12V battery log

Since I use PriiDash for daily commuting, the routine is to turn the car to IG-ON first, not READY, to let the program to start recording before the car goes to READY mode. Back around May 2014, the MFD would throw a warning at IG-ON, after the car sat in the garage over the weekend. Then a few weeks later it got worse, almost every morning I got a warning at IG-ON. The car still started fine, but I knew the 12 V battery was on its way out. So I changed it with a new OEM battery from the dealer at end of May.

The new battery measured 12.87 V off the shelf - seems OK considering the date code was 071013W (October 2013; the red round sticker said 4/14 but that's probably just the date the dealer/distributor received it from Japan). The old battery measured 12.41 V after it was taken out and sat overnight. At this voltage the old battery didn't seem that bad - definitely not dead yet - just seemed old.

Looking back at the 12 V battery log, initially using ScanGauge, later using PriiDash, that recorded the OFF, IG-ACC, IG-ON, IG-ON + headlight, and IG-ON + headlight + rear defogger, we see some interesting trends. PriiDash recording started ~ 2010/12, and captured the lowest voltage during IG-ON when pressing the brake pedal. The abrupt drop ~ 2012/03 was due to an increase in sampling rate, not the battery suddenly got worse.



The most interesting part is that the new battery and the old one performs pretty much the same under load (the yellow and the red data points). They differ by merely 0.1 V. The lowest recorded IG-ON voltage did seem to have ~ 0.5 V change before and after the battery replacement, though the data is noisier. I had assumed that my old battery was weak starting from day one, but that seems to be a wrong assumption, unless the new battery is also weak, since the initial performance of the two batteries look about the same. The old battery seems just slowly getting weaker, no sudden death. At various times I tried using BatteryMinder and adding water, none gave any benefit to the old battery as far as I could tell. To be sure, I also used BatteryMinder on the new battery, and it had no significant effect on the voltage of the battery under load. For the new battery there are two sets of green data points: the lower ones are the lowest voltage during IG-ON (when pressing the brake pedal) and the higher ones are the initial steady IG-ON voltage (can be compared with the ScanGauge data in 2009 and 2010).

Early warning sign of a dying 12 V battery | PriusChat