Brushless Motors

[KDTrey5]

Hi Everyone,

I recently competed at a tournament and saw some vehicles using brushless motors that performed well. Given that AA batteries provide much fewer amps than the LiPo/NiMH batteries typically required by brushless motors/ESCs, I am confused about how it is possible to reliably use them. Are people using some kind of module to increase the amps of the power supply or is the voltage requirement of the motor/ESC low enough where multiple AA batteries can be connected in parallel to provide the needed amps? Any help would be greatly appreciated.

Thank you

[Nydauron]

For the record, AA batteries that are NiMH do exist. AA batteries simply refer to a standard form factor and not the internal chemistry. Traditional 1.5v AA batteries are typically alkaline, which is what you are thinking of, but the 1.2v NiMH AA batteries are explicitly allowed by rule 3.a (since the only constraint is the voltage stated per AA battery).

As you may know, alkaline AA batteries are mostly rated to safely draw 0.5 to 1.0 A (over an extended period of time) because they all have an internal resistance of ~0.15 ohms, which is primarily due to the battery chemistry. This is something you cannot change without changing the chemistry of the battery you use. AA NiMHs, on the other hand, have a much lower internal resistance (~0.03 ohms), so the maximum current throughput is higher (max sustained current is about 4-8 times higher).

I haven't seen any EVs this season myself, but I would say it's very unlikely that teams using BLDC motors are using alkaline batteries in the first place, so if you are considering using a BLDC+ESC setup, you should use NiMH AAs instead of trying to solve a more complicated problem that is likely not to provide any benefit. 8 NiMHs would give 9.6V, which should be enough to control most 12V BLDC motors (granted, not at their rated top speed).

That being said, with only Alkalines, you can't simply increase the amperage of a circuit beyond the spec of the battery without some drawback (consider the following scenarios):

• You can technically put batteries in parallel instead to double the maximum current, but you halve the voltage. With 8 alkalines and assuming alkalines can draw 1A safely, this would only give you a maximum of around 2A safely while only having now 6 volts instead of 12. The minimum voltage for a motor will vary, but most require 7-8 V minimum. (e.g. The Emax XA2212 BLDC Motor (Datasheet) requires at least 8 V.)
• This was mainly a thought experiment, but consider the case where the batteries could charge capacitors, and then the capacitors discharge to get boosts of high current. However, the issue is that the large amount of current is brief and falls off quickly as the capacitors discharge. Higher farad capacitors can help to store more charge (), but it thus takes longer to charge ( where would also include the internal resistance of the batteries only while charging), resulting in . Even if you were to have large farad capacitors and used the time before each run to charge up the capacitors to their maximum, not only will the capacitors have to be massive, but if the amount of power being pulled is greater than what the batteries alone can supply, then the voltage across the capacitors will converge to 0V and the circuit will eventually just be powered by whatever voltage and current the battery outputs. This overall feels like an extra layer of complication for what is a little gain at the end of the day.