http://www.vicimeter.com/en/product/VC8145
Yep, this is the DMM I mentioned and I think it's a good candidate. Turns out to be easily modded (I've done a mod so the annoying buzzer only beeps in continuity mode, described here
https://www.eevblog.com/forum/testgear/vc8145-thread-it_s-here-finally/msg3448652/#msg3448652 )
I've done some preliminary work on it. It have four relays, so it consumes a couple of mA more than Daves GW Instek.
PSU---------------------------------------
Measured with: BK 2709B DMM.
9 Vdc single rail from a 7809. About 13.5V on the unregulated side.
It has a "low bat" segment on the LCD and the case has screw towers and such
that are unpopulated. A battery option is certainly possible, but the "low bat"
segment seems NOT to be connected/used as it never comes on.
Winding down the input voltage dims the backlight and the LCD starts fading pretty soon.
The measurement seems stable and reliable down to 7.55V.
I connected a PentaRef (that haven't been calibrated for 5-6 years) set to 3.9001V and the display (Vdc range) showed a stable 3.9009V down to 7.55V, and then started drifting upwards (as expected).
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Current consumption--------------------
Measured with: uCurrent on mA range, a BK 2709B DMM.
About 11-12 mA in the following modes:
mAdc, Vdc, Adc
About 44 mA in the following modes:
Vac, mVdc, Ohm (add 0.9mA on short), Cap, Temp, Out
74 mA in:
Diode/Cont
Back-light: add 9,6 mA
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My goal differs a bit from Daves. He wants a primary cell-driven low power DMM that lasts for decades with intermittent use. Li-SOCl2 as Dave suggested is an excellent choice for this and I understand the reasoning. But they are pretty expensive and my own user case is a bit different. So I want a rechargeable Li Ion system anyway (or would have selected alkaline or Li-FeS2 primary cells instead, as the DM8145 takes more power).
I've tested to power the DM8145 from a boost converter (this may be critical. There is a reason mains-driven measure equipment always have good old iron transformers and linear regulation) and nothing bad seems to be happening, but I haven't checked all the ranges and scenarios. But I know that many are gonna puke on my "solution" with buck and boost DCDCs, and rightly so. This is just a, eh, shall we call it "design study"?
The upper part (circled in red) of the schematics is the real PSU of the DM8145. The lower part is my quick and dirty add on for a 3.7V li ion battery (bank). It's just a sketch, I haven't done all the calculations, tests or other "brain task" necessary. The only practical tests I've done is measure the currents and a superficial test to power the DMM from a boost converter. The relay inhibit the boost converter when mains power is available. It may not be needed, but without it the battery sees a light duty as long as power is on (driving a useless DCDC) preventing some cycling. Without it the thing works like an online UPS with seamless switchover. I decided to stuff in a large cap (C5) to make the switchover seamless with the relay.
The case have several screw towers not in use. I've discovered that one was suitable to mount a microswitch for the battery mod. Attached are some pictures. Yes, the "block" used to push the micro switch is a large SMD cap I glued on the power putton steel arm.
Edit: It's a bit silly that they have chosen 5V relays (Omron, no less!) driven from a 5V rail, when they have 9V available and of course use linear regulation. 9V relays would draw less power. A mod is certainly possible with either reed relays or driven from 9V, to minimize the power consumption.