http://wiki.mikrotik.com/wiki/Monitor_i ... B333/433AH
I've been using that script on the 450G I have on solar.
I made a modification that "corrects" the RouterOS reading to match the reading we got with a volt-meter at that time. It may not be accurate all the time, but it got closer. I know approximate state of charge of the batteries. It is close enough to know when to start someone on the 2 hour drive out there to charge the batteries and clean the panels.
# the following to create a voltage correction ratio.
# multimeter measured voltage 12.5
:global mmv 125
# mikrotik health voltage while measuring with the multimeter 10.8
:global mtv 108
...
:global voltage ( ( [/system health get voltage] * $mmv ) / $mtv )
Each RouterBoard may need a different "correction" factor, so I planned on setting the script on each device to use measured values for that device. We have not, yet, put another 450G on batteries.