Posted by Sherman in Idaho [72.47.153.112] on Sunday, February 02, 2014 at 21:03:24 :
I know we'd all like to have Fluke multimeters and Snap-On wrenches, but some of us don't. In my case, I keep half a dozen $5 multimeters scattered around various vehicles and places so that I'll always have one handy, usually just to check battery voltage or continuity. For the last few days I've been making sure all the batteries in my idled rigs are either fully charged or removed before the next cold snap hits. I had a couple where I just couldn't get enough voltage out of the charging system, even though I could have sworn it used to be good. Turns out the cheap meters were reading low. The worst one read 11.7 for 12.0 volts. This makes a difference of about 50% in charge level of a battery.
I decided to take them all back to the lab and check them and calibrate them at 12 volts on the 20V DC range, since that's all I typically use. The good news is they're easy to calibrate, since there's only one adjustment (probably the reference voltage). Rather than dig out the Fluke 5100 for this, I just used a good stable HP power supply with an HP 3457A DVM to monitor it. The power supply held to within .001 V over an hour or so, which is plenty good enough for this. Just for kicks I checked my old workhorse Fluke 77 multimeter that's been beaten around in my toolbox for probably 20 years, and it read 11.99. No calibration needed there!
So, my first warning, is if you're using a $3.99 Harbor Freight coupon special and it tells you your charging system's not putting out enough juice, don't automatically believe it. If you don't have a good bench voltmeter and power supply, or a calibrator, you can compare your cheap meter to a friend's good (Fluke) meter just using the same car battery as a test source. If yours is off by more than 100 mV or so, it's worth calibrating it. Just take the back off and find the only adjustable thing in there. Another thing I learned is that even when the cheapo meters look the same on the outside, the innards can be quite different from one to the next.
The second thing I discovered about these cheapo meters, which is even worse, is that as the battery gets weak, the meters read too high. Not just a little too high, but way too high. With a battery at 7 volts, 12 volts shows up as 15 volts on the meter. This is obviously because the reference circuit is the cheapest imaginable, probably just a resistor plus a 6.2V zener, so as soon as there isn't enough juice in the battery, the reference voltage starts dropping, which makes the measurement circuit think the test voltage is higher than it is (because it's all relative to the reference), and yet there's nothing to warn you that the reading is no longer accurate. The lesson there is when the "low battery" indication comes on, replace the battery. The meter will appear to still work fine, but the readings will be too high.
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