DC Voltmeter help

[navyflyer]

Hello everybody,
1) I have a 2008 Precedent. 4X12 2011 batteries. Added a voltmeter (as pictured below) wired correctly (thanks to this forum). My question is, after about 5-10 minutes of driving, it goes way down, like to 50%, and eventually into 30-40% range...is this right? Should it not be staying up in the 90% range and SLOWLY show drain?
Am I interpreting the info wrong?
2) The orange factory LOW BATT light (I was told) comes on at less than 50% charge. Is that true? If so, why would my new voltmeter show less that 50% and the factory light not come on?

I'm thinking either the new voltmeter is defective, or I'm just not undertanding what's going on with DC battery drain.

Thanks for your help.