HybridFan — Let's try to get to the bottom of this Auxiliary battery potential misbehavior, please. I have repeated my experiments a number of times now, with basically the same result each time. Waking up my ScangaugeII by pressing its Home button while my TCH is still OFF, I can monitor the auxiliary battery's voltage after the car has stood overnight. I find that I get a reading around 12.3 V each time. This has remained true even though the ambient air temperature has varied quite a bit over this period, from -6 to +6 degrees Celsius. As soon as I power the car ON (but not READY), it drops to around 11.9 V. Then when the car is started (i.e., READY), it rises to 14.2 V as the dc-to-dc converter starts charging the auxiliary battery. All lights and other ancillary devices have been switched "off" for these experiments. Can you confirm similar behavior from your TCH? I'd appreciate hearing from you in this regard. Thanks! I suspect that the battery is behaving correctly, but having more than just my own data would be really useful.

It has been said in this thread that this is an AGM (Absorptive Glass Mat) lead-acid battery. Do we know this for a fact? Toyota doesn't say in their literature. Could it be a gel-cell type? From the Web, I see that both types are apparently rather similar in their characteristics, and somewhat different from flooded-cell lead-acid batteries. [For example, their voltage apparently rises rather than falls at low temperatures.]

Stan