NiCds (NiCads) are long gone; NiMH packs might still around thanks to RTRs, but overall, the LiPo is the most prevalent battery chemistry in RC. Yet, despite how widespread its use is, a lot of people are still intimidated by the technology. To make LiPos more user-friendly, almost every speed control is outfitted with a low-voltage cutoff. Over discharging is one of the more common mistakes users make with LiPos, and over discharging can not only ruin the battery, it can also cause legitimate safety issues. Low-voltage cutoffs solve a very real problem, but it turns out the typical low-voltage cutoff leaves a little bit to be desired. That’s where SMC’s Precision LiPo Alarm comes in.
While it would seem low-voltage cutoffs built into speed controls would solve the problem and be best for the battery, it unfortunately isn’t always that simple. The battery experts at SMC (Superior Matching Concepts) noticed low-voltage cut-off and alarms were often inaccurate. Voltage drop while under heavy acceleration wasn’t being accounted for. As you get towards the end of a run, a heavy load (big tires, lots of trigger) placed on the battery can briefly pull the voltage below the cutoff setting and shut the speed control down or trigger the alarm, if equipped. The problem is the battery isn’t really at the set, desired voltage. After many months of testing and development, SMC came up with the Precision LiPo Alarm that incorporates a delay. The end result is your batteries are safe, but are also being allowed to run to reach the desired cutoff voltage.
According to SMC, its alarm is more precise than built-in speed control cutoffs because the SMC Precision LiPo Alarm measures voltage at the cells (via the balance connector). This is hard to argue with. Using the balance connector eliminates the voltage drop encountered in the battery connectors, the power wires and the solder joints. It’s also more accurate due to the preciously mentioned delay. The purpose of the delay being to eliminate false alarms.
The SMC Precision LiPo Alarm is a small black box with a LCD screen on top and a single pushbutton at one end and a plug port at the other. It measures in at 40 mm long and 25 mm wide. It is 12 mm thick. It is intended to be attached to the vehicle or battery with the included self-adhesive hook-and-loop fastener. If you’re attaching it to your vehicle, servo tape could certainly be used, but since the device isn’t waterproof, you’ll want to be able to easily remove as needed. The hook-and-loop also makes it easier to use it with multiple vehicles, not just multiple packs. The SMC Precision LiPo Alarm works with 2S to 8S packs. It’s also worth pointing out the SMC Precision LiPo Alarm differs from traditional cutoffs in that it is measuring each cell individually. It sounds the alarm when either cell gets to the preset voltage. When measuring total pack voltage, theoretically, a strong cell could mask a weak cell. The weaker cell will continue to get weaker if it’s continually drained lower than it should be.
Adjusting the SMC Precision LiPo Alarm is pretty easy–just press the single button. Since it’s powered by the battery it’s connected to, the alarm needs to be plugged in to make adjustments. Pressing the setup button once displays the current cutoff setting. If you quickly press the button again, the setting will increase by 0.1. The selectable range is 2.7 volts to 3.8 volts. You can also opt for no cutoff.
TESTING & FINAL ASSESSMENT
I installed the SMC Precision LiPo Alarm in a Traxxas Stampede that is outfitted with a high Kv brushless system. This setup is fun, but with heavier-than-stock tires, a lot of motor and frequent use on grass and loose dirt, it really eats up the volts. Low-voltage detection is simply a must-have.
Reading the voltage is easy. When plugged in, each cell voltage value flashes five times, so running a 2S pack, the voltage of cell one is flashed five times and then the voltage of the second cell is flashed five times. After the cutoff alarm goes through the cells, the total voltage is flashed once and then sequence starts over. I quickly found I liked having the visible screen on the SMC Precision LiPo Alarm. It was like having a fuel gauge. The screen is big to easily see when peering in the body.
The first time I used the SMC Precision LiPo Alarm I didn’t do anything special, I just ran the vehicle until the alarm went off. I had the factory cutoff turned off on the speed control. I knew the alarm seemed plenty loud when bench testing, but I wanted to hear it (or not hear it) well while bashing. It passed the is-it-loud-enough test. It won’t make your ears bleed, but it was loud enough that I easily heard it at the normal distance I drive when bashing around.
My next test consisted of another regular run, but with the factory cutoff on. I wanted to see if the speed control cutoff kicked in before the alarm went off. It didn’t with this test vehicle. The current setup I have exhibits cogging at really slow speeds, so I knew when the battery’s voltage was getting low. Set at the recommended 3.7 volts, the alarm went off slightly after I started noticing cogging. Shortly after that the speed control cutoff engaged.
For my third run, I left the factory cutoff on and drove in grass, on dirt, took jumps, did wheelies, did a ton of stop-and-go driving and did when I did go, I did all hard launches. I treated the throttle like a light switch. I was either full throttle or full brakes. I did worse than ‘drive it like ya stole it.’ I drove like my 12-year-old son drives. This time, the speed control cutoff went off, I paused long enough to unplug and replug the battery and then drove again. I didn’t, however, continue to drive like a madman and I got slightly over three minutes of continued runtime before the alarm sounded. As expected, the hard driving caused hard voltage draws that triggered the speed control’s cutoff. It’s worth noting that I tested the SMC Precision LiPo Alarm with 3S in this same vehicle with no issues.
The downside (sort of) is the SMC Precision LiPo Alarm is just that, an alarm, and not a cutoff. If you turn off your built-in cutoff and, for some reason, don’t hear the alarm go off, you could damage your battery. I call this only sort of a downside because SMC doesn’t claim the alarm to be a cutoff. I can’t really fault it for not being something it’s not claiming to be. The upside of the SMC Precision LiPo Alarm is it’s far more accurate than typical cutoffs. Measuring each cell independently at the balance plug is simply better than measuring at the speed control. Add in the delay feature to prevent false alarms and that it’s triggered by the lowest cell of the pack and not the total voltage and the SMC Precision LiPo Alarm is better at its job than the alternatives. At around $18, it’s also cheap insurance.