850mAh unit that is rated for 70C maximum discharge (59.5 amps). On the LiHV side is a G7 3-cell (13.05 volts) 900mAh battery rated for 60C discharge (54 amps). Before getting into my results from using these batteries, we should cover a few things to think about regarding LiHV cells.
The question of whether or not to use LiHV batteries is somewhat complicated by the ancillary effects of their higher voltage. Lithium-based batteries of all types are very sensitive to overcharge and over-discharge. Consequently, the widgets that charge and utilize these cells monitor their voltage very diligently. Introducing cells with different voltages may confuse components that have hard-coded voltage limits.
The first potential issue is charging LiHV batteries. Technically, you could charge a LiHV pack using a standard LiPo charger. But it would only charge the battery to 4.2 volts per cell…negating the very advantage that these batteries offer. You need a LiHV-compatible charger to unlock the real potential of these batteries. If your current charger is not up to the LiHV challenge, you'll need to shop for a new one.
I have half a dozen different LiPo chargers, but all were purchased before LiHV batteries emerged onto the scene. None of them are LiHV-compatible. Nor do they offer firmware updates to make them so. I've always been a fan of Hitec chargers, so I picked up their X1 Mini ($45). This little AC-powered unit has been working well for me.
Another consideration when using LiHV cells is how they will be recognized by various Electronic Speed Controls (ESC). Most modern ESCs calculate the number of LiPo cells in your battery based on the initial input voltage. The ESC then determines the appropriate voltage to warn you that the battery is almost depleted. This signal (usually called "low-voltage cut off" - LVCO) is often presented in the form of pulsing or a complete shut-down of the motor. Either LVCO signature can be a hazard with flying models.
The concern here is that some ESCs could recognize a fully charged 3-cell LiHV as a partially-charged 4-cell LiPo. Assuming LVCO is set to trigger at 3.2 volts per cell, such a misread would cause the pilot to deal with a compromised power situation when the battery hits 12.8 volts…which wouldn't take long. This situation could cause a crash. At the very least it would prevent practical use of the battery.
This potential problem turned out to be a non-issue for me. The two ESCs that I used with the LiHV properly recognized the battery as a 3-cell and set the appropriate LVCO value. With that said, I have friends who have had to deal with auto-detection problems when using LiHV. It seems that some ESCs overlook the extra voltage, while others do not. There are also ESCs which can have the LVCO manually programmed. I don't know how a modeler can predict which way their particular ESCs will react. But the potential for battery misreads is certainly something to be aware of and prepared for.
The final consideration I want to bring up is headroom. If your RC vehicle is already pushing the limits of its power system when using LiPo batteries, going to LiHV could be the final straw. More voltage also means higher current and greater power. Make sure that your motor and ESC can handle the extra juice.
My first step in comparing the Hyperion batteries was to get them charged. The packs come with XT-30 power connectors installed. I only needed to solder an XT-30 charging lead for the X1 Mini.
The menus on the X1 Mini are similar to those on my other Hitec chargers. So the X1 feels familiar and is easy for me to navigate. Before charging, I exercised the charger's battery resistance feature on both packs. The LiHV battery measured significantly lower internal resistance than the LiPo (55 milliohms vs 83 milliohms). This should help the LiHV to experience less of a voltage drop under load.
I charged the G5 and G7 batteries on the X1 Mini using the appropriate settings (LiPo and LiHV respectively). While a LiHV battery can be partially charged on a standard LiPo charger, charging a regular LiPo with LiHV settings could be bad news. LiPos simply do not tolerate overcharging very gracefully. You might kill the battery. You might start a fire…it's tough to predict. Let's try to not find out for real. The X1 Mini has an extra confirmation step when using the LiHV setting to help mitigate this concern.
My first vehicle for comparing these batteries was the ViFly R130 racing quad. This machine is designed to run on 3-cell or 4-cell LiPos. So the slight extra voltage of a 3-cell LiHV was not a concern.
I logged a number of flights using both batteries. They performed well and delivered very similar flight times. Quite honestly, I couldn't tell any difference in performance between them. Physics dictates that there must be some difference, but I'm just not able to detect it.
I think the main factor in my blindness is that I'm not pushing the R130 very hard. It has so much power, even with the LiPo, that any additional boost the LiHV allows is going untapped by me. I suspect that someone who can really wring out a racing quad would be calibrated precisely enough to detect some differences in performance. I'm not that person. In my hands, with this quad, the LiPo and LiHV are effectively equivalent.
The next vehicle I tried was a little fixed-wing park flyer called the Parallax. I typically fly this model using 3-cell 850mAh or 1000mAh LiPo batteries. The Parallax is sporty, but I certainly would not call it overpowered. It isn't uncommon for me to spend a significant portion of any given flight at full throttle.
Again, I logged several flights with both the LiPo and LiHV. Flying with the LiPo felt like, well…flying with a LiPo. The Parallax had the performance that I've become accustomed to from years of flying this model.
I easily detected a difference in performance whenever I flew with the LiHV battery. The Parallax had more speed in level flight and climbed higher during vertical ascents. It was not a huge increase in power, but I certainly noticed the uptick every time. I must say that I enjoyed the extra kick in the pants provided by the LiHV.
Any differences in flight time or flying weight imposed by the LiHV were marginal. In this application, I felt like the LiHV delivered noticeably better flight performance with no tradeoffs. It's a win-win situation.
Subsequent testing in my workshop provided numbers that backed up my subjective observations. When using the G5 LiPo, the Parallax pulls 10.7 amps and produces 118 watts of power. Switching to the G7 LiHV bumps the numbers to 11.4 amps and 132 watts. That's nearly a 12% increase in power. I'll take it!
No matter what kind of RC toy we fly or drive, we're usually looking for a simple way to get better performance. LiHV batteries are a tempting path to that destination. In cases like my Parallax, it really is that simple. You swap the LiPo for a LiHV and enjoy the turbo boost. The upgrade may not be so simple, or even worthwhile, in other applications. It is important to pause and consider the angles before lunging forward.
Based on my positive experience with the Parallax, I think there are some other fixed-wing models in my fleet that would benefit from swapping to LiHV batteries. The only question remaining for me is the long-term durability of these cells. My initial experience has been excellent. I hope performance remains the same after a few dozen cycles. Time will tell.
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.