In basic terms, helicopters convert energy into mechanical power in order to maintain flight. To expand, a specific helicopter flying a specific way will require a specific amount of energy to perform the task. This energy can be quantified as wattage on an electric model, and is basically a constant we can use to compare different power systems in order to show the benefits and drawbacks of each.
The power system I am referring to is comprised of the motor, esc and battery. In order to change from say a 3s to a 6s power system on a 450 sized heli, one would have to replace all three components. A higher voltage esc is needed, a properly sized lower Kv motor is needed and of course a different sized battery pack is needed. Sometimes certain components can be reused if they match up properly.
Starting with some known numbers
Lets start with a common application where the power systems can range anywhere from 6s to 12s, the 600 class helis. The most popular model currently available is only available in a 6s power system from the factory and is designed specifically to use a 6s 5000mAh flight pack. Assuming a moderate head speed, we'll assume a 5min flight time using 80% of the pack. Now lets calculate the average wattage over this 5min flight.
Note: The following is covered in other sections on this site.
5000mAh * 80% = 4000mAh or 4Ah
60min / 5min = 12
12 * 4Ah = 48A
48A * 22.2V = 1065W
Now that we have the average wattage over a 5min flight, we can use it to compare different power systems that will work on this heli.
Increasing voltage by adding cells
What does adding cells do? Well the more cells in series, the higher the voltage but how does that help? Since wattage is basically a constant, manipulating the voltage directly effects the current needed to achieve that wattage.
Take the above example. The heli averaged 1065W or 48A on a 6s pack. Now lets change the pack voltage and see how that effects the amperage.
Watts = Volts * Amps
or we can rewrite it as
Amps = Watts / Volts
and in this case everything is an average over a 5min flight so
Avg. Amps = Avg. Watts / Avg. Volts
Lets start by lowering the voltage by moving to a 4s system.
Avg. Amps = 1065W / 14.8V = 72A
Notice how the amp draw rises? Now lets raise the voltage by moving to a 8s system.
Avg. Amps = 1065W / 29.6V = 36A
As you can see the higher the voltage, the lower the amp draw. So what happens if we go to even higher voltages? You guessed it, the amp draw will yet again drop. Lastly lets look at the highest voltage system that is commonly available for helis, 12s.
Avg. Amps = 1065W / 44.4V = 24A
On first thought, it seem that the higher the voltage the better but there is a practical limit. Most of this comes from the fact that the only way to increase the voltage is to increase the number of cells and that has limits.
Also as the number of cells raises, the capacity of those cells needs to drop. The total amount of energy needs to remain similar so the weight and size remain similar. For example a if you are changing over the above example heli from a 6s power system to a 12s power, the pack will need to change from a 6s 5000mAh pack to a 12s 2500mAh pack. Both packs offer the exact same energy (or Wh = Ah*V) and should weight about the same.
Benefits of high voltage systems
There are many benefits to using a high voltage system (8s+) in helis and there are also some drawbacks. Lets start with the benefits.
Lower amp draw
One of the biggest benefits is the lower amperage needed. As we covered above a 600 class heli with a 12s system only draws half the amperage of a 6s system. This means the esc, wires and connectors can be much smaller.
Higher voltage under load
This is where hardcore pilots jump on board. Because the amp draw is lower, there is less of a voltage drop under load and that means higher sustained voltage during hard flying. Simply put it makes it harder to bog the heli.
Larger motors available for high voltage systems
This really has nothing to with the higher voltages but rather is a byproduct of higher voltages being used in larger helis. You may have noticed that the bigger/more powerful a motor is, the lower the kv (rpm per volt) rating. Because each heli has a basically fixed, or at least very limited adjustable, motor to main rotor ratio, a motor with the proper kv is needed. By raising the voltage of the power system, a lower kv motor is needed and this opens up more options.
For example take a 600 class heli. One of the most popular models ships with a 1220kv, 1600W motor designed for 6s. If you wanted to directly replace it with a higher performance motor, the largest you could choose is likely a 1100kv, 2200W motor from a popular motor company. But if you were to move to a 12s power system, a motor with roughly half the kv of the stock motor would be needed and that would open you up to lots of new options including the largest heli motor this company makes, a 630kv model rated at 4200W.
Drawbacks of high voltage systems
Although the drawbacks are small and few in comparison to the benefits, they do exist.
This biggest drawback is the cost. If you are starting from scratch, the cost is not much more but if you already have a functioning power system, the whole system will have to be replaced. These days the battery cost is similar but the larger motor and HV esc will definitely be more than the lower voltage models.
Powering the other components
Another problem commonly faced how to power the receiver, servo and other electronics. For 6s power systems, there are many BECs to choose from but for 10s and 12s systems your choices become few. Another option is to run a separate battery and regulator but that adds cost, weight and an additional battery to charge. The good news is that HV BECs are becoming more popular, so this is becoming less of an issue.
Most pilots are aware of the spark from connecting the lipo to the esc in a 6s power system and most of them don't like it. That said, the spark from a 10s or 12s system is many times greater. This scares most pilots but it is normal and should not be a concern.
Charging high voltage packs
Of course if you have a high voltage heli, you are going to have to charge the high voltage packs. Chargers for high voltage packs can be expensive and most are barely powerful enough to charge large packs at 1C. So this must be taken into account.
This leads me into related subject, using 2 lower cell count packs in series instead of 1 high cell count pack in high voltage setups.
Using 2 packs for HV setups
The idea of using 2 smaller packs in series is becoming more and more popular. Some believe that a single factory assembled pack is the only option but this is simply not the truth. As such I believe the days of huge packs, like a 12s 5000mAh pack, is soon coming to an end. Instead it is better to use a pair of 6s 5000mAh packs in series.
This is a multi-part thing. The cost of the packs may or may not be cheaper, that will depend on the packs chosen. Running 6s and smaller packs opens up the option to use cheaper brand packs. Most of the 8s+ packs are only available from a very limited number of assembles, most of which only offer high end packs. That causes these packs to be very expensive. So using 6s and smaller packs opens up many more brands including many very cheap ones.
When you plant a 10s 600 sized heli from 100ft up, you can pretty guarantee you taco'd the pack. If this was a 10s pack then the whole pack is unusable but if it was a pair of 5s packs, there is a good chance one of the 5s packs will survive. That saves money.
Having a pair of flight packs allows you move them around. In some cases you can separate them by placing one in the front of the heli and one in the rear. This greatly aids the cg adjustment, as well as give you more placement options for things like scale helis.
This is one of the biggest benefits, not having to buy a 10s or 12s charger and instead allowing you to use a smaller charger or a pair of them for faster charging. I am not going to explain all the shortcomings of the current 12s chargers out there but just let me say that they are expensive and limited. There are many more options for 10s chargers but compared to the 6s charger market, they come up pretty short. Right now the 6s chargers rule the roost. Using a pair of smaller packs lets you choose from these chargers.
Much lower charge time
This is related to the charger choices. Because of the selection of 6s chargers out there, it is easily possible to buy a pair of chargers and use one for each pack. This allows much higher charge rates and much lower charge times.
A few potential drawbacks
Because you are using a pair of packs, care needs to be taken to make sure the packs are well matched to each other. If you are buying new packs then this is not a problem but if you have older packs or mis-matched packs, it can affect performance. Next up is the more complicated wiring. A pair of packs will have to wired to the esc with a y-adapter cable. This slightly complicates things and adds additional points of failure. These are both small issues that are easily outweighed by the benefits.
Sample wiring setups for dual packs
The future will be full of big electric helis and most will run on HV power systems. Why? Because it makes no sense to use lower voltage systems and the efficiency of a HV system offers so many benefits.