I have a RB450G that I use at a remote solar site. I noticed this weekend that I had a low voltage showing on the Health meter of 10.9. Since this was a new setup I thought that I had to many devices running and that the battery pack was extreamly low. I was afraid that the low voltage disconnect was going to shut the intire system down. So , I made a mad dash up to the mountain to disconnect equipment and keep only the critical stuff running until I could get a generator up there. The next morning I made another trip up there with a volt meter and started to check stuff out. What I found is that the voltage reading from the battery to the RB showed no drop (20 ft run of 10awg) from the battery to the injector. No suprizes there considering the wire size distance and small current draw. The voltage at the injector is 13.5vdc the reading on the Health meter showed 12.25
I pulled the RB and brought it back to the shop and ran some test. What I found is that a Power source showing 14.90 vdc showes 14.2 vdc on the RB when pluged in to the power jack and shows 12.8vdc when ran thru the poe/ether1 port with just 1 foot of cat5 pigtail.

Questions are.
1. Why is there a big difference in readings, as to where the power is injected into the RB?
2. Is there a setting that can be adjusted in the ROS to make adjustments as to what the voltage is suppose to be reading?

RB450G voltage monitor get only approx. value. Volatge reading offset is directly related to CPU load. You can try different RouterOS version and check what is more precise. All new RB version have more preicse voltage monitor.