Routerboard voltage monitor(how precise?)

RouterOS General
1

Hi, I would like to measure the voltage using rb433ah, to know when system is going on battery.
When there is real voltage 14,5V mikrotik shows 12,5V.
When I put it just on battery voltage real is 13,5V , mikrotik shows 11,5V.

So it seems that it measures the voltage real -2V.
Is that like I suppose or it measures voltage everytime different(not precise?).

2

Ok, it monitors voltage right, but in my case: it shows less than 1,8V than real voltage.
Could anybody help me, what is wrong with this script?:

/system health 
:if ([get voltage]<15) do={/:local subject "POZOR VYPADOK ELEKTRINY $[/system clock get date] $[/system clock get time] aktualne napatie: $[/system health get voltage]"
:local recipients { "yyyz@gmail.com";
"yyy@gmail.com";
 "xxx@gmail.com" }
:foreach r in=$recipients do={
   :put ("Sending email to " . [:tostr $r])
   /tool e-mail send tls=yes to=[:tostr $r] subject=[:tostr $subject]
}