Agilent 34461a - Fluke 8846A opinions & recommendations welcome

[TechJunkie]

With the screen saver enabled, what is the period of idle time before it goes into saver mode?  I've left mine on for 70 minutes and it has not gone into save mode - that I can tell.  Does it blank the screen or dim it and if dimming, by about how much?

If enabled, the unit will blank the screen after 8 hours of inactivity. LCD screens are not really susceptible to burn-in like the older CRT or even Plasma technologies. If the LCD screen was left on for weeks with numbers of letters, you might a slight weakness in those areas but using if would rapidly even this out. It's really not a concern. If you are going to leave your DMM on for a couple days then I would keep it enabled. Can't hurt!

[TechJunkie]

BVH,

  Wanted to show you how accurate these Agilent DMM's are even over a considerable length of time. I attached an image of my older Agilent which was produced in late 2005, but has low hours. Comparing it to my newer 34461A using a voltage reference of 2.5 VDC after a warmup time of 30 minutes each. The older Agilent was out by only out by 1-2 counts down to 5 places. Should give you some confidence! 

[BVH]

Thanks Eric for both posts.  I won't worry about my screen.  That's great news on the very long term accuracy on the two meters.  I'm most likely not going to worry about an annual calibration for my uses.

I've found that when in 5.5 and more digit display mode, the rapidly changing digits when not measuring anything bug me.  I've learned to simply press the Run/Stop button when I'm done and press it again when I need to measure (assuming there's 5 or 10 minutes between uses).  Are there any issues with this?  Would it technically necessitate another 30 minute warm-up after each cycle and does it technically lengthen the life of the constantly processing electronic components by not constantly triggering? ("technically" not real-word)

[TechJunkie]

Pressing the Run/Stop will not require additional warm-up nor will it shorten or prolong runtime. Keep in mind, "Runtime is Runtime!", regardless...

[BVH]

Can my new meter measure Internal Resistance of my Lithium Polymer RC battery cells without being damaged by the approximate 4.2 VDC cell voltage? Typical IR as measured by the chargers I have is between 1.5 and 5 mOhms so spec-wise it can do it but am just concerned about the Voltage level doing damage.  I have the Agilent 4-wire Kelvin probes.

[TechJunkie]

You can't directly measure the internal resistance of the battery with the DMM. There are two ways to measure internal battery resistance.

1) Use an industry standard 1KHz battery impedance meter. They go from around 50-60 bucks out of China up to thousands for a real good one. I found the cheaper models from China seem to do a good job and they are even 4-wire.

2) Use 2 meters, such as your Agilent and another DMM. Measure current on one with .5A, 1A and maybe 1.5A depending on your battery. You can use an electronic load for this if you have one or just a power resistor that can handle the current. Use the other DMM to measure the voltage drop across your battery

For step 2, make sure you select a resistor that will not discharge the cell to quickly. Typically around 1A for 18650's and .5A for smaller cells.

First, measure the cell voltage in an open circuit. (Nothing connected), then measure the resistor you are using. Next, connect the load resistor and battery to make a complete circuit. Measure the voltage drop across the battery, then subtract the open circuit voltage across the battery to the loaded circuit voltage across the battery and subtract to get the difference. Also make note of the current during the loaded test. Then use ohms law to calculate the internal resistance of the battery.

Here is an example I did with an old 18650:

1) Measure open circuit voltage across battery. (3.883V)
2) Connected 4.7 Ohm load across battery, measured both current and voltage drop across battery. (3.625V) and (690mA)
3) Subtract (3.883)-(3.625) = (0.258), this is your voltage drop across the battery.
4) Use Ohms law to calculate resistance: Resistance (R) = Voltage (V)/ Current (I). So (.258) / (.690) = .373913 or 373.913 mOhm.

A typical GOOD Lion cell like an 18650 should have an internal resistance of around 60 to 100 mOhms. The cell above is worn and at lifes end which is typical for an 18650 that is 8+ years old with hundreds of cycles to be higher in internal resistance. Keep in mind this is a DC resistance test and many prefer this over a 1KHz AC impedance test. I choose to do both methods and compare the results.

Hope this helps!

[BVH]

Thanks T J for the detailed reply.  I'll play around with the method #2 you detailed out.