I wanted a 130-amp load tester because--at least per the manufacturers' claims--these units are more accurate than 100-amp models.  The OTC 3181 looks exactly like the Actron CP7614 (except for brand markings on the front), but is far less expensive.The OTC 3182 is a digital 130-amp load tester for just a bit more money than this OTC 3181, and it has a couple more features (an alternator test lead and a ripple detection light), but I tend to have better luck with analog products over digital products, in terms of reliability. Also, when you use the OTC 3182, it shows you just a number (the voltage reading), and you're on your own to interpret it. The OTC 3181, on the other hand, has "replace/weak/good" markings (in red/yellow/green), which make it very easy to interpret the results, without having to remember or think about what voltage range is acceptable during testing.I was a little surprised at the size of the unit when I opened the box.  It's a bit large!  The clamps are large and strong.  The overall feel is quality.When testing a battery (unless it's a totally dead battery), the unit will heat up very significantly.  Through the vent holes in the unit, you can actually see metal coils getting red hot, like little heater elements.  That's how the load is applied--in the form of heat.Testing is easy: With the vehicle and the key both off, attach the negative clamp to the negative post of the battery, then attach the positive clamp to the positive post of the battery.  (People with mechanical experience may think this is backwards, since you normally attach the positive clamp first.  I assure you, I am quoting the manufacturer here on the order advised.)  Press and hold the rocker switch for 10 seconds, and then quickly look at the needle while still holding the switch down.  (They advise against holding the rocker switch for more than 15 seconds.) To test the charging system, begin with the clamps attached as described above, then start the car and run it at 1,200 - 1,500 RPM, and read the needle.  Remove the positive clamp, and then the negative.When I was choosing a load tester, I kept reading about "zero adjust" features.  I didn't know what that meant.  Now that I have a unit in hand, I can see that it refers to a little screw in the face of the unit; turning this screw one way or the other (with the clamps not connected to anything!) allows you to adjust the needle so that it is accurately zeroed.  That's all.