Trying to get a guess at which of my meters are most accurate since I do not have a reference source to calibrate against. Significant standard deviation here. I may have to say my Tek, BK, and Fluke are pretty darn close together and very close to the average of all my meters and probably close to the real voltage...
3½=1999 count
3¾=3999 count
4½=19999 count
MeterAmplifier or ADC ICMethod3Display12V SLA
BK Precision 2833ICL7129INTLCD 4½??11MΩSlow6xUM2 (measured ~ 5mA, est 800h), or 7.5VDC"Portable" benchtop, true RMS
Centech HFT#92020 aka Mastech M830B w/lit displayICL7106 clone dieINTbacklit LCD 3½12.926.391MΩNAPP3 (200h)hFE, Backlight
Centech P33499 aka Mastech M300ICL7106 clone dieINTLCD 3½13.006.431MΩFastA23 (est. 20h)attached probes, pocket
Centech 96308 1ICL7106 clone die?INTLCD 3½12 ?6 ?9MΩSlow3xCR2032, measured 2.33mA, est 90hClamp 20A/200A/400A AC, 300V AC/DC, 2000/200KΩ
Centech P98674 aka Mastech MS8229FS9721 dieINTbacklit LCD 3¾ (3999)12.966.4110MΩFast3xUM4 (est. 300h, backlight will reduce to ~70h)Autorange+Lux/dB/RH/Temp/Backlit, 10A AC/DC, BROKEN/FAILED
Fluke 771CustomINTLCD 3¾ (3199)??11MΩSlowPP3 9V (rated 1600h for CZn)handheld Autorange
HP 3466A1CustomINTLED 4½??10MΩNA120V AC, 3xCyclon 2V 2.5Ah SLA, est. 3 hr"Portable" benchtop, True RMS
Tektronix DM502LD111 DIPINTLED 3½12.946.4010MΩNA120V ACModule for TM50x cabinet, hacked standalone
Tenma 72-4025UM7108F QFPINTLCD 3½12.916.3810MΩFastPP3 (Alkaline 565mAh/2.6mA drain = about 200h)hFE,Hz,10A AC/DC
Viz WD-750A VoltohmystLD111 DIPINTLED 3½12.886.3110MΩNA120V/240V
4xNiCd C (4h?)
"Portable" LED, metal case
AW Sperry SP-5A (legacy meter)1NONEMagMvMirrored Scale Analog, AC=Halfwave136.22KΩ/V-DUT, UM3 for Rx1KΩPocket Analog
Eico Model 5551NONEMagMvAnalog, AC=Halfbridge??20KΩ/V-DUT, 1xUM1 for Rx1Ω Rx100Ω Rx10KΩ, 4xUM3 for Rx10KΩStandard Analog multimeter
Radio Shack 22-2161SSI CMOS, NJM062M, LM2902, LM4558MagMvMirrored Scale Analog, precision op amp rectifier??10MΩ or 1MΩ in mV rangeFast4xUM3 est. 250 hoursFETVM Autorange
Heathkit V7A112AU7 Vacuum TubeMagMvAnalog AC=6AL5 halfbridge--11MΩNA120VAC, 1xUM2 for Ω modesVTVM SOLD
Craftsman Engine Analyzer1NONE/icl7611MagMvAnalog??625Ω/VNADUT, 1xPP3 for Rx1/Rx1KΩAnalog Car engine Analyzer, included because it can measure volts, amps, ohms just like a multimeter...
Simpson 200mV1,2ICL7107 DIPINTLED 3½--1MΩNo Ω5VDCPanel Meter SOLD
Breadboard Test1,2TC7116CPL DIPINTLCD 3½--1MΩNo Ω9VDCBreadboard, plan transfer to PCB
Homemade Digital PSU1,2AVR90S4433SARLCD HD44780about 12.5about 6.2~50KΩNo Ω120VACuncalibratable
Homemade Analog PSU1,2NONEMagMvAnalogabout 12.6about 6.21KΩ/VNo Ω120VACuncalibratable
1 Not used in calibration
2 Not a multimeter, added for comparison
3 Integrating, Magnetic Movement, Successive Approximation Register
4 Continuity Beeper. Mode where it sounds a buzzer when a low resistance is connected. Fast = no lag, Slow = latched/slight lag. NA = this multimeter requires power during use but does not have continuity beeper. A hyphen means that this feature is not practical due to power requirements.

The FS9721 supports RS232-out and was hacked into my P98674.
The UM7108F supports serial-out but requires a microcontroller to support RS232-out to convert the 16-bit raw data to RS232. This may be a project in the future - a RS232 capable microcontroller with 2 input ports (receive data, End of Conversion) and 1 output port (clock) is needed. I have plenty of AVR90S2313's that could be used... A tiny AVR with software RS232 may work too.

Average 12V battery: 12.935V
Standard Deviation: 0.0382

Average 6V battery: 6.387V
Standard Deviation is 0.0377

I probably shouldn't trust my WD750A and the P33449? Tried to calibrate the P33449 to the Tektronix, unfortunately now my average and std deviation changed...
New Voltage Analysis