My understanding is the HVB inverter will only take the 12V to around 12V, where the trickle charger will take it to 13.5+, and regular external charging will improve the expected 3-year life. All I know is the experience of being on the road with a dead 12V in 2016 (just at 3 years) was not a pleasant experience which I hope to avoid. And will replace the 12V again in early 2019. Rather spend ~$125 than be caught with a DOA car.
Did the dealer reset the battery age to 0 days with the install in 2016? Do you know the current SOC of the 12V battery? Rather than guess what is happening, get ForScan.
The DC/DC converter (not inverter) charges the 12V battery. From my observations, I've never seen the 12 V reading at any module below 13.85 V where desired voltage at the converter usually starts out high at 14.4 / 14.5 V which allows faster 12 V battery charging. After several minutes, my desired drops to 14.3 / 14.1 V and charging current drops. My SOC is currently at 58%. Car can probably sit at airport for a month and start upon return. However, this was not the case until I reset the battery age of the dealer replacement battery.
The BCM monitors the current through the negative cable of the battery via a battery monitoring sensor. I don't know the 12 V charging algorithm but I do know that the combination of battery age and the cumulative battery flowing current over time (+ and -) is used to track the SOC and it appears by my observations that the charging algorithm likely change as the battery ages and the energy capacity of the 12 V battery declines. My battery age was not reset by the dealer when my battery failed at about 3+ years. The SOC of my new 12 V battery very rarely got above 20% unless I put in on my battery charger. My battery age was 1700 days + (can't recall exactly when I reset the battery age) because the dealer did not reset my battery age at 3 + years. Below are observations from a 28 hour, 1900+ mile trip where a monitored 12 V battery charging before I reset my battery age.
The charging algorithm acts like a 3 stage battery charger (initial faster charging state, longer slow charge state, and a float charge state). There was no abrupt change in current just a slow drop early with the charge rate decling even slower as SOC increased. For the first 4 hours of driving, current flow through the 12 V battery averaged around 0.75 A (early it's mid / low single digits) and SOC went from 3% to about 50%. During the next 8 hours, current averaged about 0.5 A and SOC went up to 80%. During the last 16 hours, current averaged about 0.19 A and state of charge ended at 92 %. From about 88% SOC, current was running about 0.12 A with 0.06 A showing up sometimes (likely float voltage applied). It took about 24 hours of charging to near full charge battery - very similar to a battery charger (not a quick charger).
After I realized that the Windows free version of ForScan allowed a reset of battery age, I downloaded it and reset my battery age to 0 months. Now my SOC runs around 55-60% unless I take longer trips when it will increase.
Bottom line: Charge your 12V battery with your charger if you desire. Yes, it is good to fully charge a lead acid battery occassionally to prevent sulfation. Find out if your battery age was reset. If not, you are likely running at a very low SOC like I was.
Edited by plus 3 golfer, 18 July 2018 - 03:11 PM.