And now for the battery mod...
Several ways to get an "battery option" is possible. I think that a standard 7.2 Ah 12V SLA battery might fit, and if so, it's a good choice if the transformer has enough voltage. There is likely some individual variations, and my transformer, when unloaded, after rectification gives about 14V. It's just about too low for a good headroom when charging a 12V SLA, but it may be a possibility.
Now, measuring equipment often have linear PSUs for a reason. Any source of noise should be avoided, if possible. I know this, so please don't crucify me for introducing DCDCs into this DMM. Just saying, because someone ought to tell me that that's no good idea (and rightly so) otherwise...
First I doodled some quick n' dirty li ion option, using cheap (and probably very noisy) China modules as a buck, a boost and a li ion charger. Attached is the schematics. The red circle compasses the original linear PSU. The rest is my draft for a 3.7V li ion battery option.
But, since I already had four 2V D-cell SLAs just laying around I decided to create a version with Pb acid batteries instead. One of the cells are bad, but the other three I successfully desulfated a few weeks ago. So attached is also my current working solution that I built (I made an error in my last upload in another thread).
In order to be able to switch the DMM on and off, I mounted a microswitch at a unused screw tower and glued a block (a large SMD HV-capacitor btw) on the steel rod going from the front panel to the power switch at the back.
Note: The pictures show four D-cell SLAs. I'm only using three of them. The last one is a dud, but I didn't bother to separate it from the pack. It adds a little good weight to the unit. Also, the little switch connected to the batteries near the transformer are not in the schematics. It serves no other function than to disconnect the battery if you service the thing.
---------- Some measurements and data about PSU power consumption ------------
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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