Linear BEC current limit dependent on battery voltage?

[tudordewolf]

I know linear BEC's are a pretty crude way to drive a servo in the age of compact UBECS and HV servos, but hear me out:

The Hobbywing 16BL30 is a neat little 2-3S, 30A brushless ESC with a "6V/1A BEC", and the manual specifies that it's a linear rather than switch-mode bec as is found on their larger ESC's. This means that it acts like a sort of variable resistor that 'burns off' the excess voltage between the input and the desired output. (switch-mode refers to dc-dc converters that typically achieve ~90% efficiency. I know all this will be redundant to forum regulars, but I figure I should include this for anyone unfamiliar with the jargon)

A 6V linear BEC on 3S is going to have to "drop" more volts within itself than it provides, which is kind of mind-blowingly inefficient. From that, though, I infer that the 1A limit is a worst-case scenario where it's running off a freshly-charged lipo at 12.6V, which would imply a ~6.6 watt average thermal limit.

On 2S, or even better, a 6-cell NiMh, the situation is a lot more favorable. The BEC would only have to drop a couple volts, less towards the end, achieving ~80% efficiency over the life of a 2S battery.

So, since the current limit to most electronic components, especially semiconductors, isn't a strict ceiling but rather an "average over time" limit primarily governed by heat buildup and dissipation, shouldn't the BEC be good for 2.75A on 2S, assuming a worst-case voltage drop of 2.4V? (6.6W / 2.4V = 2.75A)

I'll field-test my theory by driving two full-sized servos off a supposedly "1A" BEC in my souped-up bashing-crawler.

 

[Greywolf74]

I know linear BEC's are a pretty crude way to drive a servo in the age of compact UBECS and HV servos, but hear me out:

The Hobbywing 16BL30 is a neat little 2-3S, 30A brushless ESC with a "6V/1A BEC", and the manual specifies that it's a linear rather than switch-mode bec as is found on their larger ESC's. This means that it acts like a sort of variable resistor that 'burns off' the excess voltage between the input and the desired output. (switch-mode refers to dc-dc converters that typically achieve ~90% efficiency. I know all this will be redundant to forum regulars, but I figure I should include this for anyone unfamiliar with the jargon)

A 6V linear BEC on 3S is going to have to "drop" more volts within itself than it provides, which is kind of mind-blowingly inefficient. From that, though, I infer that the 1A limit is a worst-case scenario where it's running off a freshly-charged lipo at 12.6V, which would imply a ~6.6 watt average thermal limit.

On 2S, or even better, a 6-cell NiMh, the situation is a lot more favorable. The BEC would only have to drop a couple volts, less towards the end, achieving ~80% efficiency over the life of a 2S battery.

So, since the current limit to most electronic components, especially semiconductors, isn't a strict ceiling but rather an "average over time" limit primarily governed by heat buildup and dissipation, shouldn't the BEC be good for 2.75A on 2S, assuming a worst-case voltage drop of 2.4V? (6.6W / 2.4V = 2.75A)

I'll field-test my theory by driving two full-sized servos off a supposedly "1A" BEC in my souped-up bashing-crawler.

Let's break down your reasoning and see if your conclusions hold up.

1. Linear BEC on 3S Battery: As you correctly point out, a linear BEC in a 3S setup (where the nominal voltage can be around 11.1V, reaching up to 12.6V when fully charged) is quite inefficient. As you said, this is because a linear BEC works like a variable resistor, essentially dissipating the excess voltage as heat. For a 6V/1A BEC, the power dissipation can be significant when stepping down from 12.6V to 6V. The power dissipated in the BEC can be calculated as Power Dissipated=(Vin−Vout)×IoutPower Dissipated=(Vin−Vout)×Iout, where Vin is the input voltage (12.6V for a fully charged 3S battery), Vout is the output voltage (6V), and Iout is the output current (1A). This indeed results in a scenario where more power is dissipated as heat within the BEC than is actually provided to the load.
2. Efficiency on 2S Battery: The situation is better with a 2S battery (nominal voltage around 7.4V, up to 8.4V fully charged). The voltage drop required by the BEC is less, resulting in less power dissipation and better efficiency.
3. Current Limit on 2S Battery: The idea that the current limit isn't a strict ceiling but an average over time is generally correct for electronic components. It's primarily governed by thermal limits. The BEC's ability to handle higher currents on a 2S battery setup due to reduced voltage drop and hence lower power dissipation is a valid hypothesis. Using your calculation 6.6W2.4V2.4V6.6W, where 6.6W is the assumed thermal limit and 2.4V is the worst-case voltage drop on 2S, yields 2.75A. This suggests that, theoretically, the BEC could handle a higher current on 2S, given the reduced thermal stress.

However, a few caveats should be noted:

• Manufacturer's Ratings: The manufacturer's rating of 1A may take into account factors other than just thermal limits, such as the design and quality of components used in the BEC. It's generally advisable to adhere to these specifications to avoid potential failure or reduced lifespan of the component.
• Thermal Management: The BEC's ability to handle higher currents will also depend on its thermal management capabilities, such as heatsinks or airflow.
• Safety Margin: Manufacturers usually build in a safety margin in their specifications. Exceeding these specifications could increase the risk of failure, especially in scenarios where the BEC is under sustained load.

In summary, while your calculation and the theory behind it are sound, I would advise caution. Exceeding the manufacturer's specifications could lead to unpredictable behavior and potential failure, especially if other aspects of the BEC's design are not suited to handle the increased load but I'm sure you already know that so, Id say "Carry on sir!" and add a fan to the ESC if possible.

 

[ahr43]

Question? Based on OP's and Wolf's posts (above), would it be prudent to install an external BEC when going from 2s to 3s power? Is a quality ESC adversely effected in power delivery to the motor, ESC heat build-up notwithstanding, when the BEC circuit is running at inefficient levels?

Asking because my program features 2s DragSpec and 3s SpeedSpec runs on the same chassis. Interchanging b/w 2s for drag and 3s for speed. External BEC recommended for 3s in this specific example? Both are short-duration, high amp draw events.

'AC'

 

[Greywolf74]

Question? Based on OP's and Wolf's posts (above), would it be prudent to install an external BEC when going from 2s to 3s power? Is a quality ESC adversely effected in power delivery to the motor, ESC heat build-up notwithstanding, when the BEC circuit is running at inefficient levels?

Asking because my program features 2s DragSpec and 3s SpeedSpec runs on the same chassis. Interchanging b/w 2s for drag and 3s for speed. External BEC recommended for 3s in this specific example? Both are short-duration, high amp draw events.

'AC'

Some ESCs have really good BECs in them and if you're not using it in some extreme build its fine to use the internal one. Personally, I find it good practice to use external BECs in most of my kits just so I dont have to worry about the ESC having to do it. For $25 bucks Castle makes a 10A external BEC that is programmable from 4.8V to 9V output. Plenty for most applications and if its not they make bigger ones

 

[Jerold]

Linear BECs are simple linear regulators. Meaning that the current in = current out. So if you draw 1A @ 6V, you still draw 1A at your supply voltage. For example.

Power in = 12.6W
Power out = 6W
Power lost in heat = 6.6W.

This is a bit less than 50% efficiency. It gets worse with higher source voltage and at 6S it would be about 25% efficiency. If you have big batteries and low current draw it's doesn't matter so much. But if you need like 5 A or 10 A it's going to be a toaster.

The ideal application is where the current requirements are low and Vin/Vout are close to each other.

Switching BECs are switching regulators and much more efficient. The efficiency curve is not flat nor linear, it depends on a lot of factors Vin, Vout, temp, loading, etc... With all that still in play it's not uncommon to see 80% efficiency from poorly design/implemented BECs.

You could buy a simple switching power supply on Amazon, but plan to replace that often as they are pretty much junk. The solder joints will break with impacts and it could even cause a fire. So I would avoid them and stick to a known brand like Castle.