How to compensate battery voltage drop with light PWM value

[hamed-ta]

I made custom hardware that has 5 PWM channels to control 5 light channels, this device uses a one-cell lithium battery which when is fully charged the voltage is about 4.2 volts and when it's getting empty it can go down to 3 volts. so when the user sets the light brightness to 50% when the battery is full the PWM value should be different than when the battery voltage is 3.7 and when it's 3.2. I should mention that I measure the battery voltage, current, and power of the device with a power meter sensor. so where and how do you think is the better approach to adjust the PWM value based on brightness and battery voltage?

[mcspr]

Could be the same approach as with colored lights? With the most basic brightness, we simply walk the existing channels and apply it as a 0...100% scaling

espurna/code/espurna/light.cpp

Lines 732 to 736 in 02581c6

	void _lightValuesWithBrightness(LightChannels& channels, long brightness) {
	const auto Brightness = LightBrightness{brightness};
	for (auto& channel : channels) {
	channel.apply(Brightness);
	}

espurna/code/espurna/light.cpp

Lines 713 to 730 in 02581c6

	// After the channel value was updated through the API (i.e. through changing the `inputValue`),
	// these functions are expected to be called. Which one is chosen is based on the current settings values.
	// TODO: existing mapping class handles setting `inputValue` & getting `target` value applied by the transition handler
	// should it also handle setting the `value` so there's no need to refer to channels through numbers?
	struct LightBrightness {
	LightBrightness() = delete;
	explicit LightBrightness(long brightness) :
	_brightness(std::clamp(brightness, Light::BrightnessMin, Light::BrightnessMax))
	{}
	long operator()(long input) const {
	return (input * _brightness) / Light::BrightnessMax;
	}
	private:
	long _brightness;
	};

You could either replace the implementation completely and make it dependent on the battery level at that moment, or chain another object into the 'apply()` of the light channel

espurna/code/espurna/light.cpp

Line 875 in 02581c6

(*it).apply(rgb, Brightness);

(i.e. it applies channel filtered through rgb object first, then brightness one)

And some periodic timer to force-apply channel values

[hamed-ta]

ok got it, or maybe adding another module to the project called for example battery support?

any idea of the approach of adjusting the PWM value according to the battery level? when for example the nominal voltage of LEDs is 3.3 volts and the battery voltage range is between 3 to 4.2 volts? which means there should be an upper limit for PWM value according to the exceeded voltage (battery voltage - 3.3) and a limited brightness value when the battery voltage is less than 3.3 to show to the user?

[mcspr]

I'd think about the separation when we already have a clear api. It would also depend where are you getting the battery info; is it analog pin, is it some digital interface, etc. But if there is, sure.

re. values... just encode it as a 2-value table with specific voltage level <-> multiplier? for example, with 0.1v step (or, through experimentation, where voltage makes the most difference)

[hamed-ta]

I'd think about the separation when we already have a clear api. It would also depend where are you getting the battery info; is it analog pin, is it some digital interface, etc. But if there is, sure.

re. values... just encode it as a 2-value table with specific voltage level <-> multiplier? for example, with 0.1v step (or, through experimentation, where voltage makes the most difference)

I read the voltage, current, and power by a digital power meter sensor.
a discrete value table can be a little problematic as it can cause a flickering issue, I am thinking of a continuous solution, for example calculating the PWM value that causes the exceded RSM voltage more than 3.3 volts based on battery level and subtracting the MAX PWM value by this surplus value?

[hamed-ta]

Or another approach could be, as I have the current, that can be possible to store the corresponding current value when the brightness is set and then adjust the PWM value when the battery level changes but in this way, I still need to limit the MAX PWM value to prevent over current problem...!