If the larger speaker has the same impedance as the small speaker then at the same output voltage level the battery consumption will be the same.

But the larger speaker will be able to be much louder (10dB is produced by 10 times the power) then people might turn down its loudness which will make the battery charge last longer.

50W into 4 ohms from an amplifier needs a sinewave of 14.2V RMS which is 40.2V p-p minus a loss of 2V but the amplifier is bridged so the total p-p output and battery voltage must be 19.1VDC.

6 Lithium cells is 25.2V fully charged or 22.2V nominal.

5 Lithium cells is 21V fully charged or 18.5V nominal.

Using 5 cells at 18.5V then the output power into 4 ohms will be 47.5W which sounds the same as 50W.

The peak current for 50W RMS into 4 ohms is 4.2A then the battery voltage might drop a little.

But the larger speaker will be able to be much louder (10dB is produced by 10 times the power) then people might turn down its loudness which will make the battery charge last longer.

50W into 4 ohms from an amplifier needs a sinewave of 14.2V RMS which is 40.2V p-p minus a loss of 2V but the amplifier is bridged so the total p-p output and battery voltage must be 19.1VDC.

6 Lithium cells is 25.2V fully charged or 22.2V nominal.

5 Lithium cells is 21V fully charged or 18.5V nominal.

Using 5 cells at 18.5V then the output power into 4 ohms will be 47.5W which sounds the same as 50W.

The peak current for 50W RMS into 4 ohms is 4.2A then the battery voltage might drop a little.

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