What voltage should I use – 12v, 24v, 36v or 48v?
We are often asked whether 12v or 24v is best for a battery powered motor system [the short answer is 24v]. For a more reasoned explanation lets think about power, putting it in car terms, if you want to accelerate harder or have a higher top speed then you need more horsepower.
In an electric vehicle the power usually comes from the battery, and is converted by the motor into energy. Electrical power is volts multiplied by amps so that 40 amps from a 12v battery is 480 watts. But 480 watts can also come from a 24v battery at a current of only 20 amps; so for any particular power, the higher the voltage, the lower the current will be.
Now electrical current causes heating. The motor, wiring and controller will all get hot which wastes power. The heat wasted is proportional to the square of the current multiplied by the resistance. Other things being equal, that would cause losses on 24v to be half those on 12v.
It is clear from this that a 24v system is always better than a 12v system – provided you can physically fit two batteries. By the same token 36 or 48v would be even better [there is a page about using our controllers at 48v and above here].
Really heavy current systems (milk floats, electric cars, fork lift trucks) often use 72v or even 96v to reduce heating. We’re designing our new Pro-160 to work up to 84v.
The amount of energy in the batteries is amps x hours x volts. Consider a 12v 60 Ampere Hour battery. Clearly this is exactly the same as two smaller 12v 30 AH batteries in parallel. But the total amount of energy in these two will not change whether we connect them in parallel or in series. So a 12v 60 AH battery can store exactly the same energy as a 24v 30 AH battery.
There is another factor against 12v operation, MOSFETs need a good voltage to fully turn them on, so most of 4QD’s controllers use an internal 9v supply rail, which is adequate to ensure proper turn-on. However, there is not much difference between 9v and 12v. It does not take much current to be drawn from the battery before it drops 2v at its terminals. A small mount of extra drop in and wiring – and the 9v supply drops. After that, the available current from the controller drops quite quickly! Remember that the battery current is actually a chopped version of the motor current, so the inductance and resistance of the batteries and battery wiring all contribute to any voltage drop.
For this reason, we would generally not advise 12v operation if the peak motor current is likely to be more than around 30-50 amps.
12v Motors on 24v
Motor current rating
Motors are specified to run at a stated rpm at a particular applied voltage with a specified loading – that at which the motor takes its maximum continuous current.
If you run the motor under a lighter load than this ‘name plate rating’ its current consumption will reduce and its speed will increase slightly.
If you increase the load, then the motor’s current consumption will increase and its speed will reduce. Obviously you are now exceeding the motor’s continuous rating so it will start to get hotter than it should. The greater the overload, the quicker the motor will heat so there is a time limit on such an overload. However it is usually safe to run a motor at a 300%-400% over current for, perhaps, a minute – although this will vary from motor to motor.
If you run a 12v motor from 24v its current drain and speed will still depend on the mechanical loading. However under no load it will now run at twice the speed at which it originally ran with 12v. Heating in the motor is still related to the current – so you can still run it at its full rated mechanical load/current. However if the motor is badly balanced you may expect noise and vibration as the general construction may be inadequate for the faster speed. There may also be a problem with brush wear since the brushes are being asked to switch the current twice as fast. These effects are, however not very likely and usually the speed increase is quite OK.
There is one caveat on this. The motor is an inductive device and the commutator and brushes are a mechanical, switch. Such a mechanical switching system will have a limit on the maximum rate at which is can work and if this is approached, the commutation breaks down. Exactly what the limits are, I would not like to say but one effect is noise – and extreme noise can, on occasion, cause a controller to fail. The effect is quite rare – but beware of excessive over-revving.
Motor speed limits
Limits on motor speed are not simply the bearing quality. If you rev a motor hard enough – centrifugal force will take over and the rotor will fly to pieces. Also brush and commutator design is important. Depending on the design these will have a maximum switching rate and operating above this speed will cause tremendous brush arcing. In extreme circumstances this will generate severe noise transients which can destroy the controller. This is unlikely: we have only ever seen one customer do this: he was running 12v motors on 36v and blew two controllers! These motor limits are not things a controller manufacturer can really comment on: you need to consult the motor manufacturer.
If you overload the motor, its current rises in the same way whether the motor is running from 12v or 24v. However on stall the current from 24v could be twice that from 12v, so the motor could get four times as hot (heating is proportional to the square of the current). This however won’t happen when you are using a good controller as the controller will limit the current to its designed value. Also the controller varies the voltage on the motor so you are probably not going to use the motor at full voltage in any case.
Another consideration is that, if you put too much current through a permanent magnet motor, it is possible to slightly demagnetise the magnets. This is cumulative: the motor’s performance will drop slightly each time you do it. However, for battery motors, is is probably fairly safe to assume that, at the rated voltage, the current drawn when the motor is stalled will not reach this demagnetisation level. If you were to run a 12v motor off a 24v battery the stall current could then be excessive if it weren’t limited by the controller.
Therefore, provided you chose a controller suitable for the motor you use, you can usually run a motor 12v motor from a 24v battery with no effect except that full speed is doubled.
MOSFET gate voltage
Common MOSFETs require about 7 or 8 volts on their gates to properly turn them on. Because if this, most 4QD controllers have an internal supply of 9v – which gives nearly 8v on the MOSFET gates.
Now if you view the terminal voltage of a 12v battery, with an oscilloscope, you will find that, when the controller draws chopped current from the battery, there is a squarewave of 2 volts amplitude shown. The battery may be 13v open circuit, but during the PWM periods when current is actually being drawn, the effective voltage is actually falling to 11 volts. If you want to know more about why there is a chopped current, see our circuits archive.
Consider also that a 12v battery may, when 80% discharged (a realistic level before recharging) has a terminal voltage (open circuit) of about 10.8v. So the PWM will be working from effectively 8.8 volts. So there is no way the 9v internal rail of the controller can stay at 9v! And that’s before we start to consider voltage drops in the battery wiring due to its resistance and also its inductance.
So it’s pretty difficult to fully use a 12v battery at high currents and get the full rated current out of the controller, as the 9v rail will drop and, with it, the available current. See our service section for details of a modification to 12v version, Pro-120 because of this effect.
Motor stall current
Consider the stall current of a motor, for instance, the Sinclair C5 motor. On a freshly charged battery, its stall current can be 120 amps. This is limited by the motor resistance, the resistance of the leads supplying it and also on the internal resistance of the battery. Adding anything else into this loop will increase the loop’s resistance. So, if you have a system that works nicely without a motor speed controller, adding a motor speed controller will inevitably reduce its peak performance. Many 12v systems are simply not designed for operation with a speed controller and adding this will greatly reduce the performance.
The overheads on a 24v system are nowhere near as critical. The 2v drop, even 4v, will still take the battery supply nowhere near to the 9v rail. Motor resistances are also higher, so the extra effect of controller and wiring is less noticeable.
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