Clamp-On Ammeter

[markopolo]

Craftsman Digital AC/DC Clamp-On Ammeter



http://www.sears.com/shc/s/p_10153_12605_03482369000P

Digital clamp-on ammeter. Measures AC/DC current up to 400 amps with 3.0 percent (VAC) accuracy without breaking the circuit. Also measures AC/DC voltage, resistance, frequency, capacitance, continuity and diode. Type K thermometer allows for surface or air temperature measurements. Display is an extra large 4000 count LCD screen. Also includes one-touch auto zero, belt holster, molded rubber holster, auto on/off, test leads and thermocouple. Category III - 600 volt.

I bought one of these from Sears in the U.S. on my last trip. $59.99 seems pretty good to me. Sears sells better meters (like Fluke) as well but they cost too much for my hobby usage.

I plan to measure the DC current output of my solar panels and measure the current drawn when running my coffee maker off my inverter etc.

It would be a handy tool to determine how high the parasitic draw from your battery is.

[markopolo]

I used this tool today. I was just testing it. I used my 15" laptop (fully charged) and a 240 watt inverter connected to a 136ah battery bank (also fully charged).

The tests were:

1. laptop on, wifi connected, plugged into grid power
2. laptop on, wifi connected, plugged into inverter

Results:

1. AC Amp: .22
2. DC Amp: 3.1

Now, if Watts = Amps x Volts then:

.22 AC Amps x 120volts AC = 26.4 watts
3.1 DC Amps x 12volts DC = 37.2 watts

If there was no loss then the watts should have been the same. (120 watts = 120 volt @ 1 amp or 12 volts @ 10 amps) In my test, I used 41% more energy powering the laptop with the inverter.

I suspect the results were so poor because of the combination of:

1. relatively low power consumption (the inverter loss would be a greater percentage)
2. the laptop power brick (AC to DC) basically I went from DC to AC to DC to power the laptop

The 3.1 DC Amp hours used to power my laptop is the figure that matters. That's probably the minimum current it would use. Basically the laptop was just logged on to Windows, I wasn't doing anything with it.

[David Smith]

When plugged into the inverter you are converting 12V (or thereabouts) into 120V in the inverter and then converting the 120V back to the laptop voltage in the laptop inverter - which may be between 12V and 18V. Most of your losses are in the form of heat (feel the inverter and laptop inverter). Also, I don't know what type of inverter you are using but if it is a modified sine wave inverter (a square wave) they are less efficient than true sine inverters which are about 95% efficient I believe.