Concerns about parasitic 12V battery drain
We purchased a 2025 RAV4 PHEV, and are loving it so far, but my only concern is with the parasitic 12V battery drain.
TL;DR - parasitic 12V battery drain & infrequent driving leads to needing to top it off with a battery maintainer 2 or 3 times/month.
After reading up on it, I understand during short trips the car normally doesn't send high enough voltage to the 12V while driving to charge it, due to how the Battery Monitoring System is set; it's recommended to drive at least 30-60 minutes a week in READY mode to sufficiently recharge the battery.
Given we don't typically drive that far that often, the other alternatives are to turn the headlights on (not Auto) to send higher voltage (not preferred as we may forget to turn them off), or disconnect the BMS, also not preferred.
I also read that when the HV battery is charging, it should also charge the 12V, but once it reaches 100% this stops and the 12V starts draining again; it's recommended to schedule the charge to reach 100% right before we leave.
The best solution I've found to avoid waking up to a dead 12V, is hooking up a battery monitor, and getting a trickle charger to top off whenever it gets low. I installed an Ancel BM300 monitor, and the initial State-of-Charge was 59%. I hooked up a NOCO Genius 5 charger and charged it back to 100%, and have been checking the SoC daily; after a few short trips the battery had decreased to 85%.
After parking it last night at 5pm, it went from 85 to 81% within a couple hours, then held at 81% until morning.
I understand one of the main culprits of the parasitic draw is the Data Communication Module always looking for a cell signal, which supports the Emergency Response SOS button, as well as the Toyota Connected Services, but we don't subscribe to it, and have opted out of any data sharing in the Toyota app.
I suppose the SOS button could be useful in an emergency, but we always have our phones, so I looked for options to completely disable the DCM; there's a DCM bypass kit, but I'm not comfortable tearing apart the dash to install it. I reached out to a car stereo mechanic and they declined because it would be a liability disabling the EMS.
Apologies for the long post; I found all this info across various threads and figured it may be useful to anyone new to this car as well. A couple questions, if you've read this far:
Is the 4% drop over a couple hours normal? I plan to continue checking daily to see if it recurs; if so, I'm wondering if it's Toyota still gathering data, even though we've opted out.
Any other non-frequent drivers have a similar experience? At this rate it seems I'll be topping off the 12V once a week/two (whenever it gets down to 60%) to avoid it dying completely.