Using DMM when voltage is lower than 14.4V

[zako 10+ year member]

Let's assume that the amplifier RMS watt rating is honest at 14.4V. What do we do if the alternator puts out something like 13.8V or so? Is there a formula to deduce the RMS watt limit at lower voltage?

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[av83 10+ year member]

Let's assume that the amplifier RMS watt rating is honest at 14.4V. What do we do if the alternator puts out something like 13.8V or so? Is there a formula to deduce the RMS watt limit at lower voltage?

Measure your alts voltage at the front battery, or alt its self. See if its higher than in the rear. You may have a wiring issue.

But the actual answer would be to aim low. I'd knock at least 15 percent off the 14.4 rms figure. Much more if its an amp known to not do rated. If not, you'll clip it pretty bad.

 

[gckless 5,000+ posts]

Many of the non-maintstream brands (SA, DC, B2, etc.) also list ratings at 12.6V. If they don't, I'm sure you could email them and they wouldn't have a problem providing that info.

 

[maylar 10+ year member]

There's no formula to calculate that, it totally depends on the amp design. This is a case where an oscilloscope is better than a DMM.

 

[Undisputed King 10+ year member]

Yes well this is my best shot at it, if everything else stays the same efficiency and such, since the amplifier is linear with respect to voltage then it should be just a ratio between (oem rms watts)/14.4=(rms watts at lower voltage)/13.8 and just solve for the rms watts at lower voltage. By multiplying 13.8 to each side.

 

[Julian 10+ year member]

You can do it by max fusing because that is the max allowable amperage in.. but it's not perfect it can give you a general idea on most reputable amps.

Lets say you have a max fuse of 250a at 14.4 (unlikely to sustain it) 14.4x250= 3600w(.65) = 2340w after average 1 ohm effeciencey.

 

[maylar 10+ year member]

Yes well this is my best shot at it, if everything else stays the same efficiency and such, since the amplifier is linear with respect to voltage then it should be just a ratio between (oem rms watts)/14.4=(rms watts at lower voltage)/13.8 and just solve for the rms watts at lower voltage. By multiplying 13.8 to each side.

Not true. My amp is 1/2 power at 12V. Your formula implies that it should be 12/14.44 or 83%. Some amps can maintain full power at lower voltages, some can't.

 

[zako 10+ year member]

Indeed, I have RMS watt spec at 12V and 14.4V. I am thinking of just taking the mean of two to be safe.

 

[maylar 10+ year member]

I have similar concerns. The system voltage is controlled by a computer in my car and I can't be certain I'll have 14V with engine running. Gotta make a compromise I guess.